Explain the Unit 1: Introduction to Instrumentation System (10 hrs)

Basic of 1.1 Typical applications of Instrument systems

Instrumentation, 1.2 Functional elements of Instrumentation and Measuring systems i.e.,
Bridge Input elements (Transducers & Electrodes), intermediate elements
Measurement and (signal conditioning) and output elements (Data display & storage)
Transducers 1.3 Errors and uncertainties in Measurements and Static performance
characteristics of instruments:
1.3.1 Introduction to errors and uncertainties in the measurement of
performance parameters of instruments.
1.3.2 Static performance parameters: Accuracy, Precision,
Resolution, Threshold, Sensitivity, Linearity, Hysteresis, Dead
band, Backlash, Drift, Span
1.3.3 Impedance loading and matching
1.3.4 Errors: Statistical analysis of error in measurement
1.3.5 Standards of measurement
1.4 Bridge Measurement:
1.4.1 DC bridges- Wheat-stone bridge
1.4.2 AC bridges – Kelvin, Hay, Maxwell, Schering and Wien bridge
1.4.3 Wagner ground Connection
1.5 Physical Variable and Transducer
1.5.1 Physical Variable and their types (Electrical, Mechanical,
Process and Biophysical)
1.5.2 Transducer principle and operation
1.5.3 Input and output characteristics and application of transducers
1.5.3.1 Resistive
1.5.3.2 Capacitive
1.5.3.3 Inductive
1.6 Measurement of mechanical variables, displacement, strain. Velocity,
acceleration and vibration
1.7 Measurement of process variables ‐ temperature pressure, level, fluid
flow, chemical constituents in gases or liquids, pH and humidity
1.8 Measurement of bio‐physical variables blood pressure and
myoelectric potentials
1.9 Calibration and error in transducers
1.10 Measurement of voltage & current (moving coil & moving iron
instruments
1.11 Measurement of low, high & medium resistances
Explain the basis of Unit II: Principle of Analog Instruments (7 hrs)
Analog instruments 2.1 Review of DC/AC voltmeter & Ammeter: The D' Arsonval Principle
and Principle of 2.2 DC Multirange Ammeters and Extending Ammeter ranges
equipment used in 2.3 DC Multirange Voltmeters and Extending Voltmeters ranges

measurement of 2.4 AC voltmeter and multi range voltmeter

electrical quantities 2.5 Ohm Meter and Multirange
2.6 Electronic Multimeter
2.7 Multimeter as a micro ammeter and dc ammeter Types pf voltmeter:
Differential type and True rms
2.8 Wattmeter: Types and Working principles
2.9 Energy Meter: Types and Working Principle
2.10 Power Factor Meter
2.11 Instrument Transformer
Explain about the Unit III: Electrical Signal Processing and Data Acquisition (7 hrs)
Signal conditioning 3.1 Basic Op‐amp characteristics
and transmission 3.2 Instrumentation amplifier
system 3.3 Signal amplification, attenuation, integration, differentiation, network
isolation, wave shaping
3.4 Effect of noise, analog filtering, digital filtering
3.5 Data Acquisition System
3.5.1 Analog Data Acquisition System
3.5.2 Digital Data Acquisition system
3.5.3 Single channel Data Acquisition system:
3.5.4 Multi-channel Data Acquisition system
3.5.5 PC based Data acquisition system
3.6 Series and Parallel transmission:
3.6.1 Features and application of RS232 cable
3.6.2 Features and application of IEEE 1248 B
3.7 Optical communication, fibre optics, electro-optic conversion device
 Explain about the Unit IV: Date Converter and Connectors (8 hrs)
analog to Digital 1.1 Analog to Digital Converter (ADC) and Digital to analog Converter
and Digital to (DAC): Principle and Specification
Analog converter 1.2 Quantization Error
in depth 1.3 Types of ADC
1.3.1 Flash type ADC
1.3.2 Counter type ADC
1.3.3 Successive Approximation Type ADC
1.3.4 Dual Slope ADC
1.3.5 Introduction to Delta-Sigma ADC
1.4 Types of DAC
1.4.1 Weighted Resistor DAC
1.4.2 R-2R Ladder DAC
1.4.3 PWM Type DAC
1.5 Probes and Connectors
1.5.1 Test Leads: Twisted pair unshielded test leads
1.5.2 Shielded Cables
1.5.3 Connectors
1.5.4 Low Capacitive Probes
1.5.5 High Voltage Probes
1.5.6 Current Probes
 Compare different Unit V: Wave Analyzers and Digital Instruments (8 hrs)
types of wave 5.1 Wave Analyzer
analyzer and 5.1.1 Frequency Selective Wave Analyzer
principle of 5.1.2 Heterodyne Wave Analyzer
Digital 5.2 Spectrum Analyzer
instrumentation. 5.2.1 Basic Spectrum Analyzer using Swept Receiver Design
5.2.2 IRF Spectrum Analyzer
5.3 Distortion Analyzer: Harmonic Distortion Analyzer-Fundamental
Suppression Type
5.4 Measurements of Frequency and Time: Decimal Count Assembles
5.5 Frequency Counter
5.6 Period Counter
5.7 Error: Counter Error and Signal Related Error
5.8 Digital Voltmeter
5.8.1 Ramp type digital voltmeter
5.8.2 Integrating type digital voltmeter
5.8.3 Servo Potentiometer type digital Voltmeter
5.8.4 Successive Approximation type digital Voltmeter
5.9 Vector Voltmeter
5.10 Digital Multimeter
5.11 Computer Based Digital Instruments: IEEE 488 GPIB Instrument
 Differentiate Unit VI: Recorders, Displays and Storage Devices (5 hrs) 6.1
different types of Oscilloscopes:
output devices used 6.1.1 Cathode Ray Tube, Vertical and Horizontal Deflection Systems,
in Delay lines, Probes and Transducers,
instrumentation 6.1.2 Specification of an Oscilloscope
6.1.3 Oscilloscope measurement Techniques
6.2 Special Oscilloscopes – Storage Oscilloscope, Sampling Oscilloscope
6.3 Recorders Basic recording systems. Strip chart recorders.
Galvanometer and Potentiometer type recorders (direct and null type)
6.4 Indicators and display Devices - Nixie, LED, LCD and seven segment
and dot matrix displays.
6.5 Magnetic tape and disc recorders
6.6 Data loggers, Dot matrix and laser printers
6.7 Compact disc/Optical disc recorders

Unit 1: Introduction to Instrumentation System (10 hrs)

Introduction to Instrument and Instrumentation System
An instrument is a device that communicates, detects, denotes, indicates, observes, measures,
records, or signals a quantity or controls or phenomenon, or manipulates another device.
Instrumentation is a collective term for measuring instruments that are used for indicating,
measuring and recording physical quantities.
An Instrumentation system exists to provide information about the physical value of some
variable being measured. These Variables are called as process variable which states a condition
of the process fluid (a liquid or gas) that can change the manufacturing process in some way. In
simple cases, the system can consist of only a single unit that gives an output reading or signal
according to the magnitude of the unknown variable applied to it. However, in more complex
measurement situations, a measuring system consists of several separate elements

Measurement
Measurement is defined as an act or the result of comparison between the unknown quantity and
a predefined standard. A measurement of any quantity is characterized by a numerical value
followed the unit. The numerical value is meaningless if it is not followed by the unit. For
example, if you measure a length of the table, it comes out to be 2.5 meter. Then, only numerical
value 2.5 is meaningless without the unit of length meter.

Intelligent vs. Dumb Measurement Systems

Intelligent instrumentation system:-In this system after a measurement has been made of the
variable, further processing whether in digital or analog form is carried out to refine the data, for
the purpose of presentation to an observer or to other computers. Use of computer system,
Artificial intelligence and microprocessor make it easy to compile it.
Dumb instrumentation system:- In this system once the measurement is made, the data must be
processed by the observer.

Methods of Measurements
There are two types of methods of measurements i.e.
1.Direct Methods: Direct measurement refers to measuring exactly the thing that you are
looking to measure. So, the unknown quantity or measurand is directly compared against a
standard. For example pressure, time, length, weight, etc. can be directly measured.
2.Indirect Methods: Indirect measurement refers to measuring something by measuring
something else. Sometimes it is not possible to measure a quantity directly. Hence we measure a
different quantity that can be easily measured and using which we can find actual measurand. For
0example, differential pressure on a pitot tube is used to determine airspeed.

Typical applications of Instrument systems

All our electronics enclosures are suitable for use in the measurement, control and
instrumentation sector and for all enclosure solutions in the automation sector.
Typical applications include:
• Data loggers
• Control systems for outdoor equipment
• System modules for automation technology
• Sensor technology and optoelectronics (e.g. control and alarm devices)
• Peripheral devices and data collection in the laboratory and research
Typical example
Dairy processing unit operations mainly involve heating, cooling, separating, drying or freezing
of the products. These unit operations are carried out under varying conditions of temperatures,
pressures, flows and physical compositions. The measurement and control of these variable
factors at the various stages of processing call for the accurate and efficient instruments, in
addition to the dependence upon human skills. With the advent of large scale milk handling
plants the automatic operation and control through efficient instrumentation and automation has
become even more necessary. Utilities such as steam, water, electricity air, fuel etc. have to be
measured and controlled at appropriate points in the plant. Automatic control instruments are
employed to measure and control the temperature, pressure, flow and level of these utilities. The
overall aim of the instrumentation/ automation is to improve the product quality and enhance the
plant efficiency for better economic returns.

Functional Elements of Instrumentation and Measuring

The block diagram of a basic instrumentation system consists of primary sensing element,
variable manipulation element, data transmission element and data presentation element.
Primary Sensing Element: The quantity under measurement makes its first contact with
primary sensing element of measurement system. The quantity is first sensed or detected by
primary sensor. Then detected physical quantity signal is converted into an electrical signal by a
transducer. Transducer is defined as a device which converts a physical quantity into an electrical
quantity. Sensor is act as primary element of transducer. In many cases the physical quantity 8 is
directly converted into an electrical quantity by a transducer. So the first stage of a measurement
system is known as a detector transducer stage.

Variable Conversion Element: The output of primary sensing element is electrical signal of any
form like a voltage, a frequency or some other electrical parameter. Sometime this output not
suitable for next level of system. So it is necessary to convert the output some other suitable form
while maintaining the original signal to perform the desired function the system. For example the
output primary sensing element is in analog form of signal and next stage of system accepts only
in digital form of signal. So, we have to convert analog signal into digital form using an A/D
converter. Here A/D converter is act as variable conversion element

Variable Manipulation Element: The function of variable manipulation element is to

manipulate the signal offered but original nature of signal is maintained in same state. Here
manipulation means only change in the numerical value of signal. Examples, 1. Voltage
amplifier is act as variable manipulation element. Voltage amplifier accepts a small voltage
signal as input and produces the voltage with greater magnitude .Here numerical value of voltage
magnitude is increased.

Data Transmission Element:The elements of measurement system are actually physically

separated; it becomes necessary to transmit the data from one to another. The element which is
performs this function is called as data transmission element. Example, Control signals are
transmitted from earth station to Space-crafts by a telemetry system using radio signals. Here
telemetry system is act as data transmission element. The combination of Signal conditioning and
transmission element is known as Intermediate Stage of measurement system

Data presentation Element: The function of this element in the measurement system is to
communicate the information about the measured physical quantity to human observer or to
present it in an understandable form for monitoring, control and analysis purposes. Visual
display devices are required for monitoring of measured data. These devices may be analog or
digital instruments like ammeter, voltmeter, camera, CRT, printers, analog and digital computers.
Computers are used for control and analysis of measured data of measurement system. This Final
stage of measurement system is known as Terminating stage.
Some applications requires a separate data storage and playback function for easily rebuild the
stored data based on the command. The data storage is made in the form of pen/ink and digital
recording. Examples, magnetic tape recorder/ reproducer, X-Y recorder, X-t recorder, Optical
Disc recording etc.

Introduction to Errors and Uncertainties in the Measurement

Errors in Measurement and its Types
An error may be defined as the difference between the measured and actual values. For example,
if the two operators use the same device or instrument for measurement. It is not necessary that
both operators get similar results. The difference between the measurements is referred to as an
ERROR.
Error = Measured value – True value
Types of Errors
There are three types of errors that are classified based on the source they arise from; They are:
• Gross Errors
• Random Errors
• Systematic Errors
Gross Errors
This category basically takes into account human oversight and other mistakes while reading,
recording, and readings. The most common human error in measurement falls under this
category of measurement errors. For example, the person taking the reading from the meter of
the instrument may read 23 as 28. Gross errors can be avoided by using two suitable measures,
and they are written below:
• Proper care should be taken in reading, recording the data. Also, the calculation of error
should be done accurately.
• By increasing the number of experimenters, we can reduce the gross errors. If each
experimenter takes different readings at different points, then by taking the average of
more readings, we can reduce the gross errors

Random Errors
The random errors are those errors, which occur irregularly and hence are random. These can
arise due to random and unpredictable fluctuations in experimental conditions (Example:
unpredictable fluctuations in temperature, voltage supply, mechanical vibrations of experimental
set-ups, etc, errors by the observer taking readings, etc. For example, when the same person
repeats the same observation, he may likely get different readings every time.

Systematic Errors:
Systematic errors can be better understood if we divide them into subgroups; They are:
• Environmental Errors
 • Observational Errors
• Instrumental Errors
Environmental Errors: This type of error arises in the measurement due to the effect of the
external conditions on the measurement. The external condition includes temperature, pressure,
and humidity and can also include an external magnetic field. If you measure your temperature
under the armpits and during the measurement, if the electricity goes out and the room gets hot, it
will affect your body temperature, affecting the reading.
Observational Errors: These are the errors that arise due to an individual‘s bias, lack of proper
setting of the apparatus, or an individual‘s carelessness in taking observations. The measurement
errors also include wrong readings due to Parallax errors.
Instrumental Errors: These errors arise due to faulty construction and calibration of the
measuring instruments. Such errors arise due to the hysteresis of the equipment or due to friction.
Lots of the time, the equipment being used is faulty due to misuse or neglect, which changes the
reading of the equipment. The zero error is a very common type of error. This error is common in
devices like Vernier callipers and screw gauges. The zero error can be either positive or negative.
Sometimes the scale readings are worn off, which can also lead to a bad reading.
Instrumental error takes place due to:
• An inherent constraint of devices
• Misuse of Apparatus
• Effect of Loading

Uncertainties in Measurements
• All measurements have a degree of uncertainty regardless of precision and accuracy. This
is caused by two factors, the limitation of the measuring instrument (systematic error) and
the skill of the experimenter making the measurements (random error).
• Errors are the difference between the measured value and real or expected value;
uncertainty is the range of variation between measured value and expected or real value.
• To calculate uncertainty, we take the accepted or expected value and subtract the furthest
value from the expected one. The uncertainty is the absolute value of this result.
• The graduated buret in Figure 1 contains a certain amount of water (with yellow dye) to
be measured. The amount of water is somewhere between 19 ml and 20 ml according to
the marked lines. By checking to see where the bottom of the meniscus lies, referencing
the ten smaller lines, the amount of water lies between 19.8 ml and 20 ml. The next step
is to estimate the uncertainty between 19.8 ml and 20 ml. Making an approximate guess,
the level is less than 20 ml, but greater than 19.8 ml. We then report that the measured
amount is approximately 19.9 ml. The graduated cylinder itself may be distorted such that
the graduation marks contain inaccuracies providing readings slightly different from the
actual volume of liquid present.
Performance Characteristics of Measurement System
The characteristics of measurement instruments which are helpful to know the performance of
instrument and help in measuring any quantity or parameter, are known as Performance
Characteristics. The performance characteristics of measuring instrument are judge by how
faithfully the system measures the desired input & how thoroughly it rejects the undesired input.
Performance characteristics of instruments can be classified into the following two types.
• Static Characteristics: value of measured variable change slowly
• Dynamic Characteristics: value of measured variable change very fast
Static Characteristics
• The characteristics of quantities or parameters measuring instruments that do not vary
with respect to time are called static characteristics. Sometimes, these quantities or
parameters may vary slowly with respect to time. Following are the list of static
characteristics.
• The static characteristics and parameters of measuring instruments describe the
performance of the instruments related to the steady-state input/output variables only.
The various static characteristics and parameters are destined for quantitative description
of the instruments perfections and they are well presented in the manufacturer's manuals
and data sheets.
• The various static characteristics are:
i) Accuracy ii) Precision iii) Sensitivity iv) Linearity
v) Reproducibility vi) Repeatability vii) Resolution viii) Threshold
ix) Drift x) Stability xi) Tolerance xii) Range or span
xiii) Hysteresis xiv) Bias xv) static error
i) Accuracy: It is the closeness with which an instrument reading approaches the true value of
the quantity being measured. Thus accuracy of a measurement means conformity to truth. It the
important static characteristic of electrical measuring instruments.
Deviation from the true value indicates the low accurate of measurement. Accuracy can be
specified in terms of inaccuracy or limits of errors and can be expressed in the following ways:
a. Point accuracy:
This is the accuracy of the instrument only at one point on its scale. The specification of
this accuracy does not give any information about the accuracy at other points on the scale
 or in the words, does not give any information about the general accuracy of the instrument.
b. Accuracy as percentage of scale range:
When an instrument has uniform scale, it's accuracy may be expressed in terms of scale
range.
c. Accuracy as percentage of true value:
The best way to conceive the idea of accuracy is to specify it in terms of the true value of the
quantity being measured within +0.5% or -0.5% of true value.

ii) Precision: It is the measure of reproducibility i.e., given a fixed value of a quantity,
precision is a measure of the degree of agreement within a group of measurements.
If an instrument indicates the same value repeatedly when it is used to measure the same quantity
under same circumstances for any number of times, then we can say that the instrument has high
precision.
The precision is composed of two characteristics:
a) Conformity: Consider a resistor having true value as 2385692, which is being
measured by an ohmmeter. But the reader can read consistently, a value as 2.4 M due to the
nonavailability of proper scale. The error created due to the limitation of the scale reading is a
precision error.
b) Number of significant figures: The precision of the measurement is obtained from the
number of significant figures, in which the reading is expressed. The significant figures convey the
actual information about the magnitude & the measurement precision of the quantity.

Q. Accuracy and precision are dependent on each other, explain.

Consider a shooter aiming at the target. Fig below illustrate difference between them,

Sensitivity: The sensitivity denotes the smallest change in the measured variable to which the
instrument responds. It is defined as the ratio of the changes in the output of an instrument to a
change in the value of the quantity to be measured. Mathematically it is expressed as,

Sensitivity =
The term sensitivity signifies the smallest change in the measurable input that is required for an
instrument to respond.
 • If the calibration curve is linear, then the sensitivity of the instrument will be a constant
and it is equal to slope of the calibration curve.
• If the calibration curve is non-linear, then the sensitivity of the instrument will not be a
constant and it will vary with respect to the input.

Linearity: Linearity is an indicator of the consistency of measurements over the entire range of
measurements. In general, it is a good indicator of performance quality.
It is also defined as ability to reproduce the input characteristics symmetrically and linearly. The
curve shows actual calibration curve and idealized straight line.

Threshold: If the instrument input is increased very gradually from zero there will be some
minimum value below which no output change can be detected. This minimum value defines the
threshold of the instruments. In specifying threshold, the first detectable output change is often
described as being any noticeable measurable change.

Resolution: The smallest change in a measurement variable to which an instrument will respond
is resolution. If the output of an instrument will change only when there is a specific increment of
the input, then that increment of the input is called Resolution. That means, the instrument is
capable of measuring the input effectively, when there is a resolution of the input.

Hysteresis-Threshold Resolution: When testing an instrument for repeatability, it is often noted

that the input-out value does not coincide with the inputs, which are continuously ascending and
descending values. This occurs because of hysteresis, which is caused by internal friction,
sliding, external friction, and free play mechanisms. Hysteresis can be eliminated by taking
readings corresponding to the ascending and descending values of the input and calculating their
arithmetic mean.
Drift
Drift is a departure in the output of the instrument over the period of time. An instrument is said
to have no drift if it produces same reading at different times for the same variation in the
measured variable. Drift is unrelated to the operating conditions or load. The following factors
could contribute towards the drift in the instruments:
• Wear and tear
• Mechanical vibrations
• Stresses developed in the parts of the instrument
• Temperature variations
• Stray electric and magnetic fields
• Thermal emf
Drift can occur in the flow meters due to wear of nozzle or venturi. It may occur in the resistance
thermometer due to metal contamination etc.
Drift may be of any of the following types;
a) Zero drift: Drift is called zero drift if the whole of instrument calibration shifts over by
the same amount. It may be due to shifting of pointer or permanent set.
b) Span drift: If the calibration from zero upwards changes proportionately it is called span
drift. It may be due to the change in spring gradient.
c) Zonal drift: When the drift occurs only over a portion of the span of the instrument it is
called zonal drift.

Drift is an undesirable quality in industrial instruments because it is rarely apparent and cannot
be easily compensated for. Thus, it must be carefully guarded against by continuous fields can be
prevented from affecting the measurements for proper shielding. Effect of mechanical vibrations
can be minimized by having proper mountings. Temperature changes during the measurement
process should be preferably avoided or otherwise be properly compensated for.

Range or span:
The input range of an measuring device is specified by the minimum and maximum values of
input variable (Xmin to Xmax). The output range of an measuring device is specified by the
minimum and maximum values of output variable (Ymin to Ymax). Span and range are the two
terms that convey the information about the lower and apa calibration points. The range of
indicating instruments is normally from zero to full scale value and the Span is simply the
difference between the full scale and lower scale value.
The minimum & maximum values of a quantity for which an instrument is designed to measure
is called its range or span. For exapmple: For a standard thermometer given the range 0° C to
100°C then span is 100°C . If the thermometers range is ¬30 to 220°C, then the span is equal to
250°C.
Backlash:
The maximum distance or angle through which any part of mechanical system may be moved in
one direction without applying appreciable force or motion to the next part in a mechanical
sequence.
It is often defined as the difference between the position of a mechanism's input and output when
the input is moved in one direction, and then in the opposite direction. Backlash is a common
problem in gear systems and can cause inaccuracies in the measurement of position or velocity.

Dead Band and Dead Time

Dead band, sometimes called a neutral zone, is an area of a signal range or band where no action
occurs, that is, the system is dead. e.g. 10 g weight on a 10 kg balance. It is the largest change in
the physical variable to which the measuring instrument does not respond. In other words it is
defined as the range of input values over which there is no change in output value.

Dead Zone: For the largest changes in the measured variable, the instrument does not respond.

Bias: Bias describes a constant error which exits over the full range of measurement of an
instrument. The error is normally removable by calibration
Dynamic Characteristics:
The characteristics of the instruments, which are used to measure the quantities or parameters
that vary very quickly with respect to time are called dynamic characteristics.
Dynamic characteristic are concerned with the measurement of quantities that vary with time.
Following are the list of dynamic characteristics.
 i) Speed of Response ii) Dynamic Error
iii) Fidelity iv) Measuring Lag v)
Bandwidth vi) Time Constant

Impedance loading and matching

Loading Effects
The ideal situation in a measurement system is that when an element is used for any purpose may
be for signal sensing, conditioning, transmission or deflection is introduced into the system, the
original signal should remain unmolested. This means that the original signal should not be
distorted in ·any form by introduction of any element in the measurement system.
However, under practical conditions it has been found that introduction of any element in a
system results, invariably, in extraction of energy from the system thereby distorting the original
signal. This distortion may take the form of attenuation (reduction in magnitude), waveform
distortion, phase shift and many a time all these undesirable features put together. This makes
ideal measurements impossible.
The incapability of the system to faithfully measure, record, or control the input signal
(measurand) in undistorted form is called the loading effect

Numerical
Impedance Matching
Impedance matching is defined as the process of designing the input impedance and output
impedance of an electrical load to minimize the signal reflection or maximize the power transfer
of the load.
An electrical circuit consists of power sources like amplifier or generator and electrical load like
a light bulb or transmission line have a source impedance. This source impedance is equivalent to
resistance in series with reactance.
According to the maximum power transfer theorem, when the load resistance is equal to the
source resistance and load reactance is equal to negative of the source reactance, the maximum
power is transferred from source and load. It means that the maximum power can be transfer if
the load impedance is equal to the complex conjugate of the source impedance.

Errors: Statistical Analysis of error in measurement

Statistical Evaluation of measured data is obtained in two methods of tests as shown in below.
Multi Sample Test: In multi sample test, repeated measured data have been acquired by different
instruments, different methods of measurement and different observer.
Single Sample Test: measured data have been acquired by identical conditions (same instrument,
methods and observer) at different times. Statistical Evaluation methods will give the most
probable true value of measured quantity.
The mathematical background statistical evaluation methods are Arithmetic Mean, Deviation
Average Deviation, Standard Deviation and variance.

• Average (mean) x
• Deviation from mean d,
d1 = x - x1
d2 = x - x2
d3 = x - x3
.
.

. dn = x - xn

• Average deviation d=
|d1 | + |d2 | + + |dN |
N

• Standard deviation s if n≥20

if n<20
2
• Variance V = s
• Probable Error PE = (+-) 0.6745 s
Normal distribution/Probability Distribution
• Probability distributions are a function, table, or equation that shows the relationship
between the outcome of an event and its frequency of occurrence.
• Probability distributions are helpful because they can be used as a graphical
representation of your measurement functions and how they behave.
• A histogram is a graphical representation used to understand how numerical data is
distributed. Take a look below at the histogram of a Gaussian distribution.
• Look at the histogram and view how the majority of the data collected is grouped at the
center. This is called central tendency.
• Now look at height of each bar in the histogram. The height of the bars indicate how
frequent the outcome it represents occurs. The taller the bar, the more frequent the
occurrence.
Numerical
1) The following set of 8 measurement was recorded during an experiment were: 532, 548, 543,
535, 546, 531, 543, 546. Calculate
a. Arithmetic mean b. Deviation from mean c. Average deviation
d. Standard deviation e. Varience f. Range
e. Probable error of one reading f. Probable error of mean reading
2) In a test, temperature is measured 100 times with variation in apparatus and procedure. After
applying the correction the result are
Temp 397 398 399 400 401 402 403 404 405
Freq of 1 3 12 23 37 16 4 2 2
Occurance
 Calculate
a. Arithmetic mean b. Deviation from mean c. Average deviation
d. Standard deviation e. Varience f. Probable error
3) How probable error is determined from Gaussian probability curve?

Standards in Measurements
A measurement standard is the point of reference to which the testing equipment refers. A
measuring device will measure the part in question and then compare this to the standard.
Without the measurement standard as a reference point, the likelihood of incorrect results greatly
increases. Depending on the functions and applications, Different Types of Standards of
Measurement are classified in categories
1. International Standards,
2. Primary Standards,
3. Secondary Standards, and
4. Working Standards

International Standards: International standards are defined by International agreement. They

are periodically evaluated and checked by absolute measurements in terms of fundamental units
of Physics. They represent certain units of measurement to the closest possible accuracy
attainable by the science and technology of measurement. These International Standards of
Measurement are not available to ordinary users for measurements and calibrations.

Primary Standards/Master Standards: The principle function of primary standards is the

calibration and verification of secondary standards. Primary standards are maintained at the
National Standards Laboratories in different countries. The primary standards are not available
for use outside the National Laboratory. These primary standards are absolute standards of high
accuracy that can be used as ultimate reference standards.

Secondary Standards/Calibration Standards: Secondary standards are basic reference

standards used by measurement and calibration laboratories in industries. These secondary
standards are maintained by the particular industry to which they belong. Each industry has its
own secondary standard. Each laboratory periodically sends its secondary standard to the
National standards laboratory for calibration and comparison against the primary standard. After
comparison and calibration, the National Standards Laboratory returns the Secondary standards
to the particular industrial laboratory with a certification of measuring accuracy in terms of a
primary standard.

Working Standards/Workshop Measuring Standards: Working standards are the principal

tools of a measurement laboratory. These standards are used to check and calibrate laboratory
instrument for accuracy and performance. For example, manufacturers of electronic components
such as capacitors, resistors, etc. use a standard called a working Standards of Measurement for
checking the component values being manufactured, e.g. a standard resistor for checking of
resistance value manufactured.
Bridge Measurement:
Bridges are often used for the precision measurement of component values, like resistance,
inductance, capacitance, etc. The simplest form of a bridge circuit consists of a network of four
resistance arms forming a closed circuit as shown in Fig. A source of current is applied to two
opposite junctions and a current detector is connected to other two junctions. The bridge circuit
operates on null detection principle and uses the principle of comparison measurement methods.
The two Types of bridges are,
1. D.C Bridges
• Wheatstone bridge
• Kelvin bridge
2. A.C Bridges
• Inductance bridge ( Hay and Maxwell bridge)
• Capacitance bridge ( Schering bridge)
• Wein bridge
The D.C bridges are used to measure the resistance while the A.C bridges are used to measure the
impedances consisting capacitance and inductances.
DC bridges- Wheat-stone bridge
Wheatstone‘s bridge is a simple DC bridge, which is mainly having four arms. These four arms
form a rhombus or square shape and each arm consists of one resistor.
To find the value of unknown resistance, we need the galvanometer and DC voltage source.
Hence, one of these two are placed in one diagonal of Wheatstone‘s bridge and the other one is
placed in another diagonal of Wheatstone‘s bridge.
Wheatstone‘s bridge is used to measure the value of medium resistance. The circuit diagram of
Wheatstone‘s bridge is shown in below figure.
Here, the resistor, R3 is a standard variable resistor and the resistor, R 4 is an unknown resistor.
We can balance the bridge, by varying the resistance value of resistor R3.

At balanced condition, when no current flows through the diagonal arm, DB. That means, there is
no deflection in the galvanometer, when the bridge is balanced.
The bridge will be balanced, the voltage across arm AD is equal to the voltage across arm AB.
VAD = VAB
I1 R1 = I2 R2

Solving we get,
R 1 R 4 = R2 R 3
R
By substituting the known values of resistors R 1, R2 and R3 in above equation, we will get the
value of resistor R4.

Kelvin Bridge
Kelvin bridge or Thompson bridge is used for measuring the unknown resistances having a value
less than 1Ω but if we want to measure the resistance below 1 – ohm, it becomes difficult
because the leads which are connected to the galvanometer adds up the resistance of the device
along with the resistance of leads leading to variation in the measurement of the actual value of
resistance. Hence in order to overcome this problem, we can use a modified bridge called kelvin
bridge. It is the modified form of the Wheatstone Bridge.

r is the resistance of the

contacts that connect the unknown resistance R to the standard resistance S. The ‘m’ and ‘n’
show the range between which the galvanometer is connected for obtaining a null point.
When the galvanometer is connected to point ‗m‘, the lead resistance r is added to the standard
resistance S. Thereby the very low
indication obtains for unknown
resistance R. And if the
galvanometer is connected to point
n then the r adds to the R, and hence
the high value of unknown
resistance is obtained. Thus, at point
n and m either very high or very
low value of unknown resistance is
obtained.
So, instead of connecting the
galvanometer from point, m and n
we chose any intermediate point say
d where the resistance of lead r is
divided into two equal parts, i.e., r1 and r2
From equation (1), we get

The presence of r1 causes no error in the measurement of unknown resistance.

As
The above equation shows that if the galvanometer connects at point d then the resistance of lead
will not affect their results.
The above mention process is practically not possible to implement. For obtaining the desired
result, the actual resistance of exact ratio connects between the point m and n and the
galvanometer connects at the junction of the resistor.

AC Bridges
AC bridges are the circuits that are used for the measurement of electrical quantities such as
inductance, capacitance, resistance.

Let, Z1 = (Z1 θ1)

Z2 = (Z2 θ2)
Z3 = (Z3 θ3)
Z4 = (Z4 θ4)
For the bridge to be balanced, considering the above-shown figure The current through detector
must be 0 that requires the potential difference Vbd to be 0.
In such a condition voltage drop from a to b will get equal to voltage drop from a to d, both in
magnitude and phase.
So, we can write the above-stated condition as, At balance, E1
= E2
I1 Z1 θ1= I2 Z2 θ2
But,
I
And
I
So,
I1 Z1 θ1= I2 Z2 θ2

The above equation can be written as

(Z1 θ1) Χ (Z4 θ4) = (Z2 θ2) Χ (Z3 θ3) So,
here impedance parameters will get multiplied and angles will be added.
 Z1 Z4 θ1+θ4 = Z2 Z3 θ2+θ3
Hence for AC circuit to be balanced, two condition must be satisfied
1) Product of Magnitude of impedance of opposite arm must be equal
Z1 Z4 = Z 2 Z3
2) Sum of angle of impedance of opposite arm must be equal
θ1+ θ4 = θ2+ θ3
Numerical
The four impedances of an AC bridge having 1000Hz has following arms:
Branch AB with impedance Z1= 400ꭥ<80˚, Branch BC with impedance Z2= 200ꭥ<40˚,
Branch CD with impedance Z3= 400ꭥ<-30˚; Branch DA with impedance Z 4= 800ꭥ< 20˚;
Find out whether bridge is balanced or not.
AC inductance bridge
Following are the two AC bridges, which can be used to measure inductance.
• Maxwell‘s Bridge
• Hay‘s Bridge
Now, let us discuss about these two AC bridges one by one.

Maxwell's Bridge
Maxwell‘s bridge is used to measure the value of medium inductance. The circuit diagram of
Maxwell‘s bridge is shown in the below figure.
i) Maxwells Inductance Capacitance Bridge

Y
Z 2 = R2
Z 3 = R3
Zx =
At balanced condition
Z1 Zx = Z2 Z3
Zx

Comparing real and imaginary parts

 Rx =
Lx = R2 R3 C1
The quality factor of Maxwell‘s bridge circuit is given as,
Q = ωL1/R1 = ωC4R4 ii) Maxwells Inductance Bridge

Let, L1 – unknown inductance of resistance R1.

L2 – Variable inductance of fixed resistance r1.
R2 – variable resistance connected in series with inductor L2.
R3, R4 – known non-inductance resistance
We have,
Z1 = R1 +jwL1
Z2 = R2 + r2+ jwL2
Z 3 = R3
Zx =
Under balanced condition (i.e., when detector shows null deflection), we have,

On equating the real and imaginary parts on both sides, we get,

Hence, the unknown self-inductance and resistance of the inductor are obtained in terms of
known standard values. Also, both the equations are independent of frequency term.

Advantages of Maxwell’s Bridge

The advantages of a Maxwell Bridge are:
1. The frequency does not appear in the final expression of both equations, hence it is
independent of frequency.
 2. Maxwell‘s inductor capacitance bridge is very useful for the wide range of measurement
of inductor at audio frequencies.

Disadvantages of Maxwell’s Bridge

The disadvantages of a Maxwell Bridge are:
1. The variable standard capacitor is very expensive.
2. The bridge is limited to measurement of low quality coils (1 < Q < 10) and it is also
unsuitable for low value of Q (i.e. Q < 1) from this we conclude that a Maxwell bridge is
used suitable only for medium Q coils.
The above all limitations are overcome by the modified bridge which is known as Hay‘s bridge
which does not use an electrical resistance in parallel with the capacitor.

Hay’s Bridge
The bridge is the advanced form of Maxwell‘s bridge. The Maxwell‘s bridge is only appropriate
for measuring the medium quality factor. Hence, for measuring the high-quality factor Q f >10,
the Hays bridge is used in the circuit.

Let,
L1 – unknown inductance having a resistance R1
R2, R3, R4 – known non-inductive resistance. C4
– standard capacitor

At balance condition,
Separating the real and imaginary term, we obtain

Solving the above equation, we have

The quality factor of the coil is
 The equation of the unknown inductance and capacitance consists frequency term. Thus for
finding the value of unknown inductance the frequency of the supply must be known. For the
high-quality factor, the frequency does not play an important role.

Substituting the value of Q in the equation of unknown inductance, we get

For greater value of Q the 1/Q is neglected and hence the equation become

Advantages of Hay’s Bridge

The following are the advantages of Hay‘s Bridge.
1. The Hays bridges give a simple expression for the unknown inductances and are suitable for
the coil having the quality factor greater than the 10 ohms.
2. It gives a simple equation for quality factor.
3. The Hay‘s bridge uses small value resistance for determining the Q factor.

Disadvantages of Hay’s Bridge

The only disadvantage of this type of bridge is that it is not suitable for the measurement of the coil
having the quality factor less than 10 ohms.

Q) Why Hay bridge is used for measurement of the coil having the quality factor high only?

Numerical
State Wheatstone principle for circuit to be balance. A 1000Hz bridge with ABCD branch
has following constants arms,
AB, R=1000ꭥ in parallel with C= 0.5μf
BC, R=1000ꭥ in series with C= 0.5μf
CD, L=30 mH in series with R= 200ꭥ Find, the
constants of arms DA to balance the bridge.

Measurement of capacitance
i) Schering Bridge
This bridge is used to measure to the capacitance of the capacitor, dissipation factor and
measurement of relative permittivity. Let us consider the circuit of Schering Bridge.
 Let, C1=capacitor whose capacitance is to be measured.
r1= a series resistance representing the loss in the capacitor C1C1.
C2= a standard capacitor.
R3= a non inductive resistance.
C4= a variable capacitor.
R4= a variable non inductive resistance.
At balance condition
Z1 Z4 = Z2 Z3

Measurement of Frequency
i)Wien bridge
The circuit consists of four arms, one arm with a series combination of resistor and capacitor and
another with a parallel combination resistor and capacitor. The other two arms compress a
resistance. The below shows the circuit diagram of Wien's bridge.
A balance detector or null indicator is connected across two junctions (i.e., across BD as shown
above). The indicator shows null deflection when the bridge is balanced i.e. when the junctions B
and D will be at the same potential. When the bridge is balanced, we have,

Equating the real terms, we get,

The above equation is used to determine the resistance ratio (R 4/R3). Now equating the imaginary
terms

If suppose the bridge components are chosen such that R1 = R2 = R and C1 = C2 = C. Then the
above equation is given as,
Measurement the low-value resistance
i) Kelvin Bridge

The r is the resistance of the contacts that connect the unknown resistance R to the standard
resistance S. The ‘m’ and ‘n’ show the range between which the galvanometer is connected for
obtaining a null point.
When the galvanometer is connected to point ‗m‘, the lead resistance r is added to the standard
resistance S. Thereby the very low indication obtains for unknown resistance R. And if the
galvanometer is connected to point n then the r adds to the R, and hence the high value of
unknown resistance is obtained. Thus, at point n and m either very high or very low value of
unknown resistance is obtained.
So, instead of connecting the galvanometer from point, m and n we chose any intermediate point
say d where the resistance of lead r is divided into two equal parts, i.e., r1 and r2

The presence of r1 causes no error in the measurement of unknown resistance.

From equation (1), we get

As

The above equation shows that if the galvanometer connects at point d then the resistance of lead
will not affect their results.
Wagner ground Connection
When performing measurements at high frequency, stray capacitances between the various
bridge elements and ground, and between the bridge arms themselves, become significant. This
introduces an error in the measurement, when small values of capacitance and large values of
inductance are measured.
An effective method of controlling these capacitances, is to enclose the elements by a shield and
to ground the shield. This does not eliminate the capacitance, but makes it constant in value.
Another effective and popular method of eliminating these stray capacitances and the
capacitances between the bridge arms is to use a Wagner Ground Connection. Figure shows a
circuit of a capacitance bridge. C1 and C2 are the stray capacitances.

In Wagner‘s Ground Connection, another arm, consisting of R w and Cw forming a potential

divider, is used. The junction of Rw and Cw is grounded and is called Wagner Ground
Connection. The procedure for adjustment is as follow
Physical Variable
A quantity whose value vary when subjected to changes and may depend from system to system.
The physical variables are the quantities required to be measured. All these quantities require
primary detection elements to be converted into another analogous form which is acceptable by a
later stage of the measurement system.
The variable may be either electrical or non electrical quantities depending upon the condition or
system we have to deal.
Physical variable can be classified as,
• Electrical Variable: E.g. voltage, current, flux, magnetic field, frequency.
 • Mechanical Variable: E.g. force, torque, weight, displacement.
• Bio-physical variable: E.g.
ECG(electro cardiogram): Records bioelectric potential of heart)
EEG(electroencephalogram): Electrical activity of brain)
EMG(electromyogram): Skeletal muscle
• Sensor/Transducer converts one form of physical variable into another form.

Transducers

Transducer is a device which converts one form of energy into another form i.e,. the given
nonelectrical energy is converted into an electrical energy.
Transducers play an important role in the field of instrumentation and control engineering. Any
energy in a process should be converted from one form into another form to make the
communication from one rectification sector to another.

Working of Transducer
In general, Transducer works on the principle of Transduction. Whereas Transduction is the
process of converting input physical quantities into proportional electrical output signal.

A Transducer uses sensors and signal conditioning unit to perform transduction function. In other
words we can say that Transducer is the combination of sensor and signal conditioning unit. The
Sensor unit is responsible for detecting any changes in input physical quantities that has to be
measured. The output of sensor is always non-electrical in nature.
Whereas signal conditioning unit converts output of sensor into an electrical signal proportional to
the magnitude of input.
Transducers types
1) On Whether an External Power Source is required or not
Active Transducer : Active transducer is a device which converts the given non-electrical energy
into electrical energy by itself i.e. does not need external source. E.g piezoelectric crystal,
photovoltaic cell, tacho-generator, thermocouples, photovoltaic cell
Passive Transducers: Passive transducer is a device which converts the given non-electrical energy
into electrical energy by external force. E.g. capacitive, resistive and inductive transducers

2) Based on transduction phenomenon

Transducers: Devices which convert a non-electrical quantity into an electrical quantity is popularly
known as Transducers.
Inverse Transducers: Devices which convert an electrical quantity into non-electrical quantity is
called Inverse Transducer. Example of Inverse transducers is piezoelectric crystal. When voltage
across the surface of a piezoelectric crystal is applied, it changes its dimension. Another example is a
coil carrying current and kept in magnetic field. Due to interaction of current of coil with the
magnetic field, it starts to rotate or translate

3) Based on the physical phenomenon

Primary and Secondary Transducer:
Primary Transducer is the detecting or sensing element which responds to the change in physical
phenomena, whereas the Secondary Transducer converts the output of primary transducer (output in
the form of mechanical movement) into electrical output.
E.g. Consider the Bourdon‘s Tube shown in below. There are two type of transduction occurs in the
Bourdon‘s tube. First, the pressure is converted into a displacement and then it is converted into the
voltage by the help of the L.V.D.T.
The Bourdon‘s Tube is the primary transducer, and the L.V.D.T is called the secondary transducer.

4) Based on Quantity to be Measured/Application

• Temperature transducers (e.g. a thermocouple)
• Pressure transducers (e.g. a diaphragm)
• Displacement transducers (e.g. LVDT)
• Humidity transducer

5) Based on the type of output the classification of transducers are made

Analog & Digital Transducer
Analog transducer: Analog Transducers are those whose output is continuous in time domain.
This essentially means that the electrical output signal will be continuous function of time. e.g.
LVDT, thermocouple, strain gauge & thermistor
Digital transducer: Transducers which convert the input quantity into an electrical output signal
which is in the form of pulse is called Digital Transducers. Note that, the output is not continuous
rather it is in the form of pulse which means that it is discrete. e.g. Shaft Encoders, Digital
Resolvers, Digital Tachometers, Hall Effect Sensors & Limit Switches

6) On the Principle of Transduction/ physical principle involved

• Resistive Transducer :
• Inductive Transducer:
• Capacitive transducer:
• Piezoelectric transducer:
• Thermoelectric transducer:
• Hall effect transducer:

A) Resistive Transducer: Here, the input being measured into change into resistance. The
resistive transducer converts the physical quantities into variable resistance. The change in
resistance is measured by the ac or dc measuring devices. The resistive transducer is used for
measuring the physical quantities like temperature, displacement, vibration etc.
Eg: Potentiometer
Strain gauge
Thermistor Resistance temperature
detectors (RTD)

b) Inductive transducer: Here, the input being measured into change into inductance.
Inductive transducers work on the principle of inductance change due to any appreciable change
in the quantity to be measured i.e. measured. A transducer that works on the principle of
electromagnetic induction or transduction mechanism is called an inductive transducer. A
selfinductance or mutual inductance is varied to measure required physical quantities like
displacement (rotary or linear), force, pressure, velocity, torque, acceleration, etc. These physical
quantities are noted as measurands.
e.g. Linear Variable Differential Transducer (LVDT)
RVDT
c)Capacitive transducer: Here, the input being measured into change into capacitance. in the
capacitive transducer, the change in the capacitance is used to measure the physical quantities.
Capacitive transducers are passive transducers that determine the quantities like displacement,
pressure and temperature etc. by measuring the variation in the capacitance of a capacitor. In
these transducers, the capacitance between the plates is varied because of the distance between
the plates, overlapping of plates, due to dielectric medium change, etc. e.g. Capacitive
Displacement transducer

Working of Different types of transducer

Piezoelectric transducer:
The piezoelectric transducer is an active transducer that converts physical quantity (force or
pressure or stress) into an electric potential. The piezoelectric transducer consists of a
piezoelectric crystal made up of piezoelectric material which develops electrical potential across
its surface on application mechanical stress. They are self-generating transducers.
The process is reversible which means if potential difference across some specified surface is
changed, the dimension of the piezoelectric material will also change. This effect is known is
Piezoelectric Effect
The electrical energy generated is proportional to the application of mechanical force on the
surface of the piezoelectric crystal. A charge will be produced on the surface of the crystal, these
results in creating a potential difference between the two surfaces of the crystal thus inducing an
electric potential.
Rochelle Salt, Ammonium Dihydrogen Phosphate, Lithium Sulphate, dipotassium tartarate,
quartz and ceramic are common example of piezoelectric material.
Basically, there are two types of piezoelectric materials:
i) Natural: Quartz and Ceramic
ii) Synthetic: lithium sulphate, ethylene diamine tartarate

Working Principle of Piezoelectric Transducer

When a mechanical force is applied on a piezoelectric crystal, a voltage is produced across its
faces. Thus, mechanical phenomena is converted into electrical signal. Piezoelectric Transducer
responds to the mechanical force / deformation and generate voltage. There may be various
modes of deformation to which these transducers can respond. The modes can be: thickness
expansion, transverse expansion, thickness shear and face shear.

Expression
• The polarity of the charge depends on the direction of the applies forces.
Q = d × F (in coulombs) …....(1)
Where, F = Force applied in Newtons
d = Charge sensitivity of the crystal.
• The young's modulus E can be defined as the ratio of stress to strain i.e.

Y= where, t = thickness
 • The force F causes a change in thickness of crystal,

F= Y A ………2)
• Using equation 1) and 2)
Q = d × F = d × Y A ………..3)
• Also, The capacitance formed by the electrodes and the piezoelectric material is given by
Cp
Cp = ……………..4)
• Due to this charge at the electrode, an output voltage E o will be generated which can be
given by

• Eo =

= g.p.t
Where, g is the voltage sensitivity of the crystals in Vm/N

Hall Effect Transducer

Hall Effect element is a type of transducer used for measuring the magnetic field by converting it
into an emf. The principle of Hall Effect transducer is that if the current carrying strip of the
conductor is placed in a transverse magnetic field, then the EMF develops on the edge of the
conductor. The magnitude of the develop voltage depends on the density of flux, and this
property of a conductor is called the Hall effect.

Consider the Hall Effect element shown in the figure below. The current supply through the lead
1 and 2 and the output is obtained from the strip 3 and 4. The lead 3 and 4 are at same potential
when no field is applied across the strip.
When the magnetic field is applied to the strip, the output voltage develops across the output
leads 3 and 4. The develops voltage is directly proportional to the strength of the material,

EH
Where, KH = Constant of proportionality called the Hall Effect Coefficient
―t‖ = T hickness of strip. I = current in ampere and the B is the flux densities in Wb/m 2
Thermoelectric transducer
Thermoelectric conversion means the conversion of thermal energy to electric energy and vice
versa. The following two transducers are the examples of thermo-electric transducers. i)
Thermistor Transducer ii) Thermocouple Transducer
The term "thermoelectric effect" encompasses three separately identified effects: a)
Seebeck effect,
b) Peltier effect, and
c) Thomson effect.

Thermocouple
―Thomas Seebeck‖ revealed that when two different metal wires were linked at both ends of one
junction in a circuit when the temperature applied to the junction, there will be a flow of current
through the circuit which is known as electromagnetic field (EMF). The energy which is
produced by the circuit is named the Seebeck Effect/Seebeck voltage.

The above thermocouple has two metals, A & B and two junctions, 1 & 2. Consider a constant
reference temperature, T2 at junction 2. Let the temperature at junction, 1 is T1. Thermocouple
generates an emf (electro motive force), whenever the values of T1 and T2 are different. That
means, thermocouple generates an emf, whenever there is a temperature difference between two
junctions, 1 & 2 and it is directly proportional to the temperature difference between those two
junctions.
Mathematically, it can be represented as E α (T 1 –T2) Where, e is emf generated by
thermocouple. The electromagnetic force induced in the circuit is calculated by the following
equation
E = a (∆Ө) + b (∆Ө)2
Where ∆Ө = temperature difference among the hot thermocouple junction end as well as the
reference thermocouple junction end, a & b are constants
Thermocouple Working Principle
The thermocouple principle mainly depends on the three effects namely Seebeck, Peltier, and
Thompson.
See beck-effect
This type of effect occurs among two dissimilar metals. When the heat offers to any one of the
metal wires, then the flow of electrons supplies from hot metal wire to cold metal wire.
Therefore, direct current stimulates the circuit.
Peltier-effect
This Peltier effect is opposite to the Seebeck effect. This effect states that the difference of the
temperature can be formed among any two dissimilar conductors by applying the potential
variation among them.
Thompson-effect
This effect states that as two disparate metals fix together & if they form two joints then the
voltage induces the total conductor‘s length due to the gradient of temperature. This is a physical
word that demonstrates the change in rate and direction of temperature at an exact position.
List of error in Thermocouple
1) Open Junction
2) De-calibration
3) Insulation Degradation
4) Galvanic Action
5) Thermal Conduction
Laws of thermocouple
1) The e.m.f. of a thermocouple depends only on the temperatures of the junctions and is
independent of the temperatures of the wires connecting the junctions. This means that the
leads connecting the instrument can be exposed to temperature fluctuations without affecting
the measurement.
2) Law of Homogeneous Circuits: According to the law of homogeneous circuits, an electric
current cannot flow in a circuit made of a single homogeneous metal when heat alone applied
to it.
In above Figure, a thermocouple is shown with junction temperatures at T1 and T2. Along the
thermocouple wires, the temperature is T3 and T4. The thermocouple emf is, however, still a
function of only the temperature gradient T2 – T1.
3) Law of Intermediate Metals :The law of intermediate metals states that the emf developed
in a circuit made of two dissimilar homogeneous metals with the junctions at two different
temperatures will not get affected when a third homogeneous metal is introduced into the
thermocouple circuit as long as the temperature of the two junctions formed by the third
metal is same as the temperature of the thermocouple metals.

4) Law of Intermediate Temperatures : The law of intermediate temperature states that the
emf developed by a thermocouple having junctions at temperatures T1 and T3 is equal to the
sum of emf's developed by two thermocouples having junctions at temperatures T1 and T2, T2
and T3 respectively. E3 = E1 + E2

Temperature Transducers
▫ Thermocouples
▫ Resistance-Temperature Detectors (RTD) ▫
Thermistors

Resistance-Temperature Detectors (RTD)

An RTD (Resistance Temperature Detector) is a sensor whose resistance changes as its
temperature changes i.e. used to determine the temperature by measuring the resistance of an
electrical wire The resistance increases as the temperature of the sensor increases. The resistance
vs temperature relationship is well known and is repeatable over time.
The variation of resistance of the metal with the variation of the temperature is given as
Rt = Ro[1+ + ………..]
Where, Rt and R0 are the resistance values at toC and t0oC temperatures. α and β are the constants
depends on the metals.
The element of an RTD sensor is the sensing component that changes in resistance when there is
a change in temperature. The most common element is platinum. Other element metals are
copper, nickel, tungsten, Balco, and iridium.
Resistance-temperature characteristics curve of the three different metals is shown below

Thermistors
The Thermistor or simply Thermally Sensitive Resistor is a temperature sensor that works on
the principle of varying resistance with temperature. Although all resistors‘ resistance will
fluctuate slightly with temperature, a thermistor is particularly sensitive to temperature changes..
The circuit symbol of the thermistor is shown in the figure.

They are made of semiconducting materials of oxides of metals such as Nickel, Manganese,
Cobalt, Copper, Uranium etc. Thermistors are a type of semiconductor, meaning they have
greater resistance than conducting materials, but lower resistance than insulating materials. The
relationship between a thermistor‘s temperature and its resistance is highly dependent upon the
materials from which it‘s composed.
The working principle of a thermistor is that its resistance is dependent on its temperature. We
can measure the resistance of a thermistor using an ohmmeter.
Thermistors are available in two types:
Negative Temperature Coefficients (NTC thermistors) Positive
Temperature Coefficients (PTC thermistors).
Negative Temperature Coefficients (NTC thermistors): NTC thermistors‘ resistance
decreases as their temperature increases and vice-verca. Hence in an NTC thermistor temperature
and resistance are inversely proportional. The relationship between resistance and temperature in
an NTC thermistor is governed by the following expression:
 RT
Where, RT = Resistance at temperature T (K) R0 = Resistance at temperature T0 (K)
T0 = Reference temperature (normally 25oC)
β is a constant, its value is dependent on the characteristics of the material. The nominal value is
taken as 4000.
Only NTC thermistors are commonly used in temperature measurement. As the temperature
increases, an NTC thermistor‘s resistance will increase in a non-linear fashion, following a
particular ―curve.‖ The shape of this resistance vs. temperature curve is determined by the
properties of the materials that make up the thermistor.

Positive Temperature Coefficients (PTC thermistors): PTC thermistors‘ resistance increases

as their temperature increases and when temperature decreases, resistance decreases. Hence in a
PTC thermistor temperature and resistance are inversely proportional.
Although PTC thermistors are not as common as NTC thermistors, they are frequently used as a
form of circuit protection. Similar to the function of fuses, PTC thermistors can act as
currentlimiting device. The temperature resistance characteristics of an NTC and a PTC are
shown in the following figure.

Measurement of Mechanical Variables (displacement, strain. Velocity.

acceleration and vibration)
Measurement of mechanical variables, including displacement, strain, velocity, acceleration, and
vibration, plays a crucial role in various fields such as engineering. These are described below.

1.Displacement: Displacement refers to the change in position of an object from its original
reference point. Displacement transducer/sensors includes such as linear variable differential
transformers (LVDTs), potentiometers.

2.Strain: Strain is the measure of deformation or elongation experienced by an object when

subjected to external forces or stress. It indicates how much an object's shape changes under the
applied load. Strain is typically measured as a dimensionless quantity or expressed as a
percentage change in length. Strain gauges, which are devices that change their electrical
resistance in response to strain, are commonly used to measure strain accurately.
3.Velocity: Velocity is the rate of change of displacement over time. It indicates how fast an
object is moving and in which direction. Various instruments which can measure velocity,
includes Doppler radar, anemometers, and tachometers.

4.Acceleration: Acceleration is the rate of change of velocity over time. It represents how
quickly an object's velocity is changing. Acceleration is also a vector quantity and is typically
measured in units such as meters per second squared (m/s²) or gravitational acceleration (g).
Accelerometers are widely used to measure acceleration in applications such as vehicle
dynamics, structural analysis, and motion tracking.

5.Vibration: Vibration refers to the oscillating or vibrating motion of an object around its
equilibrium position. Vibration measurement involves analyzing the frequency, amplitude, and
other characteristics of the vibration. Vibration sensors, such as accelerometers or piezoelectric
sensors are commonly used to measure and analyze vibrations in machines, structures, and
various mechanical systems.

Measurement of Process Variables ‐ (Temperature Pressure, Level, Fluid Flow,

Chemical Constituents in Gases or Liquids, pH and Humidity)
Measurement of process variables such as temperature, pressure, level, fluid flow, chemical
constituents in gases or liquids, pH, and humidity is essential in various industrial processes,
laboratories, and environmental monitoring. Here's an overview of these measurements:

Temperature: Temperature measurement is crucial in a wide range of applications. It is

typically measured using temperature sensors such as thermocouples, resistance temperature
detectors (RTDs), or thermistors.

Pressure: Pressure measurement is used to monitor and control the pressure of gases or liquids
in different processes. Pressure sensors or transducers, such as pressure gauges, manometers,
or pressure transmitters, are commonly employed for measuring pressure.

Level: Level measurement is used to determine the height or volume of liquids, solids, or slurries
in containers or tanks. Various techniques are employed for level measurement, including float
switches, capacitance sensors, ultrasonic sensors, and radar sensors.

Fluid Flow: Measurement of fluid flow is crucial in processes that involve the transportation or
control of fluids. Flow meters, such as electromagnetic flow meters, turbine flow meters, or
thermal mass flow meters, are commonly used to measure the flow rate of liquids or gases.

Chemical Constituents: Measurement of chemical constituents in gases or liquids is necessary

for quality control, process optimization, and safety purposes. Techniques such as gas
chromatography, spectrophotometry, or titration are used to measure the concentration or
composition of specific chemicals or compounds.
pH: pH measurement is used to determine the acidity or alkalinity of a solution. pH meters or
electrodes are employed to measure the hydrogen ion concentration in a solution, providing
information about its chemical properties. pH is typically measured on a scale from 0 to 14.

Humidity: Humidity measurement is used to determine the moisture content or water vapor
concentration in the air or gases. Hygrometers, such as capacitive or resistive humidity
sensors, are commonly used to measure relative humidity (RH) expressed as a percentage.
Measurement of Bio‐Physical Variables Blood Pressure & Myoelectric Potentials

Measurement of bio-physical variables such as blood pressure and myoelectric potentials is

important in medical diagnostics, research, and monitoring. Here's an overview of these
measurements:

Blood Pressure: Blood pressure is a measure of the force exerted by circulating blood against
the walls of blood vessels. It is typically expressed as two values: systolic pressure and diastolic
pressure. Systolic pressure represents the maximum pressure in the arteries during a heartbeat,
while diastolic pressure represents the minimum pressure between heartbeats. Blood pressure is
measured using devices called sphygmomanometers, which can be manual (mercury or aneroid)
or digital.

Myoelectric Potentials: Myoelectric potentials refer to the electrical signals generated by

muscle activity. These signals can provide insights into muscle function, activity, and health.
Two common types of myoelectric measurements are electromyography (EMG) and
electroneurography (ENG).
Electromyography (EMG): EMG measures the electrical activity produced by skeletal muscles
during contraction and relaxation. It involves placing surface electrodes or needle electrodes into
the muscle of interest. EMG signals can help diagnose and assess muscle disorders, nerve
injuries, and neuromuscular diseases.
Electroneurography (ENG): ENG measures the electrical activity in peripheral nerves that
control muscle movement. It involves stimulating a nerve and recording the electrical
response. ENG can be used to diagnose and assess nerve damage, such as peripheral neuropathy
or nerve entrapment syndromes.
EMG and ENG measurements are typically performed using specialized equipment that
amplifies and records the electrical signals generated by muscles and nerves. The recorded
signals are then analyzed to evaluate muscle and nerve function.

Calibration and Error in Transducer

The accuracy of the measurement depends upon various factors. The equipment used for
measurements can lose their precision when used at higher temperatures, high moisture or
humidity conditions, subjected to degradation, subjected to external shocks, etc…This can be
observed as the error in the measurement. To tackle this error and make necessary changes to the
equipment calibration methods are used. Calibration plays a crucial role in removing the errors in
sensor measurements and increasing the performance of the sensor.
Sensor calibration is an adjustment or set of adjustments performed on a sensor or instrument to
make that instrument function as accurately, or error free, as possible.

Calibration Methods
There are three standard calibration methods used for sensors. They are- •
One point calibration.
• Two-point calibration.
• Multi-Point Curve Fitting.
One point calibration is used to correct the sensor offset errors when accurate measurement of
only a single level is required & sensor is linear. Temperature sensors are one point calibrated.

Two-point calibration is used to correct both slope and off-set errors. This calibration is used in
the cases when the sensor we know that the sensor output is reasonably linear over a
measurement range. Here two reference values are needed- reference High, reference Low.

Multi-point Curve fitting is used for sensors that are not linear over the measurement range and
require some curve-fitting to get the accurate measurements. Multi-point curve fitting is usually
done for thermocouples when used in extremely hot or extremely cold conditions.

Errors seen in sensors/Transducers

• Due to improper zero-reference
• Errors due to shift‘s in sensor range,
• Error due to mechanical damage/wear
• Scale error
• Error on account of noise and drift • Errors due to change in frequency.
Measurement of Voltage & Current (Moving Coil & Moving Iron Instruments)
Measuring Current: Ammeters
To measure current, the circuit must be broken at the point where we want that current to be
measured, and the ammeter inserted at that point. In other words, an ammeter must be connected
in series with the load under test.

It is very important that the insertion of the ammeter into a circuit has little effect the circuit‗s
existing resistance and, thus, alter the current normally flowing in the circuit, ammeters are
manufactured with very low values of internal resistance. Because ammeters have a very low
internal resistance, it is vitally important that they are never inadvertently connected in parallel
with any circuit component —and especially with the supply. Failure to do so will result in a
short-circuit current flowing through the instrument which may damage the ammeter (although
most ammeters are fused) or even result in personal injury.

Measuring Voltage: Voltmeters

To measure potential-difference, or voltage, a voltmeter must be connected between two points at
different potentials. In other words, a voltmeter must always be connected in parallel with the
part of the circuit under test.

In order to operate, a voltmeter must, of course, draw some current from the circuit under test,
and this can lead to inaccurate results because it can interfere with the normal condition of the
circuit. We call this the loading effect and, to minimize this loading effect‗ (and, therefore,
improve the accuracy of a reading), this operating current must be as small as possible and, for
this reason, voltmeters are manufactured with a very high value of internal resistance —usually
many megohms

The classification of ammeter and voltmeter based on effect is as follows.

• Moving Coil Instruments
Permanent Magnet Moving Coil [PMMC] instrument
Electrodynamic or Dynamometer types instrument
• Moving Iron Instruments
Attraction type M.I. instrument
Repulsion type M.I. instrument

Moving-Coil Meters
Figure is an exploded view of a moving-coil movement. It can be seen that a coil free to rotate is
suspended in the field of a permanent magnet. The coil ends are connected to a suspension
system so that current can be passed through the coil.

Figure: Exploded view of a moving-coil movement The

suspension system may consist of one of two methods:
1. A coiled spring as shown in Figure. Sometimes called a hair spring, the outer end is
attached to an adjustable arm so that the pointer of the movement can be adjusted to align
itself up with the zero on the meter scale.
2. The second method is called ‗taut band suspension‘ and is considered a more robust
method for suspending the moving coil. With this method the rigid coil pivot is replaced
with two separate thin metal strands under tension. The hair springs are no longer
necessary so are usually removed. Zero adjustment of the meter is achieved by a similar
movable arm attached to one of the bands.
It must be noted that a current is passed through the coil—not a voltage. The current that flows
through the coil is governed by the value of the applied voltage. The coil sets up its own field
which reacts with that of the permanent magnet and causes the coil to rotate. A pointer attached
to the coil gives a voltage reading against a scale.
The meter movement can only work satisfactorily on DC. If AC is applied to the movement, it
tries to turn the coil rapidly in the opposite directions with the result that the coil effectively
remains stationary.
The meter can only operate on AC if the AC is rectified to DC before it flows through the meter.
Because of these factors, the moving-coil meter always reads average values of current.
Moving-Iron Instruments
Figure shown is an exploded view of a moving-iron meter to illustrate its operating principle.

Figure: Moving-iron instrument/meter exploded view, construction, and non-linear scale

There are two magnetically soft iron vanes in the movement. One vane is fixed and the other
pivoted and free to rotate. A pointer attached to the moving vane moves across a scale as an
indicator.
When an electric current is passed through the coil, the fixed and moving vanes are magnetized
and have like poles at adjacent ends. Like poles repel each other and the movable vane moves
away from the fixed vane. The attached pointer then indicates a value against a calibrated scale.
A restraining spring provides opposing torque so that the vane movement can be stabilized. Like
the moving-coil instrument, the moving-iron meter is current operated. The current that flows
through the coil is governed by the applied voltage.
As a voltmeter, the coil impedance is very low when compared with the required series
resistance. Consequently the meter movement can be considered as resistive only and the current
through the meter is directly proportional to the applied voltage (Ohm’s law).
The meter will operate on both DC and AC, although it might need to be calibrated differently.
Because the two vanes are magnetized by the same current, the moving-iron meter operates on
root-mean-square (RMS) values of current.
Measurement of low, high & medium resistances
Depending upon the value of resistance they are classified into three categories,
Low Resistance - Resistance of the order of 1Ω and below are classified as low resistance.
Medium Resistance - Resistance ranging from 1Ω to 100Ω are classified as medium resistances
High Resistance - Resistance of the order of 100kΩ and above are classified as high resistances
Different techniques are applied for measurement of low, medium, and high values of
resistances.

Measurement of Low Resistance (<1Ω) :

The various methods that can be employed for the measurement of low resistance are,
- Ammeter-voltmeter Method
- Kelvin's Double Bridge Method
- Potentiometer Method

Ammeter – Voltmeter Method

In this method, current through the unknown resistor (Rx) and the potential drop across it are
simultaneously measured. The readings are obtained by ammeter and voltmeters respectively.
There are two ways in ammeter and voltmeters may be connected for measurement as,
Case 1 – When voltmeter is directly connected across the resistor, then the ammeter measures
current flowing through the unknown resistance (Rx) and the voltmeter.

Current through ammeter = Current through( x) + Current through voltmeter

I==IRx+IV

Therefore, the value of unknown resistance,

Case 2 – When the ammeter is connected such that it measures only the current flowing through
the unknown resistor (Rx), then the voltmeter measures voltage drop across the ammeter and Rx.

Therefore,

V=I RA + I Rx = I(RA+ Rx)

Rx=V/I-RA
Potentiometer Method
In the potentiometer method, the unknown resistance is compared with a standard resistance of
the same order of magnitude.

The circuit consists of an unknown resistance (R x), a rheostat (R) and a standard resistance (R s)
all are connected in series across a low voltage, high current supply. The value of Rs should be
known and of the same order of R x. The current flowing in the circuit is adjusted ,so that the
potential difference across each resistor is about 1 V.
Now, the voltage drop across both the standard resistance (R s) and unknown resistance (Rx) are
measured by a potentiometer. The ratio of the two potentiometer readings gives the ratio of R x
and Rs, i.e.
RX/RS = VRX /VRS

Kelvin Double Bridge Method

The Kelvin double bridge is a modified version of Wheatstone bridge and used to measure the
low resistances with higher accuracy. This bridge is called double bridge since the circuit
contains a second set of ratio arms (p and q). This second set of ratio arms connects the
galvanometer (G) to a point f at the appropriate potential difference between c and d and this
eliminates the effect of yoke resistance r. The galvanometer shows zero reading when potential at
a equals to the potential at f, i.e. the bridge is balanced.

Therefore, the value of unknown resistance can be given by,

Rx

Since, the ratio of resistances of arms p and q is the same as the ratio of P and Q. Thus,
p/q=P/Q
Substituting in the above expression, we get,
Rx=PS/Q…...(4)

The eq. (4) is the work equation of kelvin double bridge.

Measurement of Medium Resistances

To measure the medium resistances following methods are used −
..Ammeter-Voltmeter Method
• Substitution Method
• Wheatstone Bridge
• Carey-Foster Bridge Method.

Ammeter Voltmeter Method

This is the most simplest method of measuring resistance. It uses one ammeter to measure
current, I and one voltmeter to measure voltage, V and we get the value of resistance as R= V/I
Two method to measured is already described above.

Substitution Method
Step 1 – In this method, first the unknown resistance (R x) is put into the circuit and note the
value of current.

Step 2 – Then the resistance Rx is removed and it is substituted by a known variable resistance R
which is varied so that the value of current is same in both the cases. This value of R is equal to
the value of unknown resistance.

Wheatstone Bridge
The Wheatstone bridge method is the most accurate method for the measurement of resistances.
The bridge consists of four resistive arms, source of emf and a galvanometer (null detector). The
current through the galvanometer depends upon the potential difference between the points B and
D. The bridge is said to be balanced when the potential difference across the galvanometer is
zero so that there is no current flows through the galvanometer.
For the balanced Wheatstone bridge,
PS=QR
R=PS/Q...(3)

Measurement of High Resistances

The following methods are employed for the measurement of high resistances −
Direct Deflection Method
• Loss of Charge Method
• Megohm Bridge
• Megger

Direct Deflection Method

In this method, a very sensitive and high resistance (more than 1 kΩ) PMMC galvanometer is
connected in series with the resistance to be measured and to a battery. The deflection of
galvanometer gives the measure of unknown resistance. This method is mainly used for the
measurement of insulation resistance.
Let us take an example of direction deflection method for measuring insulation resistance of a
cable.

Refer the figure, the galvanometer (G) measures the current I R between conductor core and metal
sheath. The leakage current IL over the surface of insulating material is carried by the guard wire
wound on the insulation and does not flow through the galvanometer. Thus, the resistance of the
cable is,
R=V/IR……..(1)
Megohm Bridge
The circuit of Megohm bridge consists of power supplies, resistances, amplifiers and indicating
instruments.
 In this instrument, the dial on R2 is calibrated 1-10-100-1000 MΩ and the R4 gives five
multipliers 0.1, 1, 10, 100 and 100. The junction of R 1 and R2 is brought on the main panel and
assigned a name as Guard terminal. The unknown resistance is given by, R3= (R 1R4) / R2…….
(4) Megger
Megger (megohmmeter) is a device used for the measurement of high resistances, mainly
insulation resistances of electric circuits with respect to earth or one another. A megger consists
of a source of emf and a voltmeter whose scale is usually calibrated in mega-ohms. The unknown
resistance Rx has to be connected across the leads of megger.

When the megger operates, the deflection of the moving system depends upon the ratio of the
applied voltage and the current in the coils of the megger. The unknown resistance is read
directly from the scale of the megger.

Unit-II
Principle of Analog Instruments
Introduction
The analogue instrument is defined as the instrument whose output is the continuous function of
time, and they have a constant relation to the input. The physicals quantity like voltage, current,
power and energy are measured through the analogue instruments. Most of the analogue
instrument use pointer or dial for indicating the magnitude of the measured quantity.
Analog instruments find extensive use in present day applications although digital instruments
are increasing in number and applications. The areas of application which are common to both
analog and digital instruments are fairly limited at present. Hence, it can safely be predicted that
the analog instruments will remain in extensive use for a number of years and are not likely to be
completely replaced by digital instruments for certain applications.

Types of Analog Instruments

1. Direct Current (DC)
2. Alternating Current (AC)
3. Direct and Alternating Current (DC/AC) or (Universal Instruments)
 DC Instruments
• The instruments, whose deflections are proportional to the current or voltage under
measurement are used for dc measurements only.
• If such an instrument is connected in an a c circuit, the pointer will deflect up-scale for one half
cycle of the input waveform and down-scale for the next half cycle.
• At lower frequencies of 50 Hz, the pointer will not be able to follow the variations in direction
and will quiver slightly around the zero mark, seeking the average value of ac i.e., zero. Example
– PMMC instrument
AC Instruments
• The instruments utilizing the electromagnetic induced currents for their operation are used
for ac measurements only.
• These instruments cannot be used for dc measurements because the electromagnetic induced
currents are not generally available in dc circuit. Example – MI type instruments
DC / AC Instruments (Universal Instruments)
The instruments having deflection proportional to the square of the current or voltage under
measurement can be used for dc as well as ac measurements.
Example – Dynamometer type moving coil, hot-wire, electrostatic instruments, moving iron
(attraction or repulsion type).
Instruments depend for their operation on one of the many effects produced by current and voltage
and thus can be classified according to which of the effects is used for their working.

The analog instruments are also classified as following

1. Indicating instruments: The indicating instrument are those which indicate by movement of
pointer over a celebration scale ( Ammeter voltmeter wattmeter)
2. Recording instruments: Recording instrument are those instrument which gives a continuous
record of variation of same electrical quantity(such as current voltage and power) with respect
to time(ECG)
3. Integrating instruments: Integrating instruments register the amount of energy or quantity of
supplied to a ckt over a period of time
The analog instruments may also be classified on the basis of method used for comparing the
unknown quantity (measurand) with the unit of measurement. The two categories of instruments
based upon this classification are:
1) Direct Measuring Instruments
2) Comparison Instruments
DC Voltmeters
DC voltmeter is a measuring instrument, which is used to measure the DC voltage across any two
points of electric circuit. If we place a resistor in series with the Permanent Magnet Moving Coil
(PMMC) galvanometer, then the entire combination together acts as DC voltmeter. The series
resistance, which is used in DC voltmeter is also called series multiplier resistance or simply,
multiplier. It basically limits the amount of current that flows through galvanometer in order to
prevent the meter current from exceeding the full scale deflection value. The circuit diagram of
DC voltmeter is shown in below figure.

We have to place this DC voltmeter across the two points of an electric circuit, where the DC
voltage is to be measured.

Review of DC/AC voltmeter and Ammeter: The D' Arsonval Principle

The D'Arsonval principle is a fundamental principle behind the operation of DC/AC voltmeters
and ammeters. The principle is based on the interaction between a magnetic field and an electric
current. The D'Arsonval principle involves the use of a moving coil and a permanent magnet.
how it is working:

1) Moving Coil: The voltmeter or ammeter consists of a coil of wire that is mounted on a
pivoting spindle. This coil is free to move within the magnetic field.
2) Permanent Magnet: A permanent magnet is positioned near the coil, creating a static
magnetic field.
3) Current Flow: When a current flows through the coil, it interacts with the magnetic field
produced by the permanent magnet.
4) Force on the Coil: According to the principle of electromagnetic induction, the interaction
between magnetic field and electric current generates a force on the coil, which causes it to
move.
4) Measurement Scale: The coil movement is proportional to the current and passing through it
or the voltage applied across it. This movement displayed on a measurement scale.

For a voltmeter
The coil is connected in parallel with a high-value resistor, which limits the current flow through
the coil and ensures that the voltmeter has a high input impedance. When a voltage is applied
across the voltmeter, the current passing through the coil is proportional to the voltage, resulting
in a corresponding deflection of the coil.

For an ammeter
The coil is connected in series with a low-value shunt resistor, which allows a small fraction of
the total current to flow through the coil. The current passing through the coil is directly
proportional to the total current, resulting in a deflection of the coil corresponding to the
measured current.

The D'Arsonval principle is widely used in analog needle-type voltmeters and ammeters,
providing accurate and reliable measurements of DC current and voltage and AC currents and
voltages. However, it should be noted that modern digital instruments have largely replaced
analog meters in many applications, improve precision, versatility, and additional measurement
features.
DC Ammeter:
The PMMC galvanometer constitutes the basic movement of a dc ammeter. Since the coil
winding of a basic movement is small and light, it can carry only very small currents. When large
currents are to be measured, it is necessary to bypass a major part of the current through a
Resistance called a shunt, as shown in Fig. The resistance of shunt can be calculated as,
Let, Rm = internal resistance of the movement.
Ish = shunt current
Im = full scale deflection current of the movement
I = full scale current of the ammeter + shunt (i.e. total current)

Fig: Basic DC Ammeter

Since the shunt resistance is in parallel with the meter movement, the voltage drop across the
shunt and movement must be the same.
Therefore ,
But
I=Ish +Im
hence

For each required value of full scale meter current, we determine the value of shunt resistance.

Multi range Ammeters:

The current range of the dc ammeter may be further extended by a number of shunts, selected by
a range switch. Such a meter is called a multi range ammeter, shown in Fig.

Fig: Multirange Ammeter

The circuit has four s hunts R1, R2, R3 and R4, which can be placed in parallel with the movement
to give four different current ranges. Switch S is a multiposition switch, (having low contact
resistance and high current carrying capacity, since its contacts are in series with low resistance
shunts). Make before break type switch is used for range changing. This switch protects the
meter movement from being damaged without a shunt during range changing. If we use an
ordinary switch for range changing, the meter does not have any shunt in parallel while the range
is being changed, and hence full current passes through the meter movement, damaging the
movement. Hence a make before break type switch is used. The switch is so designed that when
the switch position is changed, it makes contact with the next terminal (range) before breaking
contact with the previous terminal. Therefore the meter movement is never left unprotected.
Multirange ammeters are used for ranges up to 50A. When using a multirange ammeter, first use
the highest current range, then decrease the range until good upscale reading is obtained. The
resistance used for the various ranges are of very high precision values, hence the cost of the
meter increases.

Extension of Range of PMMC Ammeter

The range of a permanent-magnet moving coil ammeter can be extended by connecting a low
resistance, called shunt which is in parallel with the moving coil of the instrument as shown in
Fig.below The shunt by passes most of the line current and allows a small current through the
meter which it can handle without burning. Let Rm, = meter resistance
S = shunt resistance
 Im = full-scale deflection current
I = full range current of the meter
Voltage across shunt = Voltage across the meter

I / Im =1+Rm / S Note- I / Im is called multiplying

factor it is denoted by( m) m-1 = Rm/S and S= Rm/m -1 . The ratio of maximum
current with shunt to full scale deflection current without shunt is known as multiplying factor
(multiplying power)

DC Voltmeter:
A basic D‘ Arsonval movement can be converted into a dc voltmeter by adding a series resistor
known as multiplier, as shown in Fig. The function of the multiplier is to limit the current
through the movement so that the current does not exceed the full scale deflection value. A dc
voltmeter measures the potential difference between two points in a dc circuit.

Fig: Basic dc voltmeter

To measure the potential difference between two points in a dc circuit or a circuit component, a
dc voltmeter is always connected across them with the proper polarity. The value of the
multiplier required is calculated as follows. Fig. above
Im = full scale deflection current of the movement
Rm = internal resistance of movement
Rs = multiplier resistance
V = full range voltage of the instrument From
the circuit

Therefore
The multiplier limits the current through the movement, so as to not exceed the value of the full
scale deflection . The above equation is also used to further extend the range in DC voltmeter.

Multi range Voltmeter:

As in the case of an ammeter, to obtain a multirange ammeter, a number of shunts are connected
across the movement with a multi-position switch. Similarly, a dc voltmeter can be converted
into a multirange voltmeter by connecting a number of resistors (multipliers) along with a range
switch to provide a greater number of workable ranges.

The Figure 1 above shows a multirange voltmeter using a three position switch and three
multipliers R1, R2, and R3 for voltage values V1, V2, and V3. Figure 1 can be further modified to
Fig. 2, which is a more practical arrangement of the multiplier resistors of a multi range
voltmeter. In this arrangement, the multipliers are connected in a series string, and the range
selector selects the appropriate amount of resistance required in series with the movement.
This arrangement is advantageous compared to the previous one, because all multiplier
resistances except the first have the standard resistance value and are also easily available in
precision tolerances:
The first resistor or low range multiplier, R 4, is the only special resistor which has to be specially
manufactured to meet the circuit requirements.

Extending Voltmeter Ranges:

A multiplier is basically a resistor connected in series with the voltmeter as shown below. The
main function of the multiplier is to limit the flow of current through the voltmeter in such a way
that the deflection of the pointer should not exceed the full-scale deflection.
 It must ensure that the voltmeter should be connected in parallel or across two points, to measure
the potential difference. Let,
• Rm = Internal resistance of the meter
• Rs = Resistance of multiplier
• Im = Full-scale deflection current of meter
• V = Voltage being measured
• Vm = Full deflection voltage of the meter
From the above figure,

The multiplying factor of the multiplier is the ratio of extended voltage range to be measured V
to the actual sustainable voltage by the voltmeter V m. If the sustainable voltage drop of the meter
Vm = Im Rm. Then multiplying factor m is,

Hence, to extend a voltmeter range for m times. The resistance of the multiplier required

AC Voltmeters
The instrument, which is used to measure the AC voltage across any two points of electric circuit
is called AC volt meter . The DC voltmeter measures only DC voltages. If we want to use it for
measuring AC voltages, then we have to follow these two steps.
Step1 − Convert the AC voltage signal into a DC voltage signal by using a rectifier.
Step2 − Measure the DC or average value of the rectifier‘s output signal.
We get Rectifier based AC voltmeter, just by including the rectifier circuit to the basic DC
voltmeter. This chapter deals about rectifier based AC voltmeters.
Types of Rectifier based AC Voltmeters
Following are the two types of rectifier based AC voltmeters.
 • AC voltmeter using Half Wave Rectifier
• AC voltmeter using Full Wave Rectifier
AC Voltmeter using Half Wave Rectifier
The block diagram of AC voltmeter using Half wave rectifier is shown in below figure.

Th0e above block diagram consists of two blocks: half wave rectifier and DC voltmeter. We will
get the corresponding circuit diagram, just by replacing each block with the respective
component(s) in above block diagram.

The rms value of sinusoidal (AC) input voltage signal is

Vrms=Vm/√2
Vm=√2 * Vrms
Vm=1.414Vrms
Where, Vm is the maximum value of sinusoidal (AC) input voltage signal.
The DC or average value of the Half wave rectifier‘s output signal is
Vdc=Vm/π
Substitute, the value of Vm in above equation.
Vdc=1.414Vrms/π
Vdc=0.45Vrms
Therefore, the AC voltmeter produces an output voltage, which is equal to 0.45 times the rms
value of the sinusoidal (AC) input voltage signal

AC Voltmeter using Full Wave Rectifier

If a Full wave rectifier is connected ahead of DC voltmeter, then that entire combination together
is called AC voltmeter using Full wave rectifier. The block diagram of AC voltmeter using Full
wave rectifier is shown in below figure

The above block diagram consists of two blocks: full wave rectifier and DC voltmeter. We will
get the corresponding circuit diagram just by replacing each block with the respective
component(s) in above block diagram.
So, the circuit diagram of AC voltmeter using Full wave rectifier will look like as shown in
below figure.

The rms value of sinusoidal (AC) input voltage signal is

Vrms=Vm/√2
Vm=√2*Vrms
Vm=1.414Vrms
Where,
Vm is the maximum value of sinusoidal (AC) input voltage signal.
The DC or average value of the Full wave rectifier‘s output signal is
Vdc=2Vm/π
Substitute, the value of Vm in above equation
Vdc=2×1.414Vrms / π
Vdc=0.9Vrms
Therefore, the AC voltmeter produces an output voltage, which is equal to 0.9 times the rms
value of the sinusoidal (AC) input voltage signal.

AC multi Range Voltmeter

Fig: multi range ac voltmeter

The above is circuit for measuring ac voltages for different ranges. Resistances R1, R2, R3 and R4
form a chain of multipliers for voltage ranges of 1000 V, 250 V, 50 V, and 10 V respectively. On
the 2.5 V range, resistance R 5 acts as a multiplier and corresponds to the multiplier R s shown in
Fig. below
 Fig: General rectifier type ac voltmeter R sh
is the meter shunt and acts to improve the rectifier operation.

Ohm Meter and Multirange

The instrument, which is used to measure the value of resistance between any two points in an
electric circuit, is called ohmmeter. There are two types of ohmmeters.
1)Series Ohmmeter ( High value resistance measurement 1 ohm to100 kilo ohm)
2)Shunt Ohmmeter (low value resistance measurement few ohm to 1 micro
ohm)
3)Multi range ohm meter
Series Ohmmeter
If the resistor‘s value is unknown and has to be measured by placing it in series with the
ohmmeter, then that ohmmeter is called series ohmmeter. The circuit diagram of series
ohmmeter is shown in below figure.

The part of the circuit, which is left side of the terminals A & B is series ohmmeter. So, we can
measure the value of unknown resistance by placing it to the right side of terminals A & B
Shunt Ohmmeter
If the resistor‘s value is unknown and to be measured by placing it in parallel (shunt) with the
ohmmeter, then that ohmmeter is called shunt ohmmeter. The circuit diagram of shunt
ohmmeter is shown in below figure.

The part of the circuit, which is left side of the terminals A & B is shunt ohmmeter. So, we can
measure the value of unknown resistance by placing it to the right side of terminals A & B.
Multi-Range Ohmmeter

This instrument provides the reading up to a very wide range. In this case, we have to select the
range switch according to our requirements. An adjuster is provided so that we can adjust the
initial reading to be zero.
The resistance to be measured is connected in parallel to the meter. The meter is adjusted so that
it shows full-scale deflection when the terminals in which the resistance connected is full-scale
range through the range switch.
When the resistance is zero or short circuit, there is no current flow through the meter and hence
no deflection. Suppose we have to measure a resistance under 1 ohm, then the range switch is
selected at the 1-ohm range at first.
Then that resistance is connected in parallel and the corresponding meter deflection is noted. For
1 ohm resistance, it shows full-scale deflection but for the resistance other than 1 ohm it shows a
deflection which is less than the full load value, and hence resistance can be measured.

Electronic Multimeter
Electronic Multimeter is a device which is used for the measurement of various electrical and
electronic quantities such as current, voltage, resistance etc. It is provided with in built power
supply necessary for the functioning of the device. Any component such as a resistor, battery can
be connected to its outer probes for the measurement of the electronic quantity.

Measurements by using Multimeter

Multimeter is an instrument used to measure DC & AC voltages, DC & AC currents and
resistances of several ranges. It is also called Electronic Multi meter or Voltage Ohm Meter
(VOM).

DC voltage Measurement
The part of the circuit diagram of Multimeter, which can be used to measure DC voltage is
shown in below figure below
The above circuit looks like a multi range DC voltmeter. The combination of a resistor in series
with PMMC galvanometer is a DC voltmeter. So, it can be used to measure DC voltages up to
certain value

DC Current Measurement
The part of the circuit diagram of Multimeter, which can be used to measure DC current is
shown in below figure.

The above circuit looks like a multi range DC ammeter. the combination of a resistor in parallel with
PMMC galvanometer is a DC ammeter. So, it can be used to measure DC currents up to certain value.

AC voltage Measurement
The part of the circuit diagram of Multimeter, which can be used to measure AC voltage is
shown in below figure.

The above circuit looks like a multi range AC voltmeter. We know that, we will get AC
voltmeter just by placing rectifier in series (cascade) with DC voltmeter. The above circuit was
created just by placing the diodes combination and resistor, R6 in between resistor, R5 and
PMMC galvanometer.
We can measure the AC voltage across any two points of an electric circuit, by connecting the
switch, S to the desired voltage range.

Resistance Measurement
The part of the circuit diagram of Multimeter, which can be used to measure resistance is shown
in below figure.

We have to do the following two tasks before taking any measurement.

• Short circuit the instrument
• Vary the zero adjust control until the meter shows full scale current. That means, meter
indicates zero resistance value.
Now, the above circuit behaves as shunt ohmmeter and has the scale multiplication of 1, i.e. 10 0.
We can also consider higher order powers of 10 as the scale multiplications for measuring high
resistances.

Wattmeter: Types and Working principles

A wattmeter is an instrument which is used to measure electric power given to or developed by
an electrical circuit. Generally, a wattmeter consists of a current coil and a potential coil.
Types of Wattmeter
• Electrodynamometer wattmeter – for both DC and AC power measurement
• Induction wattmeter – for AC power measurement only

Working Principle of Electrodynamometer Wattmeter

The electrodynamometer wattmeter works on a current-carrying conductor experiences a

magnetic force when it is placed in a magnetic field. Hence there will be a deflection of
pointer that took place due to the mechanical force. It contains two coils such as fixed coil
(current coil) and moving coil ( pressure coil or voltage coil).
The fixed coil is connected in series with the circuit in order to measure power consumption. The
supply voltage is applied to the moving coil. Current across the moving coil is controlled with
the help of a resistor, which is connected in series with it. Moving coil on which pointer is fixed
is placed in between fixed coils. Two magnetic fields are generated due to the current and
voltage in the fixed coil and moving coil. The pointer deflects as the two magnetic fields
interact. The deflection is proportional to the power that is flowing through it.
Advantages of Dynamometer Type Wattmeter
• These instruments are made to give very high accuracy and these are used as a standard
for calibration purposes.
• These instruments provide full accuracy on direct current (DC).
Disadvantages of Dynamometer Type Wattmeter
• These instruments cannot provide full accuracy on alternating Current (AC).
• These instruments cause errors at low power factor.
Working Principle of Induction Wattmeter
The induction type wattmeter can be used to measure AC power only.
The working of induction type wattmeter is based on the principle of electromagnetic induction.
The induction wattmeter consists of two laminated electromagnets viz. Shunt Magnet and
Series Magnet. The shunt magnet is connected across the supply and carries a current
proportional to the supply voltage. The coil of shunt magnet is made highly inductive so that
the current in it lags the supply voltage by 90°. The series magnet is connected in series with
the supply and carries the load current. The coil of series magnet is made highly non
inductive. A thin disc (made up of aluminum) mounted on a spindle is placed between the
two magnets so that it cuts the flux of the two magnets.
 When the wattmeter is connected in an AC circuit, a current flows through the coil of the shunt
magnet that is proportional to the supply voltage and the series magnet carries the load current.
The fluxes produced by the two magnets induce eddy currents in the aluminium disc by the
action of electromagnetic induction. Due to the interaction between the fluxes and eddy currents,
a deflecting torque is produced on the disc, causing the disc to move and hence, the pointer
connected to the disc moves over the scale. The pointer comes to rest when the deflecting torque
becomes equal to the controlling torque.
Let V = Supply voltage
IV = Current carried by shunt magnet
Ic= Current carried by series magnet
cosΦ = Lagging power factor of the load
You can see the phasor diagram in the picture below. The current IV in the shunt magnet lags the
supply voltage V by 90° and so does the flux ΦV produced by it. The current IC in the series
magnet is the load current and hence lags behind the supply voltage V by Φ.

The flux ΦC produced by this current (that is I C ) is in phase with it. It is clear that phase angle θ

Mean deflecting torque, Td ∝ ΦV ΦC sin θ

between the two fluxes is 90° – Φ that is θ = 90° – Φ
Therefore,
∝ VI sin (90° − Φ)
[ ∵ ΦV ∝ V and ΦC ∝ I ]
∝ V I cos Φ
∝ a. c. power
Since the instrument is spring controlled, TC ∝ θ

Therefore, θ ∝ a. c. power
For steady deflected position, Td = TC.

Hence such instruments have uniform scale. So let‘s now know about the energy meter, also
known as the integrating meters.

Advantages of Induction Type Wattmeter:

The scale is uniform.
• They provide good damping.
• There is no effect of stray fields.

Disadvantages of Induction Type Wattmeter:

• Can be used only for ac power measurements.
• Low accuracy due to heavy moving system and Power consumption is more  Temperature
changes can affect the readings by introducing errors.
Energy Meter: Types and Working Principle
Energy Meter or Watt-Hour Meter is an electrical instrument that measures the amount of
electrical energy used by the consumers.
The energy meter has four main parts. They are the
1. Driving System
2. Moving System
3. Braking System
4. Registering System
Driving System – Electromagnet is the main component of driving system which is the
temporary magnet and is excited by the current flow through their coil. The driving system has
two electromagnets. The upper one is called the shunt electromagnet, and the lower one is called
series electromagnet.
The series electromagnet is excited by the load current flow through the current coil. The coil of
the shunt electromagnet is directly connected with the supply and hence carry the current
proportional to the shunt voltage. This coil is called the pressure coil.
The center limb of the magnet has the copper band. These bands are adjustable. The main
function of the copper band is to align the flux produced by the shunt magnet in such a way that
it is exactly perpendicular to the supplied voltage.
Moving System – The moving system is the aluminum disc mounted on the shaft of the alloy.
The disc is placed in the air gap of the two electromagnets. The eddy current is induced in the
disc because of the change of the magnetic field. This eddy current is cut by the magnetic flux.
The interaction of the flux and the disc induces the deflecting torque.
When the devices consume power, the aluminum disc starts rotating, and after some number of
rotations, the disc displays the unit used by the load. The number of rotations of the disc is
counted at particular interval of time. Disc measured the power consumption in kilowatt hours.
Braking system – The permanent magnet is used for reducing the rotation of the aluminum disc.
The aluminum disc induces the eddy current because of their rotation. The eddy current cut the
magnetic flux of the permanent magnet and hence produces the braking torque.
This braking torque opposes the movement of the disc, thus reduces their speed. The permanent
magnet is adjustable due to which the braking torque is also adjusted by shifting the magnet to
the other radial position.
Registration (Counting Mechanism) – The main function of the registration or counting
mechanism is to record the number of rotations of the aluminum disc. Their rotation is directly
proportional to the energy consumed by the loads in the kilowatt hour.
Working of Single Phase Induction Type Energy Meter

When the load is not connected, no flux is produced in the series magnet and only a shunt field is
present. This alternating flux Φ p links with the disc and induces an emf E p in the disc, due to
this emf an eddy current I p flows in the disc, which produces an alternating field Φ p' in the disc.
But, no torque will be produced in the disc due to these two fluxes, because both the fluxes are
180° out of phase.
When the load current IL flows through the current coil, the series magnet is magnetized and an
alternating flux flows through it, and this flux links with the disc, which also produces an emf E se
resulting in the flow of eddy current I se. Ise sets up a field Φse' in the disc which interacts with the
field due to Ip and hence torque is produced in the disc due to this interaction of both the fields.
The torque produced is proportional to the difference of the torques due to Ip and Ise.
The phasor diagram of the energy meter is shown below.
Therefore, average torque is given as,

But,
Ip ∝ Φp ∝ Vph
Ise ∝ φse ∝ IL
From this,

T ∝ Vph IL cosΦ
Since A + B is a constant,

From the above, the average torque produced in the disc is proportional to the actual power
consumed in the load.
The above equation is derived assuming that Φ p lags behind Vph by exactly 90°. So, if Φ p is not
exactly in quadrature with Vph the above relation fails. Hence the copper shading rings or bands
must be provided to hold the above relation good. Let the torque produced by the braking magnet

∴ TB ∝ N
be TB. TB will be proportional to the speed of the disc (i.e., N).

TB = K2 N

Since,

At steady-state condition braking torque is equal to the driving torque.

Total number of revolutions is,

Therefore, total number of revolutions is proportional to the integral of true power i.e., energy.

Power Factor Meter

The power factor of the transmission line is measured by dividing the product of voltage and
current with the power. And the value of voltage current and power is easily determined by the
voltmeter, ammeter and wattmeter respectively. The power factor meter is of two types. They
are
1. Electrodynamometer
• Single Phase Electrodynamometer
• Three Phases Electrodynamometer
2. Moving Iron Type Meter
• Rotating Iron Magnetic Field
• Number of Alternating Field

The different types of power factor meter are explained below in details.

Single Phase Electrodynamometer Power Factor Meter

The construction of the single phase electrodynamometer is shown in the figure below. The
meter has fixed coil which acts as a current coil. This coil is split into two parts and carries the
current under test. The magnetic field of the coil is directly proportional to the current flow
through the coil.
The meter has two identical pressure coils A and B. Both the coils are pivoted on the spindle.
The pressure coil A has no inductive resistance connected in series with the circuit, and the coil
B has highly inductive coil connected in series with the circuit.
The current in the coil A is in phase with the circuit while the current in the coil B lag by the
voltage nearly equal to 90º. The connection of the moving coil is made through silver or gold
ligaments which minimize the controlling torque of the moving system.
The meter has two deflecting torque one acting on the coil A, and the other is on coil B. The
windings are so arranged that they are opposite in directions. The pointer is in equilibrium when
the torques are equal.
Deflecting torque acting on the coil A is given as

θ – angular deflection from the plane of reference.

Mmax – maximum value of mutual inductance between the coils. The
deflecting torque acting on coil B is expressed as

The deflecting torque is acting on the clockwise direction.

The value of maximum mutual inductance is same between both the deflecting equations.

This torque acts on anti-clockwise direction. The above equation shows that the deflecting torque
is equal to the phase angle of the circuit.

Instrument Transformer
A transformer that is used to measure electrical quantities like current, voltage, power, frequency
and power factor is known as an instrument transformer. These transformers are mainly used
with relays to protect the power system.
The Purpose of the instrument transformer is to step down the voltage & current of the AC
system because the level of voltage & current in a power system is extremely high. So designing
the measuring instruments with high voltage & current is difficult as well as expensive. In
general, these instruments are mainly designed for 5 A & 110 V. Instrument transformers are
classified into two types such as
• Current Transformer
• Potential Transformer

Current Transformer (CT)

• Current transformers are generally used to measure currents of high magnitude. These
transformers step down the current to be measured, so that it can be measured with a
normal range ammeter.
• A Current transformer has only one or very few numbers of primary turns. The primary
winding may be just a conductor or a bus bar placed in a hollow core. The secondary
winding has large number turns accurately wound for a specific turn‘s ratio. Thus, the
current transformer steps up (increases) the voltage while stepping down (lowering) the
current.

• Now, the secondary current is measured with the help of an AC ammeter. The turns ratio
of a transformer is K = Ns / Np = Ip / Is
I1 (or, Ip) = K. Is (or I2)
If CT ratio 1000/5A i.e. k= 1000/5 = 200
Secondary is connected directly to an ammeter. As the ammeter is having very small
resistance. Hence, the secondary of current transformer operates almost in short circuited
condition. One terminal of secondary is earthed to avoid the large voltage on
secondary with respect to earth.
• Note: The secondary winding of C.T shouldn't be left without ammeter. If we did so
secondary current I2 will be zero and opposing flux in the iron core will be zero therefore
magnetic flux in the core due to I1 will be very high. Hence, high voltage will be induced
in primary as well as secondary winding. Because of this high voltage the insulation of
primary and secondary winding will get damage. Therefore, if we want to remove the
ammeter the secondary winding must be short circuited by a thick wire.
Potential Transformer (PT)
• Potential transformers are also known as voltage transformers and they are basically
stepdown transformers with extremely accurate turns ratio.
 • Potential transformers step down the voltage of high magnitude to a lower voltage which
can be measured with standard measuring instrument. The range of this transformer is
110v. These transformers have large number of primary turns and smaller number of
secondary turns.

• Secondary of P.T. is having few turns and connected directly to a voltmeter. As the
voltmeter is having large resistance. Hence the secondary of a P.T. operates almost in
open circuited condition. One terminal of secondary of P.T. is earthed to maintain
the secondary voltage with respect to earth, which assures the safety of operators.
• If N1 and N2 are turn in primary & secondary, then turn ratio =N 2/N1 which will be given
on rating plate of PT.
• If V2 is reading of voltmeter then, V1 = V2/K can be calculated.

Numerical
1) Design a multi range d.c. mille ammeter using a basic movement with an internal
resistance Rm = 50Ω and a full scale deflection current Im =1mA . The ranges
required are 0-10mA; 0-50mA; 0-100mA and 0-500mA.
Solution:
2) A moving coil instrument gives a full scale deflection of 10mA, when the potential
difference across its terminal is 100mV. Calculate (a) The shunt resistance for a full
scale deflection corresponding to 100A (b) The resistance for full scale reading with
1000V. Calculate the power dissipation in each case?
Soln,

3) The pointer of a moving coil instrument gives full scale deflection of 20mA. The
potential difference across the meter when carrying 20mA is 400mV.The instrument
to be used is 200A for full scale deflection. Find the shunt resistance required to
achieve this, if the instrument to be used as a voltmeter for full scale reading with
1000V. Find the series resistance to be connected it?
Soln,
4) A 150 v moving iron voltmeter is intended for 50HZ, has a resistance of 3kΩ. Find
the series resistance required to extent the range of instrument to 300v. If the 300V
instrument is used to measure a d.c. voltage of 200V. Find the voltage across the
meter?
Solution:
Rm = 3kΩ,Vm =150V,V = 300V

5) Design an Aryton shunt to provide an ammeter with current ranges of 1A,5A,10A

and 20A. A basic meter with an internal resistance of 50w and a full scale deflection
current of 1mA is to be used.
Solution: Data given

6) A basic d’ Arsonval meter movement with an internal resistance Rm =100Ω and a

full scale current of Im =1mA is to be converted in to a multi range d.c. voltmeter
with ranges of 0-10V, 0- 50V, 0-250V, 0-500V. Find the values of various resistances
using the potential divider arrangement.
Solution:
 Unit III: Electrical Signal Processing and Data Acquisition
OP-Amp
An operational amplifier is an integrated circuit that can amplify weak electric signals.
An operational amplifier has two input pins and one output pin. Its basic role is to amplify and
output the voltage difference between the two input pins.

Op-amps are linear devices that are ideal for DC amplification and are used often in signal
conditioning, filtering or other mathematical operations (add, subtract, integration and
differentiation).
Ideal Op Amp Characteristics
 Open loop voltage gain is Infinite
1. Input impedance is zero i.e. without any leakage of current from the supply to the inputs.
  Output impedance is infinite i.e. supply full current to the load connected to the output.
 Zero offset voltage i.e. output will be zero when the voltage difference between the
inverting and the non-inverting inputs is zero
 Infinite bandwidth i.e. so that it can amplify any frequency from DC signals to the highest
AC frequencies 
  Infinite CMRR. CMRR=Ad/Ac
 Infinite slew rate i.e. changes in the output voltage occur simultaneously with the changes in
the input voltage i.e. ensures zero common mode gain. Due to this common mode noise
output voltage is zero for an ideal op-amp.
 Zero power supply rejection ratio i.e. reflects how well the op amp can reject noise in its
power supply from propagating to the output

Instrumentation Amplifier
An instrumentation amplifier allows an engineer to adjust the gain of an amplifier circuit without
having to change more than one resistor value. Compare this to the differential amplifier, which
requires the adjustment of multiple resistor values.

Working of Instrumentation Amplifier

The output voltage Vout of the difference amplifier is actually the output stage of the
instrumentation amplifier. This output voltage Vout is then the difference between the input
signals applied at the input terminals.
Let us consider Vo1 and Vo2 the output voltages of the two op-amps 1 and 2 respectively. Then
the value of voltage Vout of difference amplifier is

The value of voltages Vo1 and Vo2 is in terms of the input voltages and resistances. The input
stage of the instrumentation amplifier is also shown below.
Consider the value of input voltage as V 1 at node A. From the virtual ground concept, the value
of the voltage at node B will be V1. At node G the potential will also be equal to V1.
Consider the value of input voltage as V 2 at node D. From the virtual ground concept the value of
the voltage at node C will be V2. At node H the potential will also be equal to V2.
The value of current at input stage is zero because we consider the ideal case. Therefore, the
value of current remains same in all the resistors R1, Rgain and R1
At node E and F from Ohm‘s Law, we can write

As there is no input current at the input stage hence, value of current between node G and H will
be

Equating both equations 1 and 2,

The output of the difference amplifier is

Therefore,

In equation 3 we substitute (Vo1 – Vo2) value

The output voltage of an instrumentation amplifier is in the above equation. The value of the
overall gain of the amplifier is (R3/R2) {(2R1+Rgain)/Rgain}.

Advantages of Instrumentation Amplifier

The advantages of the instrumentation amplifier include the following.
• The gain of a three op-amp instrumentation amplifier circuit can be easily varied by
adjusting the value of only one resistor Rgain.
• Gain of the amplifier depends only on the external resistors used.
• Input impedance is very high due to the emitter follower configurations of amplifire
Output impedance of the instrumentation amplifier is very low due to difference
amplifier.
• CMRR of the op-amp is very high and almost all of the common mode signal will be
rejected.

Signal Amplification, Attenuation, Integration, Differentiation, Network Isolation,

Wave Shaping

Signal Amplification
• A signal amplifier is a circuit that uses electrical power to increase the amplitude of an
incoming signal voltage or current signal, and output this higher amplitude version at its
output terminals.
• The ideal signal amplifier creates an exact replica of the original signal that is larger but
identical in every other way. In practice, a ―perfect amplifier is not possible, because no
circuit can perfectly and proportionately scales up all aspects of a signal past a certain point.
• Signal amplifiers are an essential component of thousands of devices, including landline and
cellular telephone systems, music and public address systems, data acquisition (DAQ)
systems, radio frequency transmitters, servo motor controllers, and countless more.

• In data acquisition (DAQ) systems, signal amplifiers are needed to increase the amplitudes
from sensors that output small signals found in today's data acquisition systems:
 Differential amplifiers
 Isolated amplifiers
 Voltage amplifiers: low voltage amplifier, high voltage amplifier, DC voltage
amplifier, AC voltage amplifier
 Current amplifiers
 Charge amplifiers
 Thermocouple amplifiers
 Strain gauge amplifiers: bridge amplifier, full-bridge amplifier, half-bridge
amplifier, quarter-bridge amplifier)

Attenuation
Attenuation, the opposite of amplification, is necessary when voltages to be digitized are beyond
the ADC range. This form of signal conditioning decreases the input signal amplitude so that the
conditioned signal is within ADC range. Attenuation is typically necessary when measuring
voltages that are more than 10 V.
Attenuation is one of the techniques used in signal conditioning to scale down the amplitude or
voltage level of a signal to match the input range or requirements of downstream components or
systems. Attenuation can be achieved using passive or active components within a signal
conditioning circuit.
Passive attenuation techniques typically involve the use of voltage dividers or attenuator
circuits that passively divide the input signal voltage by a fixed ratio. These circuits consist of
resistors arranged in a specific configuration to achieve the desired attenuation. The voltage
divider circuit is a common example of passive attenuation, where the output voltage is a fraction
of the input voltage determined by the resistor values.
Active attenuation involves the use of active components such as operational amplifiers
(opamps) to actively amplify or attenuate the signal. In the case of attenuation, an op-amp can be
configured as an inverting amplifier or a voltage follower with appropriate gain settings to
achieve the desired signal reduction. Active attenuation techniques offer more flexibility and
precision in controlling the attenuation level and can be adjusted dynamically based on system
requirements.
Attenuation in signal conditioning is often used to protect sensitive components from excessive
signal levels, prevent signal clipping or saturation, or match signal levels between different
stages of a system. For example, in analog-to-digital converters (ADCs), where the input voltage
range is limited, attenuation can be applied to scale down the input signal to fit within the ADC's
range without losing important information.
Network Isolation:
Network isolation refers to the separation or protection of one part of a network from another to
prevent unwanted interactions or disturbances. It can involve techniques like using isolation
transformers, optocouplers, or galvanic isolation methods to electrically separate different parts
of a circuit or network.
Network isolation is an important aspect of signal conditioning because it helps ensure the
integrity and accuracy of the conditioned signal.
Here are a few reasons why network isolation is important in signal conditioning:
Grounding and Ground Loops: Network isolation helps address grounding issues and ground
loops, which can introduce unwanted noise or voltage differences between different parts of a
network. By isolating the signal path, you can prevent these ground-related problems and
maintain a clean signal.
Noise and Interference Rejection: Signals in a network can be susceptible to various sources of
noise and interference, such as electromagnetic interference (EMI) or radio frequency
interference (RFI). Network isolation techniques, such as using isolation transformers or
optocouplers, can help block or attenuate these unwanted signals and provide a cleaner signal for
further conditioning.
Voltage Level Differences: In some cases, signal conditioning circuit may operate at a different
voltage level than the surrounding network. Network isolation allows for the conversion or
adaptation of voltage levels while maintaining the separation between the circuit and the
network.
Protection: Isolation can provide protection against voltage spikes, transients, or other electrical
disturbances that may occur in the network.
Overall, network isolation in signal conditioning helps ensure the accuracy, reliability, and
integrity of the conditioned signal by separating it from unwanted noise, interference, and
disturbances present in the network.
Wave Shaping
• Wave shaping circuits are the electronic circuits, which produce the desired shape at
the output from the applied input wave form. These circuits perform two functions −
• Attenate the applied wave
• Alter the dc level of the applied wave.
• Wave shaping technique include clipping and clamping.
In op-amp clipper circuits a rectifier diode may be used to clip off a certain portion of the input
signal to obtain a desired o/p waveform.

Clippers
A clipper is an electronic circuit that produces an output by removing a part of the input above
or below a reference value. Clippers can be classified into the following two types based on the
clipping portion of the input.
• Positive Clipper
• Negative Clipper
Positive Clipper: A positive clipper is a clipper that clips only the positive portion(s) of the
input signal.

Negative Clipper: A negative clipper is a clipper that clips only the negative portion(s) of the
input signal.
Clamper Circuit: A Clamper Circuit is a circuit that adds a DC level to an AC signal. Actually,
the positive and negative peaks of the signals can be placed at desired levels using the clamping
circuits. As the DC level gets shifted, a clamper circuit is called as a Level Shifter.
 : Integration is a mathematical operation applied to a signal that calculates the
Integration
cumulative sum of the signal over time. It essentially
e area
measures
underththe signal curve.
Integration is commonly used in applications such as measuring accumulated quantities or
determining average values.
The following circuit shows a basic/ideal integrator
-amp,using op

The non-inverting input terminal is at ground potential and hence, the inverting terminal is
appearing to be at ground potential. The current 'I' through the resistance R is given as,

The input current to op-amp is zero so same current 'I' flows through the capacitor 'C' in feedback
path also and is given as,
 Comparing the above two equations for current 'I' we get,

Integrating both the sides, we get,

where – 1/RC= Gain / scale factor of an integrator. Thus output

voltage is nothing but time integration of the input signal and hence acting as an
integrator.
Now let us see what is the response of the integrator to the different types of input signals.
1) Vin = Step signa
l

2) Vin = Square Wave

3) Vin = Sine Wave

Let Vin=Vm sinωt
 : Differentiation is the mathematicalion
Differentiation operat
that calculates the rate of
change or slope of a signal with respect to time. It provides information about the instantaneous
change in the signal. Differentiation is frequently used in applications such as finding the
velocity or acceleration from
ition
posdata.

V+ =0
V+ - V- = 0

V- = 0

Applying kcl at node V-

Ic + IR= 0

C d (V1)/dt + V0/R= 0
V0/R= -C d ( V1)/ dt
V0= -RCd/dt(V1)
Now let us see what is the response of the differentiator for the different types of input signals.
Effect Of Noise, Analog Filtering, Digital Filtering
Noise refers to unwanted or random signals that can interfere with the desired signal. In signal
conditioning, noise can degrade the quality of the signal and make it more difficult to extract
useful information. It can arise from various sources, such as electromagnetic interference,
thermal noise, or even limitations of electronic components.
The effect of noise can be mitigated through filtering techniques, both analog and digital.
Filtering helps to reduce the impact of noise on the signal and improve its quality for further
processing or transmission.
Analog Filtering:
Analog filtering involves the use of analog circuits, such as resistors, capacitors, and inductors, to
modify the characteristics of the signal. Analog filters are commonly used in signal conditioning
to attenuate unwanted frequencies, remove noise, or shape the frequency response of the signal.
Analog filters can be designed as low-pass, high-pass, band-pass, or band-reject filters to
selectively allow or block certain frequency components of the signal. By employing analog
filters, the undesired frequency components, including noise, can be attenuated, improving the
signal quality for subsequent processing stages.

Digital Filtering:
Digital filtering involves processing the signal in the digital domain using digital signal
processing (DSP) techniques. Digital filters are implemented through algorithms and
computations performed by microprocessors, digital signal processors (DSPs), or dedicated
hardware.
Digital filters offer several advantages in signal conditioning.
• They provide flexibility in terms of filter characteristics and can be easily reconfigured or
modified.
• Digital filters can implement a wide range of filter types, including finite impulse
response (FIR) filters and infinite impulse response (IIR) filters.
• Digital filtering allows precise control over filter parameters, such as cutoff frequencies,
stop band attenuation, and filter order.
 • It enables efficient noise removal, signal enhancement, and frequency response shaping.
• Digital filters can also be adaptive, adjusting their characteristics based on changing
signal conditions or specific requirements.
Overall, noise, analog filtering, and digital filtering all contribute to signal conditioning by
reducing noise, improving signal quality, and shaping the frequency response as needed. The
choice of filtering techniques depends on the specific application, requirements, and available
resources.

Data Acquisition System

A data acquisition system (DAS) is an information system that collects, stores and distributes
information. It is used in industrial and commercial electronics, and environmental and scientific
equipment to capture electrical signals or environmental conditions on a computer device. A data
acquisition system is also known as a data logger.
These data acquisition systems will perform the tasks such as conversion of data, storage of data,
transmission of data and processing of data for the purpose of monitoring, analyzing, and/or
controlling systems and processes. Data acquisition systems and instruments are either the
combination of a number of data acquisition components that make up a complete system or a
self-contained instrument. A data acquisition system is a system that acquires data, generally by
digitizing analog channels and storing the data in digital form.

Types of Data Acquisition Systems

Data acquisition system is measurement system by which data is acquired economically and
efficiently in desired form. The data can be acquired either in analog or digital form hence data
acquisition systems can be classified into the following two types on the basic of data how it is
acquired.
1) Analog Data Acquisition Systems
2) Digital Data Acquisition Systems

Analog Data Acquisition Systems

The data acquisition systems, which can be operated with analog signals are known as analog
data acquisition systems. Following are the blocks of analog data acquisition systems.

The component of analog data acquisition system are:

Transducer − It converts physical quantities into electrical signals. The most common
transducers convert physical quantities to electrical quantities, such as voltage or current.
Transducer characteristics define many of the signal conditioning requirements of a DAS system.
Signal conditioner −It takes the output of the transducer and makes it into a suitable form of
condition so that rest the rest of the DAS process. It performs the functions like amplification and
selection of desired portion of the signal.
Multiplexer − Multiplexer is the process of showing single channel with more than one input
and multiplier accept multiple analog input and connects then sequentially to one measuring
output. Multiplexing uses same transmission channel for transmitting more than one quantity and
it becomes necessary if distance between transmitting and receiving point is large and many
quantities are to be transmitted by separate channel.
Amplifier − An amplifier is increase the power of a signal (a time-varying voltage or current).
It uses electric power from a power supply to increase the amplitude of a signal applied to its
input terminals, producing a proportionally greater amplitude signal at its output. The amount of
amplification provided by an amplifier is measured by its gain: the ratio of output voltage,
current, or power to input. An amplifier is a circuit that has a power gain greater than one
Display Device/Analog Recorder − It displays the input signals for monitoring purpose.
Graphic recording instruments – These can be used to make the record of input data
permanently.
Magnetic tape instrumentation − It is used for acquiring, storing & reproducing of input data.

Digital Data Acquisition Systems

The data acquisition systems, which can be operated with digital signals are known as digital
data acquisition systems. So, they use digital components for storing or displaying the
information.
Mainly, the following operations take place in digital data acquisition.
• Acquisition of analog signals
• Conversion of analog signals into digital signals or digital data
• Processing of digital signals or digital data
Following are the blocks of Digital data acquisition systems.

Fig: Digital data acquisition system

Transducer − It converts physical quantities into electrical signals.

Signal conditioner − It performs the functions like amplification and selection of desired portion
of the signal.
Multiplexer − connects one of the multiple inputs to output. So, it acts as parallel to serial
converter.
Sample and Hold Circuit − It is usually used with an Analog to Digital Converter to sample the
input analog signal and hold the sampled signal, hence the name ‗Sample and Hold‘. In the S/H
Circuit, the analog signal is sampled for a short interval of time, usually in the range of 10µS to
1µS. After this, the sampled value is hold until the arrival of next input signal to be sampled. The
duration for holding the sample will be usually between few milliseconds to few seconds.
Basically the sample and hold circuit, samples the analog signal and the capacitor present holds
these samples. This sampled value when provided to the ADC, it generates a discrete signal from
an analog one.
Analog to Digital Converter − It converts the analog input into its equivalent digital output.
Display device − It displays the data in digital format.
Digital Recorder − It is used to record the data in digital format.

Single Channel Data Acquisition System:

A Single Channel Data Acquisition System consists of a signal conditioner followed by an
analog to digital (A/D) converter, performing repetitive conversions at a free running, internally
determined rate. The outputs are in digital code words including over range indication, polarity
information and a status output to indicate when the output digits are valid.

Fig: Block Diagram of Single Channel DAS

A Single Channel Data Acquisition System is shown in Fig. (a) The digital outputs are further
fed to a storage or printout device, or to a digital computer device, or to a digital computer for
analysis.
The popular Digital panel Meter (DPM) is a well-known example of this. However, there are two
major drawbacks in using it as a DAS.
1. It is slow and the BCD has to be changed into binary coding, if the output is to be
processed by digital equipment.
2. While it is free running, the data from the A/D converter is transferred to the interface
register at a rate determined by the DPM itself, rather than commands beginning from the
external interface.

Multi-Channel Data Acquisition System

There will be many subsystems in a data acquisition system. They can be time shared by two or
more input sources. The numbers of techniques are used for time shared measurements
depending on the desired properties of the multiplexed system. It has a single A/D converter
preceded by a multiplexer.
There can be number of inputs. Each signal is given to individual amplifiers. The output of the
Amplifiers is given to Signal condition circuits. From the output of the signal conditioning
circuits the signals go to the multiplexer'. The multiplexer output is converted into digital signals
by the A/D converters sequentially.

Fig: Multi Channel Data Acquisition System

The multiplexer stores the data say of the first channel in the sample hold circuit. It then seeks
the second channel. During this interval the data of the first channel will be converted into digital
form. This permits utilization of time more efficiently.
When once the conversion is complete, the status line from the converter causes the sample/hold
circuit to return to the sample mode. It then accepts the signal of the next channel. After
acquisition of-data either immediately or on a command the sample hold circuit will be switched
to the hold mode. Now conversion begins and the multiplexer selects the next channel.
This method is slow. Sample hold circuits or A/D converters are multiplexed for faster operation.
However this method is less costly as majority of subsystems are shared. If the signal variations
are very slow satisfactory accuracy can be obtained even without the sample hold circuit.
PC Based Data Acquisition System
The most visible trends can be seen as the effects of transition to PC-based DAQ. Now, all that
processing is being done inside computers, so the instruments are interfaced to a computer with
analysis being done through computer software. Thus we see a more software-defined approach
to DAQ, as well as the emergence of high-speed USB-enabled DAQs.

PC-based DAQ (Data Acquisition) refers to a system where data acquisition hardware is
connected to a computer for the purpose of acquiring and processing analog or digital signals.
Here's an overview of PC-based DAQ systems:

Data Acquisition Hardware: The data acquisition hardware serves as the interface between the
physical signals and the computer. It typically consists of analog-to-digital converters (ADCs)
for converting analog signals into digital data, digital-to-analog converters (DACs) for
converting digital data into analog signals, and various input/output (I/O) channels for handling
different types of signals (analog, digital, counter/timer, etc.).
The hardware may also include signal conditioning components like amplifiers, filters, and
isolation circuits to enhance the quality and reliability of the acquired signals. Some PC-based
DAQ systems offer modular designs, allowing users to customize and expand the system by
adding or removing modules as needed.

PC Interface: The data acquisition hardware is connected to a computer via a suitable interface,
such as USB (Universal Serial Bus), PCIe (Peripheral Component Interconnect Express),
Ethernet, or wireless connections. The interface enables the transfer of acquired data between the
hardware and the computer.

Driver and Software: To communicate with the data acquisition hardware, the computer
requires device drivers and appropriate software. The device drivers establish the necessary
communication protocols and provide an interface for accessing and controlling the hardware
from the software applications.
The software typically includes development tools and libraries that facilitate data acquisition,
signal processing, visualization, and analysis. These tools allow users to configure acquisition
parameters, implement real-time processing algorithms, and visualize the acquired data in
various formats.

Benefits of PC-based DAQ:

Versatility: PC-based DAQ systems offer flexibility and adaptability due to the ability to utilize
a wide range of software and hardware options.
Processing Power: PCs provide ample computing power for real-time signal processing,
analysis, and visualization.
Integration: PC-based DAQ systems can easily integrate with other software and hardware
components, making them suitable for complex measurement and control systems.
Cost-Effectiveness: PC-based DAQ systems are often more cost-effective compared to
dedicated standalone data acquisition systems.

Series and Parallel transmission:

The process of sending data between two or more digital devices is known as data transmission.
Data is transmitted between digital devices using one of the two methods − serial transmission or
parallel transmission.
Serial Transmission: In Serial Transmission, data-bit flows from one computer to another
computer in bi-direction. In this transmission, one bit flows at one clock pulse. In Serial
Transmission, 8 bits are transferred at a time having a start and stop bit.
Parallel Transmission: In Parallel Transmission, many bits are flow together simultaneously
from one computer to another computer. Parallel Transmission is faster than serial transmission
to transmit the bits. Parallel transmission is used for short distance.

Difference between Serial and Parallel Transmission

Key Serial Transmission Parallel Transmission

Definition Serial Transmission is the type of Parallel Transmission is the mode of

transmission in which a single transmission in which multiple
communication link is used to transfer parallel links are used that transmits
the data from one end to another. each bit of data simultaneously.

Bit In case of Serial Transmission, only one In case of Parallel Transmission, 8bits
transmission bit is transferred at one clock pulse. transferred at one clock pulse.

Key Serial Transmission Parallel Transmission

Cost As single link is used in Serial Multiple links need to be

Efficiency Transmission, it can be implemented implemented in case of Parallel
easily without having to spend a huge Transmission, hence it is not cost
amount. It is cost efficient. efficient.

Performance As single bit gets transmitted per clock 8-bits get transferred per clock in case
in case of Serial Transmission, its of Parallel transmission, hence it is
performance is comparatively lower as more efficient in performance.
compared to Parallel Transmission.

Preference Serial Transmission is preferred for long Parallel Transmission is preferred only
distance transmission. for short distance.
 Serial Transmission is less complex as Parallel Transmission is more
Complexity compared to that of Parallel complex as compared to that of Serial
Transmission. Transmission.
Features and application of RS232 cable
The term RS232 stands for "Recommended Standard 232" and it is a type of serial
communication used for transmission of data normally in medium distances, it is used for
connecting computer and its peripheral devices to allow serial data exchange between them. As it
obtains the voltage for the path used for the data exchange between the devices. It is used in
serial communication up to 50 feet with the rate of 1.492kbps. As EIA defines, the RS232 is used
for connecting Data Transmission Equipment (DTE) and Data Communication Equipment
(DCE).

RS232 Features
1. RS232 uses Asynchronous communication so no clock is shared between PC and
MODEM.
2. Logic ‗1‘ on pin is stated by voltage of range ‗-15V to -3V‘ and Logic ‗0‘ on pin is
stated by voltage of range ‗+3V to +15V‘. The logic has wide voltage range giving
convenience for user.
3. MAX232 IC can be installed easily to establish RS232 interface with microcontrollers.
4. Full duplex interface of RS232 is very convenient.
5. Two pin simplex RS232 interface can also be established easily if required.
6. A maximum data transfer speed of 19 Kbps(Kilobits per second) is possible through
RS232
7. A maximum current of 500mA can be drawn from pins of RS232 8. The interface can be
established up to a distance of 50 feet.
 Applications of RS232 Cables
 Serial Communication between Computers and Peripherals such as modems, printers,
barcode scanners, and industrial control devices. This enables data exchange and control
between the computer and the peripheral device.
 Configuration and Programming of Devices including routers, switches, network devices,
and embedded systems. It allows users to access the device's command-line interface or
configuration menu.
 Industrial Automation and Control to enables communication between programmable
logic controllers (PLCs), human-machine interfaces (HMIs), and other control devices.
 Point-of-Sale Systems for connecting cash drawers, barcode scanners, and receipt printers
to the main terminal. This allows the exchange of transaction data between different
components of the system.
 Data Acquisition and Instrumentation used for connecting data acquisition devices, such
as sensors, data loggers, and measuring instruments, to a computer or control system.
This enables the collection and analysis of data from various sensors and instruments.
Features and application of IEEE 1248 B
IEEE 1248 -2020is titled "Standard for Analog
-to-Digital Converter (ADC) Testing
- Methods
and Metrics." This standard provides guidelines and methods for testing the performance of
analog
-to-digital converters (ADCs)
used in various applications.

Key fea
tures of IEEE 1248
-2020
• ADC Testing Methods: The standard outlines various methods for evaluating the
performance of ADCs. This includes testing the accuracy, linearity, resolution, dynamic
range, and other important parameters of the ADC.
• Performance Metrics: IEEE 1248-2020 defines several performance metrics for
characterizing the quality and reliability of ADCs. These metrics help in assessing the
ADC's ability to convert analog signals into digital representation accurately.
• Test Procedures: The standard provides recommended test procedures for conducting the
tests specified in the standard. It covers aspects such as test setup, test signals,
measurement techniques, and data analysis.
• Reporting Requirements: IEEE 1248-2020 specifies the information that should be
included in test reports to ensure clear and consistent documentation of ADC
performance.
Optical communication, fibre optics, electro‐optic conversion devices
Optical Communication
Optical Fiber Communication is the method of communication in which signal is transmitted
in the form of light and optical fiber is used as a medium of transmitting those light signal from
one place to another. The signal transmitted in optical fiber is converted from the electrical signal
into light and at the receiving end, it is converted back into the electrical signal from the light.
The data sent can be in the form of audio, video or telemetry data that is to be sent over long
distances or over Local Area Networks.

Transmitter side: On the transmitter side, first if the data is analog, it is sent to a coder or
converter circuit which converts the analog signal into digital pulses of 0,1,0,1…(depending on
how the data is) and passed through a light source transmitter circuit. And if the input is digital
then it is directly sent through the light source transmitter circuit which converts the signal in the
form of light waves.
Optical Fiber Cable: The light waves received from the transmitter circuit to the fiber optic
cable is now transmitted from the source location to destination & received at the receiver block.
Receiver Side: Now on the receiver side the photocell, also known as the light detector, receives
the light waves from the optical fiber cable, amplifies it using the amplifier and converts it into
the proper digital signal. Now if the output source is digital then the signal is not changed further
and if the output source needs analog signal then the digital pulses are then converted back to an
analog signal using the decoder circuit.
The whole process of transmitting an electrical signal from one point to other by converting it
into light using Fiber optic-cable as transmission source is called Optical Fiber
Communication.

Fiber Optics:
Fiber optics involves the transmission of light signals through optical fibers, which are typically
made of glass or plastic. These fibers have the ability to guide light through multiple reflections,
allowing for efficient and high-speed transmission of data over long distances. The Fiber optic
cable is made of high-quality extruded glass (si) or plastic, and it is flexible. The diameter of the
fiber optic cable is in between 0.25 to 0.5mm (slightly thicker than a human hair).
 Fig: Fiber Optic Cable
Key features of fiber optics include:
• Low Loss enabling data to be transmitted over long distances without significant
degradation.
• Large Transmission capacity
• Easy Amplification
• High Bandwidth allowing for the transmission of large amounts of data simultaneously.
• Immunity to Electromagnetic Interference making them suitable for use in
environments with high electrical noise.
• Lightweight and Flexible making them easy to install and for various applications.
Electro-optic Conversion Devices:
Electro-optic conversion devices, also known as optoelectronic devices, are essential components
in optical communication systems. These devices facilitate the conversion of electrical signals to
optical signals and vice versa. Some commonly used electro-optic conversion devices include:
• Light Emitting Diodes (LEDs): LEDs are used as light sources in optical
communication systems. They convert electrical signals into light signals, typically in
infrared or visible spectrum.
• Laser Diodes: Laser diodes are another type of light source used in optical
communication. They produce coherent & highly focused light, making them suitable for
long-distance transmission.
• Photodiodes: Photodiodes are used to convert optical signals back into electrical signals.
They detect and convert light intensity variations into electrical current, allowing for
signal detection and processing.
• Modulators: Modulators are devices that modulate the intensity, phase, or frequency of
an optical signal in response to an electrical signal. They are used to encode information
onto the optical carrier signal for transmission.
These electro-optic conversion devices enable the transmission, reception, and processing of data
in optical communication systems. They are integrated into transmitters, receivers, and other
system components to ensure efficient and reliable communication.
Overall, fiber optics and electro-optic conversion devices are crucial components in optical
communication systems, enabling high-speed, long-distance, and secure data transmission in
various applications.
 Unit IV: Data Converter and Connectors
Analog to Digital Converter (ADC) and Digital to analog Converter (DAC):
Principle and Specification
All the real world quantities are analog in nature. Therefore, this system needs an intermediate
device to convert the analog data into digital data in order to communicate with digital
processors like microcontrollers and microprocessors. Analog to Digital Converter (ADC) is an
electronic integrated circuit used to convert the analog signals such as voltages to digital or
binary form consisting of 1s and 0s. Most of the ADCs take a voltage input as 0 to 10V, -5V to
+5V, etc., and correspondingly produces digital output as some sort of a binary number.
There are two types of data converters
Analog to Digital Converter
Digital to Analog Converter

Analog to Digital Converter

A converter that is used to change the analog signal to digital is known as an analog to digital
converter or ADC converter. This converter is one kind of integrated circuit or IC that converts
the signal directly from continuous form to discrete form.
ADC Block Diagram

The analog signal is first applied to the ‗sample‗ block where it is sampled at a specific
sampling frequency. The sample amplitude value is maintained and held in the ‗hold‗ block. It
is an analog value. The hold sample is quantized into discrete value by the ‗quantize‗ block. At
last, the ‗encoder‘ converts the discrete amplitude into a binary number.

Analog To Digital Conversion Steps

The conversion from analog signal to a digital signal in an analog to digital converter is
explained below using the block diagram given above.
Sampler: Sampler is a circuit that takes samples from the continuous analog signal according to
its sample frequency. The sampling frequency is set according to the requirement. Basically, the
sampler converts the continuous-time-continuous amplitude signal into a continuous amplitude
discrete time signal.
Holding Circuit: Holding circuit does not convert anything it just holds the samples generated
by the sampler circuit. It holds the first sample until the next sample comes from the sampler.
Once the new sample comes from the sampler to the holding circuit it releases the old sample to
its next block.
Quantizer: Quantizer quantized the signal which means it converts the continuous amplitude
discrete - time signal into discrete time-discrete amplitude signal. It breaks or splits the samples
into small parts.
 Encoder: Encoder is the circuit that actually generates the digital signal into binary form. The
output from encoder is fed to the next circuitry. Here, is the end of the analog to a digital circuit.
as we know that the digital devices operate on binary signals so it is necessary to convert the
digital signal into the binary form using the Encoder. This is the whole process of converting an
Analog signal into digital form using an Analog to Digital Converter. This whole conversion
occurs in a microsecond.

Fig: Analog to Digital Conversion Process

Characteristics of ADC (Analog to Digital converter)

some of the important characteristics of ADC are:
1. Resolution:
• Resolution is defined as the maximum number of digital output codes. This is the same as that of
a DAC.
Resolution=2n
• Alternatively, resolution can be defined as the ratio of the change in the value of the input analog
voltage VA, required to change the digital output by 1 LSB.
Resolution
2. Conversion Time:
• It is the total time required to convert analog signal into a corresponding digital output.
• As we know, the conversion time depends on the conversion technique used for an ADC.
• The conversion time also depends on the propagation delay introduced by the circuit components.
• Conversion time should ideally be zero and practically as small as possible.
3. Quantization Error:
• Quantization error is a type of error that occurs during the process of digitizing analog signals. It
arises due to the discrete nature of digital representation, where the continuous range of analog
values is converted into a finite set of discrete digital values.
• As shown in the figure, the digital output is not always an accurate representation of the analog
input.
• For example, any input voltage between 1/8 and 2/8 of full scale will be converted to a digital
word of ―001‖. This approximation process is called quantization and the error due to
quantization is called quantization error.
• The maximum value of quantization error is ±1/2 LSB.
• The quantization error should be as small as possible. It can be reduced by increasing the number
of bits. The increase in the number of bits will also improve resolution.

Types of ADC
ADCs all perform the same function, but with different converter circuit architectures and
capabilities. Two of the primary capabilities where these types of ADCs differ are in their sample
rate and resolution, which arises due to the different conversion circuitry used in these
components.

a) Flash type/Parallel ADC

Flash ADCs are, perhaps, the simplest in concept of the ADCs and are very fast, but they tend to
be large and expensive. They derive their name from the parallel configuration of ‗comparator‘
reference voltages used in the conversion process. A flash type ADC produces an equivalent
digital output for a corresponding analog input in no time. So, flash type ADC is fastest ADC.
Basic operation:
• The analog input voltage is compared with a set of (known) fixed reference voltages operating in
parallel—the more reference voltages used, the better the resolution
 To generate an n-bit result, 2n – 1 reference voltage comparators are required—e.g., 4-bit
resolution requires 15 reference voltages, 8-bit resolution requires 255 reference voltages
comparators, and so on.
One end of the comparator array is connected to the analog voltage, while the other end is
connected to a series of resistors set up as voltage dividers.
The circuit diagram of a 3-bit flash type ADC is shown in the following figure

The working of a 3-bit flash type ADC is as follows.

• The voltage divider network contains 8 equal resistors. A reference voltage V R is applied
across that entire network with respect to the ground. The voltage drop across each
resistor from bottom to top with respect to ground will be the integer multiples (from 1 to
8) of VR8.
• The external input voltage Vi is applied to the non-inverting terminal of all comparators.
The voltage drop across each resistor from bottom to top with respect to ground is applied
to the inverting terminal of comparators from bottom to top.
• At a time, all the comparators compare the external input voltage with the voltage drops
present at the respective other input terminal. That means, the comparison operations take
place by each comparator parallelly.
• The output of the comparator will be ‗1‘ as long as Vi is greater than the voltage drop
present at the respective other input terminal. Similarly, the output of comparator will be
 ‗0‘, when, Vi is less than or equal to the voltage drop present at the respective other input
terminal.
All the outputs of comparators are connected as the inputs of priority encoder. This
priority encoder produces a binary code (digital output), which is corresponding to the
high priority input that has ‗1‘.
Therefore, the output of priority encoder is nothing but the binary equivalent (digital
output) of external analog input voltage, Vi.
Pros:
• Fastest ADC method, capable of sampling rates in the gigahertz range Cons:
• The higher the resolution, the larger the flash ADC needs to be, requiring more power and
limiting the sample rate—an 8-bit resolution tends to achieve a sensible balance between
power and precision and is a popular configuration
• Larger and more expensive than other ADC configurations Q) Design a 2-bit flash type

ADC with its working principle.

b) Ramp/Counter type ADC

A counter type ADC produces a digital output, which is approximately equal to the analog input
by using counter operation internally.
The block diagram of a counter type ADC is shown in the following figure −

The counter type ADC mainly consists of 5 blocks: Clock signal generator, Counter, DAC,
Comparator and Control logic.
The working of a counter type ADC is as follows −
• The control logic resets the counter and enables the clock signal generator in order to send
the clock pulses to the counter, when it received the start commanding signal.
• The counter gets incremented by one for every clock pulse and its value will be in binary
(digital) format. This output of the counter is applied as an input of DAC.
 • DAC converts the received binary (digital) input, which is the output of counter, into an
analog output. Comparator compares this analog value,Va with the external analog input
value Vi.
The output of comparator will be ‘1’ as long as is greater than. The operations
mentioned in above two steps will be continued as long as the control logic receives ‗1‘
from the output of comparator.
The output of comparator will be ‘0’ when Vi is less than or equal to Va. So, the control
logic receives ‗0‘ from the output of comparator. Then, the control logic disables the
clock signal generator so that it doesn‘t send any clock pulse to the counter.
• At this instant, the output of the counter will be displayed as the digital output. It is
almost equivalent to the corresponding external analog input value Vi.

c) Successive Approximation Type ADC

The Successive Approximation Register (SAR) type ADC is an extremely popular
implementation for a long time. SAR is a very popular ADC configuration since it offers a good
balance between speed, resolution, and fidelity for a wide variety of signal types. They are
slower than flash ADCs, however, since they must pause and reset after each trial. The block
diagram of SAR ADC, it is similar to the Counter ADC except that in place of the main Counter,
we have a Register and Latch Circuit.

The working of a successive approximation ADC is as follows −

• The control logic resets all the bits of SAR and enables the clock signal generator in
order to send the clock pulses to SAR, when it received the start commanding signal.
• The binary (digital) data present in SAR will be updated for every clock pulse based on
the output of comparator. The output of SAR is applied as an input of DAC.
• DAC converts the received digital input, which is the output of SAR, into an analog
output. The comparator compares this analog value Va with the external analog input
value Vi.
 • The output of a comparator will be ‗1‘ as long as Vi is greater than Va. Similarly, the
output of comparator will be ‗0‘, when Vi is less than or equal to Va.
• The operations mentioned in above steps will be continued until the digital output is a
valid one.
The digital output will be a valid one, when it is almost equivalent to the corresponding external
analog input value Vi.
Pros:
 
Relatively simple circuit design (only one comparator required)
• Offers a good balance between speed and resolution
• Versatile for different signal types (wave shapes) Cons:
• Only intermediate speeds can be achieved (slower than flash but faster than delta-sigma
ADCs)
• Limited bit resolution—typically 8 to 18 bits—which is lower than delta-sigma ADCs
• Not good at handling spikes in the analog input voltage
• Requires separate (external) anti-aliasing filtering  Susceptible to high-frequency
quantization noise

Steps.
(1) The MSB is initially set to 1 with the remaining three bits set as 000. The digital
equivalent voltage is compared with the unknown analog input voltage.
(2) If the analog input voltage is higher than the digital equivalent voltage, the MSB is
retained as 1 and the second MSB is set to 1. Otherwise, the MSB is set to 0 and the second
MSB is set to 1. Comparison is made as given in step (1) to decide whether to retain or reset
the second MSB.
Let us assume that the 4-bit ADC is used and the analog input voltage is V A = 11 V and
reference voltage is 16V.
Step 1:
When the conversion starts, the MSB bit is set to 1. i.e. i/p = 1000
Let, input to DAC = 1000 =d3 d2 d1 d0
Vout = {8+0+0+0}
= 8 V < Input Voltage (V)
Since the unknown analog input voltage VA is higher than the equivalent digital voltage VD,
as discussed in step (2), the MSB is retained as 1 and the next MSB bit is set to 1 as follows
Step: 2
So, Let, input to DAC = 1100 =d3 d2 d1 d0
Vout = { 8 + 4 + 0 + 0 }
= 12 V > input voltage
Here now, the unknown analog input voltage V A is lower than the equivalent digital voltage V D.
As discussed in step (2), the second MSB is set to 0 and next MSB set to 1 as
Step: 3
Let, input to DAC = 1010 =d3 d2 d1 d0
So, Vout = { 8 + 0 + 2 + 0 }
= 10V < input voltage
Again as discussed in step (2) VA>VD, hence the third MSB is retained to 1 and the last bit is set
to 1.
Step: 4

Prepared By: Damodar Bhandari United Technical College Comp II sem

Let, input to DAC = 1011 =d3 d2 d1 d0
Vout = {8+0+2+1}
= 11V = input voltage
Now finally VA = VD, and the conversion stops. So,
The nearest digital input is 1011.

Q) Find digital output of 4 bit SAR if input is 3.2V and reference voltage is 5 V.
Solution:

For 4 bit,
Vout =
Step: 1
Let, input to DAC = 1000 =d 3 d2 d1 d0 Vout
= {8+0+0+0}
= 2.5V < input Voltage (3.2V)
So, set d3 and set d2
Step: 2
So, Let, input to DAC = 1100 =d3 d2 d1 d0
Vout = +4+0+0}

= 3.75V < input voltage (3.2V)

So, reset d2 and set d1
Step: 3
Let, input to DAC = 1010 =d3 d2 d1 d0

So, Vout = +2+0}

= 3.125V < input voltage (3.2V)
So, set d1 and set d0
Step: 4
Let, input to DAC = 1011 =d3 d2 d1 d0
Vout =
= 3.4375V < input voltage (3.2V)
So, reset d0.
Step: 5
Let, input to DAC = 1010 =d3 d2 d1 d0
So, Vout = +2+0}
= 3.125V < input voltage (3.2V)
Repeted,
So, The nearest digital input is 1010.

Q) Find the digital output from the SAR if the input voltage is 11.1 V.

d) Dual Slope ADC/integrating ADC

In this type of ADC converter, comparison voltage is generated by using an integrator circuit
which is formed by a resistor, capacitor, and operational amplifier combination. By the set value
of Vref, this integrator generates a saw tooth waveform on its output from zero to the value Vref.
When the integrator waveform is started correspondingly counter starts counting from 0 to 2 n-1
where n is the number of bits of ADC.

Fig: Dual Slope Analog to Digital Converter

When the input voltage Vin equal to the voltage of the waveform, then the control circuit captures
the counter value which is the digital value of the corresponding analog input value.
This Dual slope ADC is a relatively medium cost and slow speed device.

e) Introduction to Delta-Sigma ADC

A newer ADC design is the delta-sigma ADC (or delta converter), which takes advantage of DSP
technology in order to improve amplitude axis resolution and reduce the high-frequency
quantization noise inherent in SAR designs.
The complex and powerful design of delta-sigma ADCs makes them ideal for dynamic
applications that require as much amplitude axis resolution as possible. This is why they are
commonly found in audio, sound, and vibration, and a wide range of high-end data acquisition
applications. They are also used extensively in precision industrial measurement applications.

Fig: Typical Delta-Sigma ADC block diagram

A low-pass filter implemented in a DSP eliminates virtually quantization noise, resulting in
excellent signal-to-noise performance.
Delta-sigma ADCs work by over-sampling the signals far higher than the selected sample rate.
The DSP then creates a high-resolution data stream from this over-sampled data at the rate that
the user has selected. This over-sampling can be up to hundreds of times higher than the selected
sample rate. This approach creates a very high-resolution data stream (24-bits is common) and
has the advantage of allowing multistage anti-aliasing filtering (AAF), making it virtually
impossible to digitize false signals. However, it does impose a kind of speed limit, so delta-
sigma ADCs are typically not as fast as SAR ADCs, for example.
Pros
• High-resolution output (24-bits)
• Over-sampling reduces quantization noise
• Inherent Anti-aliasing filtering
Cons
• Limited to around 200 kS/s sample rate
• Do not handle unnatural shape waveforms as well as SAR
Digital to Analog Converter
Digital to Analog Converter (DAC) is an integrated circuit that converts digital signal to analog
voltage/current which is necessary for further Analog Signal Processing. DAC basically converts
digital code that represents digital value to analog current or voltage. Fig. shows a block diagram
of DAC circuit which shows 8-bit digital inputs converted to Analog Signal.

Unmute

Working of DAC
The digital binary data exists in the form of bits. Each bit is either 1 or 0 & they represent its
weight corresponding to its position. The weight is 2 n where the n is the position of the bit from
right hand side & it start from 0.
Bit Weight = 2n
Bit weight of 4th bit from left= 2n = 23 = 8
The bit weight is multiplied by the bit value. Since the bit could be either 0 or 1, it means;
Bit value of 1 x bit weight = 1 x 2n = 2n
Bit value of 0 x bit weight = 0 x 2(n-1) = 0
Let,
Consider n bit digital number = a b c d = d3 d2 d1 d0
 d3 = MSB
d1 = LSB
Its respective analog value = d020 + d121 +d222+ ………..+dn-12n-1
Suppose a four bit system having full range E R volts, then for different combination of digital
input, analog voltage is given by
Digital Input Analog Value
1000 ER/2 i.e. range of MSB= ER/2
0100 ER/22
0010 ER/23
1000 ER/24 i.e. range of LSB= ER/2n
Full scale o/p voltage is given by
E
E0 = ER { 1*2-1 + 1*2-2 + 1*2-3 + 1*2-4 }
E0 = ER { d3*2-1 + d2*2-2 + d1*2-3 + d0*2-4 }

E
Thus, for n bit
E +…………+ d1*21 + d0*20 }
This is how the digital to analog converter DAC works by adding the weights of all corresponding
bits with its value to generate the analog value at its output.

Types of Digital to Analog Converter

• Binary Weighted Resistor D/A Converter Circuit
• Binary ladder or R–2R ladder D/A Converter Circuit
• Segmented DAC
• Delta-Sigma DAC

a) Weighted Resistor DAC

DAC converts binary or non-binary numbers and codes into analog ones with its output voltage
(or current) being proportional to the value of its digital input number.
The circuit diagram of a 3-bit binary weighted resistor DAC is shown in the following
figure
The digital switches shown in the above figure will be connected to ground, when the
corresponding input bits are equal to ‗0‘. Similarly, the digital switches shown in the above
figure will be connected to the negative reference voltage, −VR when the corresponding input
bits are equal to ‗1‘.
In the above circuit, the non-inverting input terminal of an op-amp is connected to ground. That
means zero volts is applied at the non-inverting input terminal of op-amp. V+ = 0
According to the virtual short
concept, V- = 0 Apply kcl,

Or,-

Or, -

Or,
So, Vo =

So, for n bit converter the output is given as

Vo = + ……………+ d121 + d020 }

Diagram of Weighted Resistor Digital to Analog Converter for 3 bit

Drawbacks
• The binary-weighted DAC has quite a large gap between LSB and MSB resistors values
and requires a very precise value of resistors.
• It becomes impractical for higher-order DACs and is suitable for less resolution DACs.
• The stability of the device is resistor-dependent and is difficult to maintain an accurate
resistance ratio with temperature variations.

b) R-2R Ladder DAC

The R-2R Ladder DAC overcomes the disadvantages of a binary weighted resistor DAC. As the
name suggests, R-2R Ladder DAC produces an analog output, which is almost equal to the
digital (binary) input by using a R-2R ladder network in the inverting adder circuit. The circuit
diagram of a 4-bit R-2R Ladder DAC is shown in the following figure
We know that the bits of a binary number can have only one of the two values. i.e., either 0 or 1.
Let the 4-bit binary input is d3d2d1d0. Here, the bits d3 and d0 denote the Most Significant Bit
(MSB) and Least Significant Bit (LSB) respectively.
Let us assume all the bits are high then,
Apply kcl we get

For n bit converter

+ ……………+ d121 + d020 }
Advantages:
• Only two resistor values are used in R-2R ladder type.
• It does not need as precision resistors as Binary weighted DACs.
• It is cheap and easy to manufacture. Disadvantages:
• It has slower conversion rate.

c) PWM Type DAC

• It is another method used in digital to analog converter & microcontrollers such as
Arduino can be easily programed to utilize its PWM function to generate an analog
output.
• Pulse Width Modulation or PWM is a method of varying the average power of a signal
by varying its duty cycle. The duty is the % turn on time of the signal, the % amount of
time for which the signal remains high. Like 40% duty cycle signal means it stays high
for 40% of time & stays low for 60%.
• We can use a binary number to generate such type signal whose duty cycle depends on
the binary digit. The PWM wave is the filtered using a low pass filter to remove the
fluctuations & provide a smooth analog voltage.
 • The low pass filter used can be a first order. 2nd order low pass filter would be a great
choice for a PWM base digital to analog converter.

Probes and Connectors

Probes and connectors are essential tools used in various fields, including electronics, electrical
engineering, and telecommunications, to facilitate accurate measurements, signal transmission,
and device connectivity. . A probe is a device that makes a physical and electrical connection
between the oscilloscope and test point. Probes are vital to oscilloscope measurements.
Depending on your measurement needs, this connection can be made with something as simple
as a length of wire or with something as sophisticated as an active differential probe.

a) Test Leads: (Twisted pair unshielded test leads): Twisted pair unshielded test leads are
commonly used in electrical testing and measurements. They consist of two insulated wires
twisted together to reduce electromagnetic interference. These test leads are suitable for
low voltage measurements and are often used in general-purpose applications.
b) Shielded Cables: Shielded cables are designed to minimize electromagnetic interference
(EMI) and radio frequency interference (RFI). They have an additional metallic layer,
called a shield, surrounding the inner conductors. The shield helps to protect the signals
from external interference, ensuring accurate and reliable measurements. Shielded cables
are commonly used in high-frequency applications or environments with high levels of
electrical noise.
c) Connectors: Connectors are used to establish a physical and electrical connection between
different components or devices. In the context of probes and measurements, connectors
 play a crucial role in connecting the test leads or cables to the measuring instrument or the
device under test. Common types of connectors used in this context include banana plugs,
BNC (Bayonet Neill–Concelman) connectors, and coaxial connectors.
d) Low Capacitive Probes: Low capacitive probes are designed to minimize the capacitance
between the probe and the circuit under test. Capacitance can affect high-frequency
measurements by introducing additional impedance or altering the frequency response.
Low capacitive probes are used in applications where accurate and high-frequency
measurements are required.
e) High Voltage Probes: High voltage probes are specifically designed to measure high
voltages safely. They feature special insulation and shielding to protect the user and the
measuring instrument from potential electrical hazards. High voltage probes typically have
a higher voltage rating and larger physical size compared to regular probes, enabling them
to handle higher voltage levels without compromising safety.
f) Current Probes: Current probes, also known as current clamps or current transformers, are
used to measure electric current without the need for breaking the circuit or inserting a
series resistor. They work based on the principle of electromagnetic induction. Current
probes are typically designed to measure AC or DC currents and come in various forms,
such as clamp-on probes or flexible Rogowski coils. They are commonly used in power
measurements, motor control, and troubleshooting electrical systems.

Numerical
1) A 5 bit converter is used for a voltage range of 0-10 V. Find the weight of MSB and
LSB. Also the exact range of the converter and the error. Find the error a 10 bit
converter is used. Solution.
Range of MSB= ½ * range of converter=1/2 x 1O=5 V.
Range of LSB: (1/25) * range of the converter= (1/25) x 10=0.3125 V,
The exact range of converter
Eo = 1O (1x2-1 + 1 *2-2 + 1 x2-3+ 1*2-4 + 1x2-5) = 9.6875 v
Error = 10-9.6875=0.3125
% error V = (0.3125/10) x 100 = 3.125%
The exact range of the converter when 1O bits are used is :
Eo = 10*(1x2-1 + 1 *2-2 + 1 x2-3+ 1*2-4 + 1x2-5 +1x2-6 + 1 *2-7 + 1 x2-8+ 1*2-9 + 1x2-10) = 9.99 V
Error= 10 - 9.99 = 0.01
% error V = (0.01/10) x 100 = 0.1%

Thus if a large number of bits are used, the error reduces considerably. But the use of a converter
with a large number of bits results in higher cost of the converter itself and also of the system
where it is used. Also, a higher number of bits add to the complexity of the system.

2) An 8-bit digital system is used to convert an analog signal to digital signal for a data
acquisition system. The voltage range for the conversion is 0-10 V. Find the resolution of
the system and the value of the least significant bit.
 soln n=8 so signal converted to 28 = 256 different
levels.
% resolution = = 0.392%
VLSB = 10/28 = 0.039V

Unit V: Wave Analyzers and Digital Instruments (8 hrs)

Wave Analyzer
A wave analyzer is an instrument designed to measure the relative amplitude of signal-frequency
components in a complex or distorted waveform. They provide a graphical representation of
signal amplitudes versus frequency, known as a spectrum. It is also called signal analyzer, since
the terms signal and wave can be interchangeably used frequently. Basic Wave Analyzer
Basic wave analyzer mainly consists of three blocks − the primary detector, full wave rectifier,
and PMMC galvanometer. The block diagram of basic wave analyzer is shown in below figure

The function of each block present in basic wave analyzer is mentioned below.
• Primary Detector − It consists of an LC circuit. We can adjust the values of inductor,
L and capacitor, C in such a way that it allows only the desired harmonic frequency
component that is to be measured.
• Full Wave Rectifier − It converts the AC input into a DC output.
• PMMC Galvanometer − It shows the peak value of the signal, which is obtained at
the output of Full wave rectifier.
We will get the corresponding circuit diagram, just by replacing each block with the respective
component(s) in above block diagram of basic wave analyzer. So, the circuit diagram of basic
wave analyzer will look like as shown in the following figure −

This basic wave analyzer can be used for analyzing each and every harmonic frequency
component of a periodic signal.
Types of Wave Analyzers
Wave analyzers can be classified into the following two types.
• Frequency Selective Wave Analyzer
• Super heterodyne Wave Analyzer
Now, let us discuss about these two wave analyzers one by one.
a) Frequency Selective Wave Analyzer
The wave analyzer, used for analyzing the signals are of AF range (audible frequency range
20Hz to 20 KHz) is called frequency selective wave analyzer. The block diagram of frequency
selective wave analyzer is shown in below.

Frequency selective wave analyzer consists a set of blocks whose function is mentioned below.
• Input Attenuator: The AF signal, which is to be analyzed is applied to input attenuator.
If the signal amplitude is too large, then it can be attenuated by input attenuator.
• Driver Amplifier: It amplifies the received signal whenever necessary.
• High Q-filter: It is used to select the desired frequency and reject unwanted frequencies.
It consists of two RC sections and two filter amplifiers & all these are cascaded with each
other. We can vary the capacitance values for changing the range of frequencies in
powers of 10. Similarly, we can vary the resistance values in order to change the
frequency within a selected range.
• Meter Range Attenuator: It gets the selected AF signal as an input & produces an
attenuated output, whenever required.
• Output Amplifier: It amplifies the selected AF signal if necessary.
• Output Buffer: It is used to provide the selected AF signal to output devices.
• Meter Circuit: It displays the reading of selected AF signal. We can choose the meter
reading in volt range or decibel range.

b) Super heterodyne Wave Analyzer

The wave analyzer, used to analyze the signals of RF range is called superheterodyne wave
analyzer. The following figure shows the block diagram of superheterodyne wave analyzer.
The working of superheterodyne wave analyzer is mentioned below.
• The RF signal, which is to be analyzed is applied to the input attenuator. If the signal
amplitude is too large, then it can be attenuated by input attenuator.
• Untuned amplifier amplifies the RF signal whenever necessary and it is applied to first
mixer.
• The frequency ranges of RF signal & output of Local oscillator are 0-18 MHz & 30-48
MHz respectively. So, first mixer produces an output, which has frequency of 30 MHz.
This is the difference of frequencies of the two signals that are applied to it.
• IF amplifier amplifies the Intermediate Frequency (IF) signal, i.e. the output of first
mixer. The amplified IF signal is applied to second mixer.
• The frequencies of amplified IF signal & output of Crystal oscillator are same and equal to
30MHz. So, the second mixer produces an output, which has frequency of 0 Hz. This is
the difference of frequencies of the two signals that are applied to it.
• The cut off frequency of Active Low Pass Filter (LPF) is chosen as 1500 Hz. Hence, this
filter allows the output signal of second mixer.
• Meter Circuit displays the reading of RF signal. We can choose the meter reading in volt
range or decibel range.
So, we can choose a particular wave analyzer based on the frequency range of the signal that is to
be analyzed.

Spectrum Analyzer
A spectrum analyzer is a device that measures and displays signal amplitude (strength) as it
varies by frequency within its frequency range (spectrum). The frequency appears on the
horizontal (X) axis, and the amplitude is displayed on the vertical (Y) axis. It looks like an
oscilloscope, and in fact, some devices can function as either oscilloscopes or spectrum
analyzers.
Types of Spectrum Analyzers
These instruments provide a display of the frequency spectrum over a given frequency band.
Spectrum analyzers use either a parallel filter bank or a swept frequency technique. So, We
can classify the spectrum analyzers into the following two types.
• Filter Bank Spectrum Analyzer
• Swept-tuned or superheterodyne Spectrum Analyzer
Basic Spectrum Analyzer using Swept Receiver Design
The spectrum analyzer, used for analyzing the signals are of RF range is called superheterodyne
spectrum analyzer. Its block diagram is shown in below figure.

The working of superheterodyne spectrum analyzer is mentioned below.

• The saw tooth generator provides the saw tooth voltage which drives the horizontal axis
element of the scope and this saw tooth voltage is frequency controlled element of the
voltage tuned oscillator. As the oscillator sweeps from f min to fmax of its frequency band at a
linear recurring rate, it beats with the frequency component of the input signal and produce
an IF, whenever a frequency component is met during its sweep.
• The RF signal, which is to be analyzed is applied to input attenuator. If the signal
amplitude is too large, then it can be attenuated by an input attenuator.
• Low Pass Filter (LPF) allows only the frequency components that are less than the cutoff
frequency.
• Mixer gets the inputs from Low pass filter and voltage tuned oscillator. It produces an
output, which is the difference of frequencies of the two signals that are applied to it.
 • IF amplifier amplifies the Intermediate Frequency (IF) signal, i.e. the output of mixer.
The amplified IF signal is applied to detector.
The output of detector is given to vertical deflection plate of CRO. So, CRO displays the
frequency spectrum of RF signal on its CRT screen.
So, we can choose a particular spectrum analyzer based on the frequency range of the signal that
is to be analyzed.

IRF Spectrum Analyzer/ parallel filter bank analyzer

An IRF (Intermediate Frequency) spectrum analyzer is a device used to analyze and display the
frequency spectrum of intermediate frequency signals. It is commonly used in communication
systems and electronic equipment for signal analysis and troubleshooting.
In a parallel filter bank analyzer, the frequency range is covered by a series of filters whose
central frequencies and bandwidth are so selected that they overlap each other, The spectrum
analyzer, used for analyzing the signals are of AF range is called filter bank spectrum analyzer,
or real time spectrum analyzer because it shows (displays) any variations in all input
frequencies. The following figure shows the block diagram of filter bank spectrum analyzer.

The working of filter bank spectrum analyzer is mentioned below.

• It has a set of band pass filters and each one is designed for allowing a specific band
ofrequencies. The output of each band pass filter is given to a corresponding detector.
• All the detector outputs are connected to Electronic switch. This switch allows the
detector outputs sequentially to the vertical deflection plate of CRO. So, CRO displays the
frequency spectrum of AF signal on its CRT screen.

Distortion Analyzer: Harmonic Distortion Analyzer-Fundamental

Suppression Type
Fundamental Suppression Type:
• Distortion analyzer measures the total harmonic power present in the test wave rather than
the distortion caused by each component
• The simplest method to suppress the fundamental frequency by means of a high pass filter
whose cut-off frequency is a little above the fundamental frequency
• Thus, the high pass filter allows only the harmonics to pass and the total harmonic
distortion (THD) can then be measured
• The most commonly used harmonic distortion analyzers based on fundamental
suppression are as follow:
(i) Employing a Resonance Bridge,
(ii) Wien's Bridge Method
(iii) Bridged T -Network Method
i) Employing a Resonance Bridge:

 The bridge is balanced for the fundamental frequency, i.e. L and C are tuned to the
fundamental frequency.
 The bridge is unbalanced for the harmonics, i.e. only harmonic power will be available at
the output terminal and can be measured.
 If the fundamental frequency is changed, the bridge must be balanced again.
 If L and C are fixed components, then this method is suitable only when the test wave has
a fixed frequency.
 Indicators can be thermocouples or square law VTVMs. This indicates the RMS value of
all harmonics.
 When a continuous adjustment of the fundamental frequency is desired, a Wien bridge
arrangement is used.

ii) Wien’s Bridge Method:

 The bridge is balanced for the fundamental frequency.

 The fundamental energy is dissipated in the bridge circuit elements.
 Only the harmonic components reach the output terminals.
 The harmonic distortion output can then be measured with a meter.
 For balance at the fundamental frequency, C1 = C2= C
 R 1 = R2 = R R 3
= 2R4.

iii) Bridged T-Network Method:

 L and C‘s are tuned to the fundamental frequency, and R is adjusted to bypass fundamental
frequency.
 The tank circuit being tuned to the fundamental frequency, the fundamental energy will
circulate in the tank and is bypassed by the resistance.
 Only harmonic components will reach the output terminals and the distorted output can be
measured by the meter.
 The Q of the resonant circuit must be at least 3-5.

 The switch S is first connected to point A so that the attenuator is excluded and the bridge
T-network is adjusted for full suppression of the fundamental frequency, i.e. minimum
output.
 Minimum output indicates that the bridged T-network is tuned to the fundamental
frequency and that the fundamental frequency is fully suppressed.
 The switch is next connected to terminal B, i.e. the bridged T-network is excluded.
Attenuation is adjusted until the same reading is obtained on the meter. The attenuator
reading indicates the total RMS distortion.
 Distortion measurement can also be obtained by means of a wave analyzer, knowing the
amplitude and the frequency of each component, the Harmonic Distortion Analyzer can be
calculated.
 However, distortion meters based on fundamental suppression are simpler to design and
less expensive than wave analyzers.
 The disadvantage is that they give only the total distortion and not the amplitude of
individual distortion components.
Measurements of Frequency and Time: Decimal Count Assembles
Frequency Counter
A frequency counter is an electronic instrument used to measure frequency and time. Frequency
counters are used for a wide range of frequency and time measurements and display many digits
of accuracy. Frequency counter helps measure the time of reputed digital signals and the
frequency correctly and associates with a wide range of radio frequencies.
In simple words, these are essential instruments that count the number of cycles per second of an
input signal.
These are widely used in electronics and telecommunication industries to measure frequency,
bandwidth, peak-to-peak voltage, or current or rise time.
Frequency counters count the pulses and transfers them into the frequency counter when the
number of pulses or events occurs in a period and displays it on the frequency range of
vibrations.
The counter then sets to zero. Frequency counters are often found in-built into other devices,
such as radio receivers, radar sets, and test equipment. It is a device that is easy to use, measures
the frequency accurately, and displays it digitally.

Block Diagram and working

The frequency counter block diagram contains input signal, input conditioning, and threshold,
AND gate, counter or latch, accurate time base or clock, decade dividers, flip-flop, and display.

Fig: Frequency Counter Block Diagram

 • A frequency counter measures a signal in the first split into the pulse setting. It operates
by counting the number of times the signal passes through the voltage point to a trigger
point in a duration.
• The trigger of frequency counters starts at zero crossing point automatically. It is a device
that sets in a clock speed with pulse per unit cycle and the pulses present send to the
device for a limited time.
• After this, vibrations/Pulses apply in a definite interval of time, counts the Pulses.
• An electric counter does the whole process, and the pulses are sent to the cycle to
represent the unidentified Signal and give it a value.
• The frequency counter works on two modes to generate the pulses and time delay.
• When we talk about the working of the frequency counter, then the pulse in this device
generates from the wave generator and microcontrollers. The timer in this device figures
as a counter.
• It sets the count of the Pulses from high and low. The final count of the pulses takes
place. It later collects in timer one, and it denotes the frequency of the vibrations by
calculating it.
• The device that converts the resultant value by multiplying it by ten frequency cycles per
second converts the value of the pulses in Hz. After the whole calculation inside the
frequency counter, the frequency of the pulses becomes visible on the LCD or LED.
Types of Frequency Counters:
Let us know more about the types of Frequency counters here:
1. Bench Frequency Counter:
Bench Frequency Counter is a type of device that is useful in applying electronic test equipment.
It helps measure the period and equal frequency precisely.
Bench frequency counter counts CPU signals, and it also provides a constant compensation for
temperature change.
The device is also known for reducing measurement errors that generally happen due to
temperature drift. It is also helpful in measuring the frequency of a Periodic electrical signal, and
these devices are beneficial for electronic labs and electronic projects.
2. PXI Frequency Counter:
A PXI frequency counter is helpful in the control system and track system for tests. It is a
valuable device in measuring the frequency and phase of an input signal as per the reference
signal.
Its application is mainly in audio, video, and RF signal. This type of frequency counter is known
for implementing standalone devices or integration with other instruments such as spectrum
analyzers and Oscilloscopes. Some of the different application of this device is testing
microwave circuit, wireless devices, and Antennas.
3. Handheld Frequency Counter:
A handheld frequency counter helps measure the frequency of cycles per second of a Periodic
waveform in the signal. Along with that, it also measures the period and time interval between
two events in the waveform.
It provides precise measurement and output. The application of this device helps measure the
radio frequencies or any other repeating signal such as audio signals, clock frequency, etc. It
represents the cycle per unit number in Hz.
4. Panel Meter:
A panel meter is a type of frequency counter available in panel mount mode. It has application in
determining the frequency of audio and radio signals.
It helps incorporate items with different kinds of equipment to count the time intervals and
frequency.
Compared to the other types of frequency count, two parameters are cheaper and valuable for
measuring the frequency of a signal in Hz.

Period Counter
Error: Counter Error and Signal Related Error
• A counter error typically indicates a problem or discrepancy in the frequency or time
measurement performed by the wave analyzer's counter. The counter is responsible for
counting the number of cycles or events within a given time period.
• A counter error might occur if there are issues with the counter circuitry, calibration, or if
the signal being measured is outside the counter's measurement range. In such cases, the
accuracy and reliability of the frequency or time measurements may be compromised.
• A signal-related error generally refers to an error or issue related to the input signal being
analyzed by the wave analyzer. This could include various factors, such as noise,
distortion, interference, or improper signal conditioning. Signal-related errors can affect
the accuracy of the measurements or distort the waveform being analyzed, leading to
incorrect or unreliable results.

Digital Voltmeter
Digital Voltmeter displays the voltage readings of a circuit numerically which is used to measure
the electrical potential difference between two points in a circuit. The below picture shows the
block diagram of the digital voltmeter.

The working of DVM is explained as follows:

1. Unknown voltage signal is fed to the pulse generator which generates a pulse whose
width is proportional to the input signal.
2. Output of pulse generator is fed to one leg of the AND gate.
3. The input signal to the other leg of the AND gate is a train of pulses.
 4. Output of AND gate is positive triggered train of duration same as the width of the
pulse generated by the pulse generator.
5. This positive triggered train is fed to the inverter which converts it into a negative
triggered train.
6. Output of the inverter is fed to a counter which counts the number of triggers in the
duration which is proportional to the input signal i.e. voltage under measurement.
7. Thus, counter can be calibrated to indicate voltage in volts directly.
The working of digital voltmeter that it is nothing but an analog to digital converter which
converts an analog signal into a train of pulses, the number of which is proportional to the input
signal. So a digital voltmeter can be made by using any one of the A/D conversion methods.

On the basis of A/D conversion method used digital voltmeters can be classified as:
• Ramp type digital voltmeter
• Integrating type voltmeter
• Potentiometric type digital voltmeters
• Successive approximation type digital voltmeter

Ramp type digital voltmeter

In a ramp type DVM, the operation basically depends on the measurement of time. The time
which a ramp voltage takes to change from the level of the input voltage to that of 0 voltage or
vice versa. An electronic time interval counter is used to measure the time interval and the count
is displayed in digits as voltmeter output.
Let us have look at the block diagram and operating principle of a ramp-type DVM.

Here, as we can see in the figure below a negative going ramp voltage is shown. This ramp
voltage is compared with the unknown voltage. An input comparator employed in the circuit
generates a pulse when ramp voltage becomes equal to the voltage under measurement.
 Now, the ramp voltage falls to reach 0 value. The ground comparator employed in the circuit
generates stop pulse. This stop pulse closes the gate.
The gate opening time duration is proportional to the value of input voltage. The sample
rate Multivibrator employed here is used to find the rate by which the measurement cycle
begins.

Integrating type digital voltmeter

In this category of Digital Voltmeter, the true value of input voltage is measured over a fixed
measuring time.
Here, an integration technique is employed that uses voltage to frequency conversion. This
voltage to frequency converter act as a feedback control system. This basically governs the
pulse generation rate is proportional to the magnitude of applied input voltage.

In voltage to frequency conversion technique, a train of pulses is generated. The frequency of

these pulses depends on the voltage being measured.
Then these pulses are counted that appears in a definite time interval. After all, the frequency of
pulses is a function of input voltage, the number of pulses is an indication of the input voltage.
Servo Potentiometer type digital Voltmeter
A potentiometric type of DVM employs voltage comparison technique. In this DVM the
unknown voltage is compared with reference voltage whose value is fixed by the setting of the
calibrated potentiometer.
The potentiometer setting is changed to obtain balance (i.e. null conditions).
When null conditions are obtained the value of the unknown voltage, is indicated by the dial
setting of the potentiometer.
In potentiometric type DVMs, the balance is not obtained manually but is arrived at
automatically.
Thus, this DVM is in fact a self- balancing potentiometer.
The potentiometric DVM is provided with a readout which displays the voltage being measured.

Successive Approximation type digital Voltmeter

In this category of DVM, the ADC employed makes use of successive approximation
converter. Thus it is named as so. These are capable of 1000 readings per second.

In the beginning, a start pulse is applied at the start/stop Multivibrator. Due to this, MSB of the
control register is set to high and all other bits to low. So, for an 8-bit control register, the
reading would be 10000000.
Thus causing the output of DAC to be half of the reference voltage.
Now, the comparator compares the output of the converter from the input voltage and produces
an output that will cause the control register to retain 1 in its MSB.
The ring converter employed in the circuit advances one count next thus shifting a 1 in the
second. This will cause the MSB of the control register and its reading to be 11000000. Thus
DAC increases its reference by one increment and another comparison of input voltage with that
of reference takes place. In this way through successive approximation the measurement cycle
proceeds. On reaching the last count, the measurement cycle stops.
The output in digital format at the control register shows the final approximation of input voltage

Vector Voltmeter
A vector voltmeter is a specialized instrument used for measuring complex voltage quantities,
particularly in RF (Radio Frequency) and microwave systems. It is designed to measure both the
magnitude and phase of a complex voltage or vector quantity. nlike a standard voltmeter that
measures only the amplitude of a voltage signal, a vector voltmeter provides additional
information about the phase or angle of the voltage. This is particularly useful in applications
where the phase relationship between multiple voltage signals is critical, such as in RF signal
analysis, network analysis, or impedance measurements.
The vector voltmeter typically consists of two channels: an in-phase channel and a quadrature
channel. The in-phase channel measures the real or resistive component of the voltage, while the
quadrature channel measures the imaginary or reactive component of the voltage.
By processing the measurements from both channels, the vector voltmeter can calculate the
magnitude and phase angle of the complex voltage. It can display these values in various
formats, such as polar coordinates (magnitude and angle) or rectangular coordinates (real and
imaginary components).
Vector voltmeters are commonly used in RF and microwave engineering for tasks like
impedance matching, network analysis, phase measurements, and evaluating the performance of
RF systems. They are especially valuable for characterizing signals with multiple components or
for measuring the properties of devices operating in the frequency domain.

Digital Multimeter
A digital multimeter is a test tool used to measure two or more electrical values—principally
voltage (volts), current (amps) and resistance (ohms). It is a standard diagnostic tool for
technicians in the electrical/electronic industries.
Digital multimeters combine the testing capabilities of single-task meters—the voltmeter (for
measuring volts), ammeter (amps) and ohmmeter (ohms). Often, they include several additional
specialized features or advanced options. Technicians with specific needs, therefore, can seek out
a model targeted to meet their needs.
The face of a multimeter typically includes four components:
• Display: Where measurement readouts can be viewed.
• Buttons: For selecting various functions; the options vary by model.
• Dial (or rotary switch): For selecting primary measurement values (volts, amps, ohms). 
Input jacks: Where test leads are inserted.
Test leads are flexible, insulated wires (red for positive, black for negative) that plug into the
DMM. They serve as the conductor from the item being tested to the multimeter. The probe tips
on each lead are used for testing circuits.

The terms counts and digits are used to describe a digital multimeter's resolution—how fine a
measurement a meter can make. By knowing a multimeter's resolution, a technician can
determine if it is possible to see a small change in a measured signal.

Computer Based Digital Instruments: IEEE 488 GPIB Instrument

Unit VI: Recorders, Displays and Storage Devices (5 hrs)

FUNDAMENTALS OF CATHODE RAY OSCILLOSCOPE
An oscilloscope is a laboratory instrument commonly used to display and analyze the waveform
of electronic signals. In effect, the device draws a graph of the instantaneous signal voltage as a
function of time. Oscopes are often used when designing, manufacturing or repairing electronic
equipment. Engineers use an oscilloscope to measure electrical phenomena and solve
measurement challenges quickly and accurately to verify their designs or confirm that a sensor is
working properly.
There are three primary oscilloscope systems: vertical, horizontal and trigger systems. Together,
these systems provide information about the electrical signal, so the oscilloscope can accurately
reconstruct it. The picture below shows the block diagram of an oscilloscope.
The first stage attenuates or amplifies the signal voltage in order to optimize the amplitude of the
signal; this is referred to as the vertical system since it depends on the vertical scale control.
Then the signal reaches the acquisition block, where the analog-to-digital converter (ADC) is
used to sample the signal voltage and convert it in a digital format value. The horizontal system,
which contains a sample clock, gives each voltage sample a precise time (horizontal) coordinate.
The sample clock drives the ADC and its digital output is stored in the acquisition memory as a
record point. The trigger system detects a user-specified condition in the incoming signal stream
and applies it as a time reference in the waveform record. The event that met the trigger criteria
is displayed, as is the waveform data preceding or following the event

BLOCK DIAGRAM OF OSCILLOSCOPE:

The major block circuit of general purpose CRO is as follows:

1) CRT

2) Vertical Line

3) Delay line

4) Horizontal amplifier

5) Time base generator

6) Trigger circuit

7) Power Supply
 Fig: Block Diagram of oscilloscope

The description is below:

1) Cathode Ray Tube (CRT):

A cathode ray oscilloscope consists of a cathode ray tube (CRT) which is the heart of the
oscilloscope, and some additional circuitry to operate the CRT. The main parts of a CRT are:
a)Electron gun assembly. b)Deflection plate assembly c)Fluorescent screen d)Glass envelope
The electron gun assembly produces a sharply focussed beam of electrons which are accelerated
to high velocity. This focussed beam of electrons strikes the fluorescent screen with sufficient
energy to cause a luminous spot on the screen.

Fig: Internal Structure of CRT

After leaving the electron gun, the electron beam passes through two pairs of "electrostatic
deflection plates". Voltages applied to these plates deflect the beam. Voltages applied to one pair
of plates move the beam vertically up and down and the voltages applied to the other pair of
plates move the beam horizontally from one side to another. Focusing anode is used to focus the
beam on the screen, and the accelerating anode makes the electron beam to move with high
velocity.

2) Vertical Amplifier:
This is a wide band amplifier used to amplify signals in the vertical section.

The vertical amplifier consists of several stages, with fixed overall sensitivity or gain expressed
in v/divisions. The advantage of fixed gain is that the amplifier can be more easily designed to
meet the requirements of stability and between the vertical amplifiers is kept within its signal
handling capability by proper selection of the input attenuator switch. The first element of the
pre-amplifier is the input stage, often consisting of a FET source follower whose high input
impedance isolates from the attenuator. This FET input stage is followed by a BJT emitter
followers to match the medium impedance of FET output with the low impedance input of the
phase inverter. The phase inverter provides two anti-phase output signals which are required to
operate the push pull output amplifier. The push pull output stage delivers equal signal voltages
of opposite polarity to the vertical plates of the CRT. The advantages of push pull operation on
CRO are similar to those obtained from push pull operation. In addition a number of focusing and
non-linear effects are reduced, because neither plate is ground potential.

3) Horizontal Amplifier:

The horizontal basically serves two purposes:

a) When the oscilloscope is being in the X-Y node, the signal applied to the horizontal input
terminal will be amplified by the horizontal amplifier.

b) When the oscilloscope is being used in the ordinary mode of operation to display a signal
applied to the vertical input, the horizontal amplifier will amplify the sweep generator output.
4) Delay Line:

It is used to delay the signal from some time in the vertical section. Comparing the vertical and
horizontal deflection circuits in the oscilloscope block diagram, we observe that the deflection
signal is initiated or triggered, by a portion of the output signal applied to the vertical CRT
plates. Signal processing in the horizontal channel consists of generating and shaping a trigger
pulse that starts the sweep generator, whose output is fed to the horizontal deflection plates. This
whole process takes time on the order of 80 ns. To allow the operator to observe the leading edge
of the signal waveform, the signal drive for the vertical CRT plates must therefore be delayed by
atleast the same amount of time. This is the function of time delay line.

CRO PROBES
The CRO probe performs the very important function of connecting the test circuit to the
oscilloscope without altering, loading or otherwise disturbing the test circuit. There are three
different probes:

a)Direct reading probe,

b)circuit isolation probe,

c)detector probe. They

are discussed below:

a) Direct reading probe This probe is the simplest of all probes and it uses a shielded coaxial
cable. It avoids stray pickups which may lead to problems when low level signals are being
measured. It is used usually for low frequency and low impedance circuits. However in using the
shielded probe, the shunt capacitance of the probe is added to the input impedance and capacity
of the scope and acts to lower the response of the oscilloscope to high impedance and high
frequency circuits.

b) Isolation probe Isolation probe is used in order to avoid the undesirable circuit loading effects
of shielded probe. The isolation probe which is used along with the capacitive voltage divider,
decreases the input capacitance and increases the input resistance of the oscilloscope. This way
the loading effects are drastically reduced. c) Detector probe When analyzing the response to
modulated signals in communication equipment like AM, FM and TV receivers, the detector
probe functions to separate the lower frequency modulation component from the higher
frequency carrier. The amplitude of the modulator carrier (which is proportional to the response
of the receiver to the much high frequency carrier signal) is displayed on the oscilloscope by
rectifying and bypassing action. This permits an oscilloscope capable of audio-frequency
response to perform signal tracing tests on communication signals in the range of hundreds of
Mhz, a range which is beyond the capabilities of all oscilloscopes except the highly specialized
ones.
Specification of an Oscilloscope
Bandwidth
Probably the most important specification of an oscilloscope is its bandwidth. The bandwidth of
the the scope governs the maximum frequency of the signal that it can capture and analyse. As
the frequency of the signal gets closer to the maximum frequency that the oscilloscope can work
with, its accuracy drops.
Sample rate
The next important specification of an oscilloscope is its sample rate. The sample rate is the
number of samples that the oscilloscope is capable of capturing per second. Obviously, the more,
the better. But higher sample rates require more and faster memory to store, and faster
electronics and processor to capture and process, driving up the price of the instrument.
Memory size/depth
Very closely related to the oscilloscope sample rate is its memory size. As the oscilloscope
samples the signal from the test circuit, it stores the waveform data in its memory.
Manufacturers report the memory size of their oscilloscopes using the term ―memory depth‖, and
instead of using the regular byte unit, they use the ―points‖ unit.
Rise time
The Rise time of an oscilloscope describes the ability of the instrument to detect and capture
rapidly rising and falling signals. This is particularly important when we work with square waves
that have very sharp edges. A square wave can rise from 0V to 5V within nanoseconds.
Channels
Oscilloscopes typically offer 2 or 4 channels.
Each channel has a separate connector where you can attach a probe, and through this probe to
monitor a signal. Trigger
The trigger of an oscilloscope is fundamental to its operation. The trigger is the mechanism
through which the oscilloscope can recognise a specific attribute of the input signal. Based on
this attribute, the oscilloscope can achieve synchronisation.
The trigger is the mechanism through which the oscilloscope can recognise a specific attribute of
the input signal.
 6.7.1 Oscilloscope measurement Techniques
6.8 Special Oscilloscopes – Storage Oscilloscope, Sampling Oscilloscope

Recorders Basic recording systems.

A recorder is a measuring instrument which records time varying quantity, even after the
quantity or variable to be measured has stopped. The electrical quantities such as voltage &
current are measured directly. The non- electrical quantities are recorded using indirect methods.
The non- electrical quantities are first converted to their equivalent voltages or currents, using
various transducers.
Electronic recorders may be classified as:
A. Analog recorders
a) Graphic recorder
i) Strip chart recorder
• Galvanometer type
• Null type
• Potentiometric recorders
• Bridge recorders
• LVDT recorders ii) Circular chart recorders iii) X-Y
Recorders
b) Magnetic tape recorders
c) Oscillographic recorders
d) Others [hybrid, paperless, ultraviolet and thermal dot matrix recorder]
B. Digital recorders
i. Incremental digital recorders
ii. Synchronous digital recorders

Strip chart recorders. Galvanometer and Potentiometer type recorders

(direct and null type)
A strip chart recorder records physical variable with respect to the independent variable time on
a long paper kept in the form of a roll. The independent variable time (t) then corresponds to the
strip-length axis and the physical variables measured (y) are related to the chart width. Tracings
are obtained by a writing process at sites on the chart short axis (y) corresponding to the physical
variables magnitudes with the strip being moved at constant velocity to generate the time axis.
Strip chart recorders consist of a roll or strip of paper that is passed linearly beneath one or more
pens. As the signal changes, the pens deflect producing the resultant chart. Strip chart recorders
are well suited for recording of continuous processes.

Strip Chart Recorder Components

The following are the components of strip chart recorder.
Pen (Stylus)
Chart Paper
Chart Paper drive mechanism
Event Marker
Signal Conditioning System
Pen: It is used for marking data.
Chart Paper: In the Strip chart recorder, this chart paper is in the form of a strip. This strip is
rolled and rotates under the pen as time passes.
Chart Paper Drive Mechanism: We need to move the chart paper under the pen to store the
data. Users can adjust the speed of the moving paper (chart).
Event Marker: This marker is used to indicate the amount of time it takes to record fluctuations.
Signal Conditioning System: Signal conditioning system consist of Discrete circuits, Filters,
Amplifiers, DAC, ADC etc.

Strip Chart Recorder Working Principle:

The working principle of a strip chart recorder is to record data on a continuous roll of chart
paper moving at a constant speed. The recorder records the difference of one of the more
variables with respect to time.
To reduce noise and interference the recorder‘s input DC signal is first filtered and then
preamplified. This conditioned signal is carried to the servo amplifier, which is constantly
compared to the feedback signal evolved by the servo potentiometer.
The difference between these two signals is a positive or negative error signal, which is used to
drive a servo motor connected to a servo potentiometer in one direction to reduce the error signal
to zero. Since the recorder plate is automatically connected to the servo motor and servo
potentiometer, its position on the graph represents the accuracy of the input signal and the
continuous registration of the graph.

The most commonly used mechanisms employed for making marks on the papers are:
(i) Pen and ink: Marking with ink-filled stylus
(ii) Thermal type: Marking with heated stylus on temperature sensitive paper (e.g. fax
paper)
(iii) Impact type: Marking with pressure sensitive paper (e.g. carbon paper)
(iv) Electrostatic stylus: Marking with charged stylus on plain paper
(v) Optical type: Marking with light ray on photosensitive paper

There are various kinds of strip chart recorders. According to their working principles, these are
divided in mainly two categories. One works on the principle of the galvanometer and other is
called null type.
(a) Galvanometric Type
Galvanometric instruments usually use a d'Arsonval galvanometer as the basic movement. This
galvanometer consists of a moving coil suspended either on pivots or a taut ligament. The coil is
then able to rotate in the field produced by a permanent magnet. When a small current is applied
to the coil, a field is created which reacts with that of the permanent magnet, and the coil rotates.
A control spring in a pivoted instrument and the ligament with a taut suspension provide an
opposing torque. Thus, depending on the current applied, equilibrium will be established.

FIG: Galvanometer type recorder

(b) Potentiometric Type
The self-balancing potentiometer type of instrument consists of a bridge circuit. Across one arm
of the bridge is a reference voltage, and across the other arm is a feedback network. Initially, the
bridge is adjusted so that the servo amplifier and its motor are in balance and stationary. When a
signal is fed to the amplifier, the output causes the servomotor to drive a balancing
potentiometer, which in turn refers a feedback voltage to the amplifier input. When the two
signals are equal and opposite, the system balances and the servomotor stops. If a pen unit is
attached to the motor/potentiometer mechanized drive, at the point of balance, the pen will show
the proportional value of the input signal. As with galvanometric instruments, this principle may
be applied in various ways.
 FIG. Potentiometric type recorder
This kind of recorders having very high input impedance, infinity at balance conditions, and a
high sensitivity.

Magnetic tape and disc recorders

The magnetic tape recorders are used for high frequency signal recording. In these recorders, the
data is recorded in a way that it can be reproduced in electrical form any time. Also main
advantage of these recorders is that the recorded data can be replayed for almost infinite times.
Because of good higher frequency response, these are used in Instrumentation systems
extensively.

Basic Components of Tape Recorder

Following are the basic components of magnetic tape recorder
1. Recording Head
2. Magnetic Tape
3. Reproducing Head
4. Tape Transport Mechanism
5. Conditioning Devices
Recording Head: The construction of the magnetic recording head is similar to Transformer
having a toroidal core with coil when the current used for recording is passed through coil
wound around magnetic core, it produces magnetic flux. When the tape is passing the head, the
flux produced due to recording current gets linked with iron oxide part ices on the magnetic tape
and these particles get magnetized.

This magnetization particle remains as it is, even though the magnetic tape leaves the gap. The
actual recording takes place at the trailing edge of the air gap. Any signal is recorded in the form
of the patterns. These magnetic patterns are dispersed any where along the length of magnetic
tape in accordance with the variation in recording current with respect to time.

Magnetic Tape: The magnetic tape is made of thin sheet of tough and dimensionally stable
plastic ribbon and is wound around a reel. This tape is transferred from one reel to another.
When the tape passes across air gap magnetic pattern is created in accordance with variation of
recording current. To reproduce this pattern, the same tape with some recorded pattern is passed
across another magnetic head in which voltage is induced. This voltage induced is in accordance
with the magnetic pattern.
Reproducing Head: The use of the reproducing head is to get the recorded data played back.
The reproducing head detects the magnetic pattern recorded on the tape. The head converts the
magnetic pattern back to the original electrical signal. In appearance, both recording and
reproducing heads are very much similar.

Tape Transport Mechanism: The tape transport mechanism moves the magnetic tape along the
recording head or reproducing head with a constant speed. The magnetic tape is wound on reel.
There are two reels; one is called as supply & other is called as take-up reel. Both the reels rotate
in same direction.
The transportation of the tape is done by using supply reel and take-up reel. The fast winding of
the tape or the reversing of the tape is done by using special arrangements. The rollers are used
to drive and guide the tape.

Conditioning Devices: These devices consist of amplifiers and fitters to modify signal to be
recorded.

Principle of Tape Recorders

When a magnetic tape is passed through a recording head, the signal to be recorded appears as
some magnetic pattern on the tape. This magnetic pattern is in accordance with the variations of
original recording current.
The recorded signal can be reproduced back by passing the same tape through a reproducing head
where the voltage is induced corresponding to the magnetic pattern on the tape.
When the tape is passed through the reproducing head, the head detects the changes in the
magnetic pattern i.e. magnetization.
The change in magnetization of particles produces change in the reluctance of the magnetic
circuit of the reproducing head, inducing a voltage in its winding.
The induced voltage depends on the direction of magnetization and its magnitude on the tape. The
emf, thus induced is proportional to the rate of change of magnitude of magnetization i.e.
e = N * (dĭ / dt)
Where N = number of turns of the winding on reproducing head
Suppose the signal to be recorded is Vm sinwt. Thus, the current in the recording head and flux
induced will be proportional to this voltage. It is given by
e = k1. Vm sinwt, where k1 = constant.
Above pattern of flux is recorded on the tape. Now, when this tape is passed through the
reproducing head, above pattern is regenerated by inducing voltage in the reproducing head
winding. It is given by e= k2 ǙVm cos wt
Thus the reproducing signal is equal to derivative of input signal & it is proportional to flux
recorded & frequency of recorded signal.

Applications of Magnetic Tape Recorders:

1. Data recording and analysis on missiles, aircraft and satellites.
2. Communications and spying.
3. Recording of stress, vibration and analysis of noise.
Indicators and display Devices - Nixie, LED, LCD and seven segment and dot
matrix displays.
Indicators and display devices play a crucial role in presenting information in various electronic
systems. They are designed to visually communicate data, status, or messages to users. Here
are some common types of indicators and display devices: Nixie Tubes:
A Nixie tube or cold cathode display is an electronic device used for displaying numerals or
other information using glow discharge. Nixie tubes are display devices that were popular in the
mid-20th century. They use neon gas-filled tubes with cathodes shaped like numerals or
symbols. The tube is filled with a gas at low pressure, usually mostly neon and a small amount of
argon, in a Penning mixture. When voltage is applied, a specific cathode lights up, displaying the
corresponding character. Nixie tubes have a vintage aesthetic and emit a warm orange glow.
They are often used in retro-style clocks and other decorative applications.
Although it resembles a vacuum tube in appearance, its operation does not depend on thermionic
emission of electrons from a heated cathode. It is hence a cold-cathode tube (a form of gas-filled
tube), and is a variant of the neon lamp. Such tubes rarely exceed 40 °C (104 °F) even under the
most severe of operating conditions in a room at ambient temperature. Vacuum fluorescent
displays from the same era use completely different technology—they have a heated cathode
together with a control grid and shaped phosphor anodes; Nixies have no heater or control grid,
typically a single anode (in the form of a wire mesh, not to be confused with a control grid), and
shaped bare metal cathodes.

LEDs (Light-Emitting Diodes):

LEDs are semiconductor devices that emit light when an electric current passes through them. It
is basically a p-n junction photodiode when excited at forward-bias condition emits light. They
are small, energy-efficient, and available in various colors. LEDs are widely used in indicators
 and displays due to their versatility. They can be used as individual indicator lights or arranged
in arrays to form alphanumeric characters or graphical displays. LEDs are commonly found in
electronic devices, signage, automotive lighting, and many other applications.

It can be easily interfaced with a simple electronic circuit and is durable and reliable. These
LEDs are often arranged in different formats to display information. Among these, the seven
segments configuration and dot matrix display are very common and widely used. The
sevensegment configuration of an LED arranged in the form of the digit 8 can be restrictive in
that it does not adequately allow the display of some alphanumeric characters. By contrast, the
versatility of a dot-matrix arrangement allows an LED unit to display more complicated shapes.
The following sections discuss the about seven-segment and dot-matrix LED display.

Seven Segment Display

Seven segment displays are the output display device that provides a way to display information
in the form of images or text or decimal numbers which is an alternative to the more complex dot
matrix displays. It consists of seven segments of light-emitting diodes (LEDs) which are
assembled like numerical 8.
Each one of the seven LEDs in the display is given a positional segment with one of its
connection pins being brought straight out of the rectangular plastic package. These individually
LED pins are labelled from a through to g representing each individual LED. The other LED
pins are connected together and wired to form a common pin.
So by forward biasing the appropriate pins of the LED segments in a particular order, some
segments will be light and others will be dark allowing the desired character pattern of the
number to be generated on the display. This then allows us to display each of the ten decimal
digits 0 through to 9 on the same 7-segment display.
There are basically two types of seven-segment displays-common cathode and common anode.
• Common anode: when the common pin is positive
• Common cathode: when the common pin is negative
 Fig: Common Anode 7 Segment Display

Fig: Common Cathode 7 Segment Display

Digital Segments for all Numbers

Then for a 7-segment display, we can produce a truth table giving the individual segments that
need to be illuminated in order to produce the required decimal digit from 0 through 9 as shown
below.
7-segment Display Truth Table

Dot Matrix Display

LEDs are arranged in matrix form-common configurations are 5 × 7, 5 × 8 and 8 × 8, as shown
in FIG. Based on the electrode connections, two kinds of LED matrices are possible, one is
common anode. All the LEDs in a row having the anode are connected together. The other one is
common cathode, having all LEDs in a row; the common cathode or cathodes are shorted. It is
easier to understand the construction and interface capabilities of an LED matrix using an
illustration. FIG. depicts a matrix construction of the common-anode type. A single matrix is
formed by thirty-five LEDs arranged in five columns and seven rows (5 × 7). The anodes of the
fi ve LEDs forming one row are connected together. Similarly, the cathodes of the seven LEDs
of a column are connected together. In this arrangement of LEDs, the cathodes are switched to
turn the LEDs of a row on or off.
The matrix (unit) illustrated in FIG. can be used to display a single alphanumeric character.
Several such units can be placed next to each other to form a larger panel to display a string of
characters.

FIG. LED Matrix with common-anode arrangement.

 FIG. 5×7 and 8×8 dot matrix display

Data loggers
Data loggers are is stand-alone devices that can record information electronically from internal or
external sensors or other equipment that provide digital or serial outputs.
1. Key Features of Data Loggers
(a) Stand-alone Operation
Most data loggers are normally configured with a PC, some models can be configured from the
front panel provided by the manufacturer. Once the data loggers are configured, they don't need
the PC to operate.
(b) Support for Multiple Sensor Types
Data loggers often have universal input type which can accept input from common sensors like
thermocouple, RTD, humidity, voltage, etc.
(c) Local Data Storage
All data loggers have local data storage or internal memory unit, so all the measured data is stored
within the logger for later transfer to a PC.
(d) Automatic Data Collection
Data loggers are designed to collect data at regular intervals, 24 hours a day and 365 days a year
if necessary, and the collection mode is often configurable.
Data logging and recording are both analog terms in the field of measurement. Data logging is
basically measuring and recording of any physical phenomena or electrical parameter over a
period of time. The physical phenomena can be temperature, strain, displacement, flow, pressure,
voltage, current, resistance, power, and many other parameters
The data logger collects information about the state of any physical system from the sensors.
Then the data logger converts this signal into a digital form with the help of an A/D converter.
This digital signal is then stored in some electronic storage unit, which can be easily transferred
to the computer for further the analysis,
A few basic components that every data logger must have which are:
1. Hardware components like sensors signal conditioning, and analog-to-digital converter,
etc.
2. Long-term data storage, typically onboard memory or a PC
3. Software for collecting data, analyzing and viewing
Dot matrix
A Dot Matrix Printer or Impact Matrix Printer refers to a type of computer printer with a print
head that runs back and forth on the page and prints by impact, striking an ink-soaked cloth
ribbon against the paper, much like a typewriter. Unlike a typewriter or daisy wheel printer,
letters are drawn out of a dot matrix, and thus, varied fonts and arbitrary graphics can be
produced.

Because the printing involves mechanical pressure, these printers can create carbon copies and
carbon less copies. Each dot is produced by a tiny metal rod, also called a ―wire‖ or ―pin‖,
which is driven forward by the power of a tiny electromagnet or solenoid, either directly or
through small levers (pawls). Facing the ribbon and the paper is a small guide plate pierced with
holes to serve as guides for the pins. The moving portion of the printer is called the print head,
and when running the printer as a generic text device it generally prints one line of text at a
time.

Most dot matrix printers have a single vertical line of dot making equipment on their print
heads; others have a few interleaved rows in order to improve dot density. These machines can
be highly durable, but eventually wear out. Ink invades the guide plate of the print head,
causing grit to adhere to it; this grit slowly causes the channels in the guide plate to wear from
circles into ovals or slots, providing less and less accurate guidance to the printing wires.
Advantages:
1. Can print on multi-part stationery or make carbon copies.
2. Impact printers have one of the lowest printing costs per page.
3. They are able to use continuous paper rather than requiring individual sheets.
4. The ink ribbon also does not easily dry out. Disadvantages:
1. Impact printers are usually noisy.
2. They can only print low resolution graphics, with limited color performance, limited quality
and comparatively low speed.
3. They are prone to bent pins (and therefore a destroyed print head) caused by printing a
character half-on and half-off the label.

laser printers
A laser printer is a popular type of computer printer that uses a non-impact photocopier
technology where there are no keys striking the paper.
When a document is sent to the printer, a laser beam "draws" the document on a selenium-coated
drum using electrical charges. The drum is then rolled in toner, a dry powder type of ink that
adheres to the charged image on the drum. The toner is transferred onto a piece of paper and
fused to the paper with heat and pressure.

Working principle
1. A photo, graphic or text image is sent to the printer, which begins the process of
transferring that image to paper using a combination of positive and negative static
electric charges.
2. The revolving drum gets a positive charge.
3. The system's electronics convert the image into a laser beam.
4. The laser beam bounces off a mirror onto the drum, drawing the image on the drum by
burning a negative charge in the shape of the image.
5. Then the drum picks up the positively charged toner from the toner cartridge. The toner
sticks to the negatively charged image on the drum.
6. Paper entering the printer receives a negative charge.
7. As the paper passes the drum, the paper's negative charge attracts toner from the
positively charged drum; the toner literally sits on top of the paper.
8. The paper's charge is removed and a fuser permanently bonds the toner onto the paper.
9. The printed paper is released from the printer.
10. The electrical charge is removed from the drum, and the excess toner is collected
Compact disc/Optical disc recorders
A compact disc is a portable storage medium that can record, store and play back audio, video
and other data in digital form. As a result, the signals captured are a replica of the original audio
stream. Text, picture images, audio, video, and software are all stored on compact discs.
It is made up of three layers.
1. Transparent Substrate with a polycarbonate wafer [plastic disc] makes up this layer.
2. Thin metallic Layer coating of aluminium alloy is applied to the wafer base.
3. Outer Layer of Protective Acrylic The CD's layout is seen in Figure.

Fig: Layout CD-ROM Disc

Working
Using a sample and hold circuit and an ADC, the signal to be recorded on CD is first amplified
and then transformed into a digital signal. The output of the ADC is also used by the Laser Beam
Generator. The control circuit and the servo system are both controlled by the signal from the
crystal oscillator and Laser beam generator.
The servo system, which is controlled by a motor, regulates the disc rotation as well as the track
and focus of the Laser beam generator. The picture depicts a block schematic of a CD recording
system.
The unexposed photoresist material is chemically removed after recording, leaving a helical
pattern across the glass disc's surface. This becomes the glass master for mass-production CDs.
 Figure: Block Diagram of CD Recording
The data retrieval system is made up of the phases listed below.
1. A servomechanism, which spins the CD.
2. A laser head that moves in a radial pattern. The laser head can both emit and detect a 70nm
laser beam.
When the disc spins, the laser beam is focused onto the playing surface, where it is reflected by
the ‗lands' and scattered by the ‗pits,' resulting in a change in the quantity of light reflected
whenever there is a pit-to-land or land-to-pit change. As a result, the pit borders are detected by a
laser beam.