UNIT II

ANALOG AND DIGITAL

MEASURING SYSTEMS

Ballistic Galvanometer
 Galvanometer

•A galvanometer is an instrument
used for detecting the presence of
small currents or voltages in a
circuit or measuring their
magnitudes.
•The ballistic galvanometer is an
instrument, which is used to measure or
indicate current in a closed circuit.
•The galvanometer also is known as
PMMC instrument, works on the
principle of permanent magnet moving
coil.
•The force is generated on the coil, due to
Lorentz Force Law
•Lorentz force is defined as the
combination of the magnetic and
electric force on a point charge due
to electromagnetic fields.

•It is used in electromagnetism and is

also known as the electromagnetic
force.
• Due to the interaction of fluxes, the pointer in the
meter is deflected.
• Once the pointer is deflected, different torques
are exerted on the pointer to make pointer stop at
its steady-state position.
• The different torques are, deflecting torque,
control torque, and damping torque.
• In the ballistic galvanometer, the damping torque
is almost zero.
• For that reason, it is called a ballistic
galvanometer. It can be used as an ammeter or
voltmeter
• The ballistic galvanometer is a current
measuring instrument with zero damping
torque.
• It is also called as frictionless, damping less
galvanometer. For this galvanometer, the
damping constant is zero.
• The basic principle of galvanometer Lorentz
Force Law, according to which, force is
exerted on the current-carrying coil when it
is placed under the influence of the
magnetic field.
Construction
 Magnetic Poles
•The two magnetic poles are required to
create the necessary magnetic field.
•The poles may be separately excited, or
in some cases, we may use permanent
magnets also.
•For the deflection of the coil/pointer,
magnetic poles are required.
 Phosphor Bronze Wire
• The wire used for the suspension of the coil is made up
of phosphor bronze.
• The reason for using phosphor bronze material is,
phosphor bronze has low torsional constant.
• It allows the coil to suspend easily. Also phosphor
bronze is nonmagnetic.
• So it does not comes under the influence of magnetic
poles.
• And phosphor bronze does not oxidize easily.
• This allows the suspension wire not to get rusted due to
atmospheric conditions.
 Mirror
• The mirror is used in the galvanometer to avoid parallax
errors.
• Parallax error means, error in taking the readings of the
meter. One may note the reading of the pointer by
looking sideways. i.e. from the left side or right side. So
different reading will come.
• So to avoid this, a mirror is placed such that, the
position of the pointer and its reflection should
superimpose, and then readings should be taken.
• This would avoid parallax errors.
 Torsional Head

•The torsional head is used to control the

position of the coil.
•This is important for accuracy in the
meter.
•The torsional head also adjusts the zero
settings of the meter.
 Springs

•The springs are used to make the

deflection of the pointer, proportional to
the magnitude of the quantity to be
measured.
•It also helps to provide restoring torque
to the galvanometer.
 Advantages
1.Linear Scale. The scale of the galvanometer
is linear.
2.It is highly sensitive.
3.It is accurate and precise
4.The toque to weight ratio is high. (This
avoids errors)
5.It is not affected by stray magnetic fields
 Disadvantages
1.Since it works on the principle of
PMMC, it can be used only of DC
measurements.

2.Due to components such as springs,

permanent magnets, etc. it develops
errors due to aging.
 Applications
1.Used in Wheatstone bridge, to detect the
presence of current in the loop
2.Can be used to measure current by connecting a
low resistance in parallel to it.
3.Can be used to measure voltage by connecting a
high resistance in series to it.
4.Detecting errors in communication cables
5.Positioning the pen in analog strip chart
recorders, electrocardiographic, etc.
D’Arsonval galvanometer
 Moving Coil
• It is the current carrying element. It is either
rectangular or circular in shape and consists of a
number of turns of fine wire.
• The coil is suspended so that it is free to turn about its
vertical axis of symmetry.
• It is arranged in a uniform, radial, horizontal magnetic
field in the air gap between pole pieces of permanent
magnet and iron core.
 Iron core
•The iron core is spherical in shape if the coil
is circular and cylindrical if the coil is
rectangular.
• The iron core is used to provide a flux path
of low reluctance and hence provides a
strong magnetic field for the coil to move in.
•This increases the deflecting torque and
hence the sensitivity of galvanometer
 Metal Former
• The moving coil is mounted on the metal former.
The metal former provides the required damping
torque.
• The damping torque is produced due to the eddy
currents developed in the metal former.
• Damping can also be achieved by connecting a
low resistance across the galvanometer
terminals.
 Suspension
• The coil is supported by a flat ribbon suspension, which
carries the current to coil.
• The other current connection is a coiled wire, also called
the lower suspension.
• The galvanometer must be levelled carefully so that the
coil hangs straight and centrally without rubbing the poles
or the soft iron cylinder.
• The upper suspension consists of gold or copper wire of
nearly 0.0125 or 0.025 mm diameter, rolled into the form
of ribbon.
• The controlling torque is usually provided by these
suspensions.
 Indication
The upper suspension carries a small mirror upon
which a beam of light is cast.
The beam of light is reflected on to a scale upon
which the deflection is measured. The scale is
usually about 1m away from the instrument.

Zero setting
A torsion head is provided for adjusting the position
of the coil and also for zero setting
 UNIT II
ANALOG AND DIGITAL
MEASURING SYSTEMS

Digital Frequency Meter

Principle of Operation
•The signal waveform is converted to
trigger pulses and applied continuously
to an AND gate.

• A pulse of 1s is applied to the other

terminal, and the number of pulses
counted during this period indicates the
frequency.
• The signal whose frequency is to be measured is
converted into a train of pulses, one pulse for each
cycle of the signal.
• The number of pulses occurring in a definite interval
of time is then counted by an electronic counter.
• Since each pulse represents the cycle of the unknown
signal, the number of counts is a direct indication of
the frequency of the signal (unknown).
• Since electronic counters have a high speed of
operation, high frequency signals can be measured.
Basic Circuit of a Digital Frequency Meter:
• The signal may be amplified before being applied to the Schmitt trigger.
The Schmitt trigger converts the input signal into a square wave with
fast rise and fall times.
• The output from the Schmitt trigger is a train of pulses, one pulse for
each cycle of the signal.
• The output pulses from the Schmitt trigger are fed to a START/STOP
gate.
• When this gate is enabled, the input pulses pass through this gate and
are fed directly to the electronic counter, which counts the number of
pulses.
• When this gate is disabled, the counter stops counting the incoming
pulses.
• The counter displays the number of pulses that have passed through it
in the time interval between start and stop. If this interval is known, the
unknown frequency can be measured.
Gate Control Method
• Initially the Flip-Flop (F/F-1) is at its logic 1 state.
• The resulting voltage from output Y is applied to point
A of the STOP gate and enables this gate.
• The logic 0 stage at the output Y̅ of the F/F-1 is
applied to the input A of the START gate and disables
the gate.
• As the STOP gate is enabled, the positive pulses from
the time base pass through the STOP gate to the Set
(S) input of the F/F-2 thereby setting F/F-2 to the 1
state and keeping it there.
• The resulting 0 output level from Y̅ of F/F-2 is applied to
terminal B of the main gate.
• Hence no pulses from the unknown frequency source can
pass through the main gate.
• In order to start the operation, a positive pulse is applied to
(read input) reset input of F/F-1, thereby causing its state to
change.
• Hence Y̅ = 1, Y = 0, and as a result the STOP gate is disabled
and the START gate enabled.
• This same read pulse is simultaneously applied to the reset
input of all decade counters, so that they are reset to 0 and
the counting can start.
Block diagram of Digital Frequency Meter
 High Frequency Measurement
• Some of the techniques used are as follows.
1. Prescaling
• The high frequency signal by the use of high speed is divided by the
integral numbers such as 2, 4, 6, 8 etc. divider circuits, to get it within
the frequency range of DFM (for example synchronous counters).
2. Heterodyne Converter
• The high frequency signal is reduced in frequency to a range within that
of the meter, by using heterodyne techniques.
3. Transfer Oscillator
• A harmonic or tunable LF continuous wave oscillator is zero beat
(mixed to produce zero frequency) with the unknown high frequency
signal. The LF oscillator frequency is measured and multiplied by an
integer which is equal to the ratio of the two frequencies, in order to
determine the value of the unknown HF.
Automatic Divider:
• The high frequency signal is reduced by some
factor, such as 100:1, using automatically tuned
circuits which generated an output frequency
equal to 1/100th or 1/1000th of the input
frequency.
Voltmeter and Ammeter
 DC Meter
• The most commonly used dc meter is based on
the fundamental principle of the motor.
• The motor action is produced by the flow of a
small amount of current through a moving coil
which is positioned in a permanent magnetic
field.
• This basic moving system, often called the
D’Arsonval movement, is also referred to as the
basic meter.
The basic meter movement becomes a dc instrument, measuring,
• dc current, by adding a shunt resistance, forming
a microammeter, a milliammeter or an ammeter.
• dc voltage, by adding a multiplier resistance, forming a milli
voltmeter, voltmeter or kilovoltmeter.
• resistance, by adding a battery and resistive network, forming an
ohmmeter.

The basic meter movement becomes an ac instrument, measuring

• ac voltage or current, by adding a rectifier, forming a rectifier
type meter.
DC Voltmeter:
• A basic D’ Arsonval movement can be converted into a
dc voltmeter by adding a series resistor known as
multiplier.
• 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 or a
circuit component.
• 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.

• Im = full scale deflection current of the movement (Ifsd)

• Rm = internal resistance of movement
• Rs = multiplier resistance
• V = full range voltage of the instrument
Multirange Voltmeter:
AC Voltmeter
AC Voltmeter using Full wave Rectifier:
Multirange AC Voltmeter:
Frequency meter
Saturable Coe Frequency meter
Energy meter calibration by direct and phantom
loading
Definition:

• Phantom Loading is a loading condition in which an

energy meter is connected to factious or phantom
load for testing of energy meter with high current
rating. Such loading is favorable to avoid wastage
of energy during the test of measurement
instrument.

• Testing of energy meters are carried out to verify

the actual registration as well as the adjustment
done to bring the meter error within acceptable
limit. An energy meter is subjected to various kind
of test like Creep Test, Starting Test etc.
 DIRECT LOADING
• Pressure coil (PC) is connected across the source and
Current coil in series with the load for the test purpose.
In another words, we can say that meter is directly
connected to the load. This is known as direct loading.

• Direct loading of meter for testing is only adopted

where the current rating of meter is considerably low.
This method of directly connecting the meter to the
circuit leads to wastage of appreciable amount of
energy when the current rating of energy meter is high.
Therefore we must devise a new way to test energy
meters having high current rating.
 Phantom loading
• In Phantom Loading, the pressure coil is
connected to the normal supply voltage and the
current coil (CC) circuit is connected to a low
voltage supply (phantom voltage).

• As the impedance of CC is low, therefore it is

possible to circulate rated current through the CC
with low voltage supply.

• The arrangement is shown below. In the figure

value of voltages are just taken for example.
These values will vary with rating of meter.
 DIGITAL VOLTMETER
• Voltmeter is an electrical measuring instruments used to measure
potential difference between two points. The voltage to be measured
may be AC or DC. Two types of voltmeters are available for the
purpose of voltage measurement i.e. analog and digital.

• Analog voltmeters generally contain a dial with a needle moving over

it according to the measure and hence displaying the value of the
same.

• With time analog voltmeters are replaced by digital voltmeters due to

the same advantages associated with digital systems. Although digital
voltmeters do not fully replace analog voltmeters, still there are many
places where analog voltmeters are preferred over digital voltmeters.

• Digital voltmeters display the value of AC or DC voltage being

measured directly as discrete numerical instead of a pointer deflection
on a continuous scale as in analog instruments
• The block diagram of a simple digital voltmeter is shown in the
figure.

• Explanation of various blocks

Input signal: It is basically the signal i.e. voltage to be measured.
Pulse generator: Actually it is a voltage source. It uses digital, analog
or both techniques to generate a rectangular pulse. The width and
frequency of the rectangular pulse is controlled by the digital
circuitry inside the generator while amplitude and rise and fall time
is controlled by analog circuitry.

• AND gate: It gives high output only when both the inputs are high.
When a train pulse is fed to it along with rectangular pulse, it
provides us an output having train pulses with duration as same as
the rectangular pulse from the pulse generator.

• Decimal Display: It counts the numbers of impulses and hence the

duration and display the value of voltage on LED or LCD display
after calibrating it.
• Unknown voltage signal is fed to the pulse
generator which generates a pulse whose
width is proportional to the input signal.
• Output of pulse generator is fed to one leg
of the AND gate.
• The input signal to the other leg of
the AND gate is a train of pulses.
• Output of AND gate is positive triggered
train of duration same as the width of the
pulse generated by the pulse generator.
• This positive triggered train is fed to the
inverter which converts it into a negative
triggered train.
• 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.
• Thus, counter can be calibrated to indicate
voltage in volts directly.
DIGITAL MULTIMETER
 DIGITAL MULTIMETER
• BUFFER AMPLIFIER
• A buffer amplifier is one that provides electrical
impedance transformation from one circuit to another,
with the aim of preventing the signal source from being
affected by whatever currents that the load may be
produced with.
• Attenuator
• An attenuator is an electronic device that reduces the
power of a signal without distorting its waveform. An
attenuator is effectively the opposite of an amplifier,
though the two work by different methods. While an
amplifier provides gain, an attenuator provides loss
• Current to Voltage Converter
• A current to voltage converter will produce a voltage
proportional to the given current.
• Analog to Digital Converter
• Analog to Digital Converter samples the analog signal
on each falling or rising edge of sample clock. In each
cycle, the ADC gets of the analog signal, measures and
converts it into a digital value. The ADC converts the
output data into a series of digital values by
approximates the signal with fixed precision.
• Digital Counter
• In digital logic and computing, a counter is a device
which stores the number of times a particular event or
process has occurred, often in relationship to a clock.
Digital Voltmeters Features and Characteristics:
Digital Voltmeters Features and Characteristics which includes measuring
instruments that convert analog voltage signals into a digital or numeric
readout. This digital readout can be displayed on the front panel and also
used as an electrical digital output signal.

Any DVM is capable of measuring analog dc voltages. However, with

appropriate signal conditioners preceding the input of the DVM, quantities
such as ac voltages, ohms, dc and ac current, temperature, and pressure can
be measured. The common element in all these signal conditioners is the dc
voltage, which is proportional to the level of the unknown quantity being
measured. This dc output is then measured by the DVM.

Various Digital Voltmeters Features such as speed, automation operation and

programmability. There are several varieties of DVM which differ in the
following ways:

1. Number of digits

2. Number of measurements

3. Accuracy

4. Speed of reading

5. Digital output of several types.

The DVM displays ac and dc voltages as discrete numbers, rather than as a

pointer on a continuous scale as in an analog voltmeter. A numerical readout
is advantageous because it reduces human error, eliminates parallax error,
increases reading speed and often provides output in digital form suitable
for further processing and recording. With the development of IC modules,
the size, power requirements and cost of DVMs have been reduced, so that
DVMs compete with analog voltmeters in portability and size.

Their outstanding qualities are their operating and Digital Voltmeters

Characteristics, as detailed below.

1. Input range from + 1.000 V to + 1000 V with automatic range

selection and overload indication

2. Absolute accuracy as high as ± 0.005% of the reading

 3. Resolution 1 part in million (1 μV reading can be read or
measured on 1 V range)

4. Input resistance typically 10 MΩ, input capacitance 40 pF

5. Calibration internally from stabilized reference sources,

independent of measuring circuit

6. 6. Output in BCD form, for print output and further digital

processing. Optional features may include additional circuitry to
measure current, ohms and voltage ratio.

Ramp Technique of Digital Voltmeter:

Ramp Technique – The operating principle is to measure the time that a linear
ramp takes to change the input level to the ground level, or vice-versa. This
time period is measured with an electronic time-interval counter and the
count is displayed as a number of digits on an indicating tube or display. The
operating principle and block diagram of a ramp type DVM are shown in Figs
5.1 and 5.2.
The ramp may be positive or negative; in this case a negative ramp has been
selected.

At the start of the measurement a ramp voltage is initiated (counter is reset

to 0 and sampled rate multivibrator gives a pulse which initiates the ramp
genera-tor). The ramp voltage is continuously compared with the voltage
that is being measured. At the instant these two voltage become equal, a
coincidence circuit generates a pulse which opens a gate, i.e. the input
comparator generates a start pulse. The ramp continues until the second
comparator circuit senses that the ramp has reached zero value. The ground
comparator compares the ramp with ground. When the ramp voltage equals
zero or reaches ground potential, the ground comparator generates a stop
pulse. The output pulse from this comparator closes the gate. The time
duration of the gate opening is proportional to the input voltage value.

In the time interval between the start and stop pulses, the gate opens and
the oscillator circuit drives the counter. The magnitude of the count indicates
the magnitude of the input voltage, which is displayed by the readout.
Therefore, the voltage is converted into time and the time count represents
the magnitude of the voltage. The sample rate multivibrator determines the
rate of cycle of measurement. A typical value is 5 measuring cycles per
second, with an accuracy of ± 0.005% of the reading. The sample rate circuit
provides an initiating pulse for the ramp generator to start its next ramp
voltage. At the same time a reset pulse is generated, which resets the counter
to the zero state.
Any DVM has a fundamental cycle sequence which involves sampling,
displaying and reset sequences.

Advantages and Disadvantages:

The ramp technique circuit is easy to design and its cost is low. Also, the
output pulse can be transmitted over long feeder lines. However, the single
ramp requires excellent characteristics regarding linearity of the ramp and
time measurement. Large errors are possible when noise is superimposed on
the input signal. Input filters are usually required with this type of converter.

Dual Slope Integrating Type DVM (Voltage to Time

Conversion):
Dual Slope Integrating Type DVM – In ramp techniques, superimposed noise
can cause large errors. In the dual ramp technique, noise is averaged out by
the positive and negative ramps using the process of integration.

Principle of Dual Slope Type DVM:

As illustrated in Fig. 5.3, the input voltage ’ei’ is integrated, with the slope of
the integrator output proportional to the test input voltage. After a fixed
time,

equal to t1, the input voltage is disconnected and the integrator input is con-
nected to a negative voltage – er The integrator output will have a negative
slope which is constant and proportional to the magnitude of the input
voltage. The block diagram is given in Fig. 5.4.

At the start a pulse resets the counter and the F/F output to logic level ‘0’.
Si is closed and Sr is open. The capacitor begins to charge. As soon as the
integrator output exceeds zero, the comparator output voltage changes
state, which opens the gate so that the oscillator clock pulses are fed to the
counter. (When the ramp voltage starts, the comparator goes to state 1, the
gate opens and clock pulse drives the counter.) When the counter reaches
maximum count,

i.e. the counter is made to run for a time ‘t1‘ in this case 9999, on the next
clock pulse all digits go to 0000 and the counter activates the F/F to logic
level ‘1’. This activates the switch drive, ei is disconnected and –er is
connected to the integrator. The integrator output will have a negative slope
which is constant, i.e. integrator output now decreases linearly to 0 volts.
Comparator output state changes again and locks the gate. The discharge
time t2 is now proportional to the input voltage. The counter indicates the
count during time t2. When the negative slope of the integrator reaches zero,
the comparator switches to state 0 and the gate closes, i.e. the capacitor C is
now discharged with a constant slope. As soon as the comparator input (zero
detector) finds that eo, is zero, the counter is stopped. The pulses counted by
the counter thus have a direct relation with the input voltage.
During charging

From Eq. 5.3 it is evident that the accuracy of the measured voltage is inde-
pendent of the integrator time constant. The times t1 and t2 are measured by
the count of the clock given by the numbers n1 and n2 respectively. The clock
oscillator period equals T and if n1 and er are constants, then Eq. 5.4 indicates
that the accuracy of the method is also independent of the oscillator
frequency.The dual slope technique has excellent noise rejection because
noise and superimposed ac are averaged out in the process of integration.
The speed and accuracy are readily varied according to specific requirements;
also an accuracy of ± 0.05% in 100 ms is available.

Integrating Type DVM (Voltage to Frequency

Conversion):
The principle of operation of an Integrating Type DVM is illustrated in Fig.
5.5.

A constant input voltage is integrated and the slope of the output ramp is
proportional to the input voltage. When the output reaches a certain value,
it is discharged to 0 and another cycle begins. The frequency of the output
waveform is proportional to the input voltage. The block diagram is
illustrated in Fig. 5.6.

The input voltage produces a charging current, ei/R1 that charges the
capacitor ‘C’ to the reference voltage er. When er is reached, the comparator
changes state, so as to trigger the precision pulse generator. The pulse
generator produces a pulse of precision charge content that rapidly
discharges the capacitor. The rate of charging and discharging produces a
signal frequency that is directly proportional to ei.

The output frequency is proportional to the input voltage ei. This DVM has
the disadvantage that it requires excellent characteristics in linearity of the
ramp. The ac noise and supply noise are averaged out.

Successive Approximation Type DVM:

The Successive Approximation Type DVM principle can be easily understood
using a simple example; the determination of the weight of an object. By
using a balance and placing the object on one side and an approximate
weight on the other side, the weight of the object is determined.
If the weight placed is more than the unknown weight, the weight is removed
and another weight of smaller value is placed and again the measurement is
performed. Now if it is found that the weight placed is less than that of the
object, another weight of smaller value is added to the weight already
present, and the measurement is performed. If it is found to be greater than
the unknown weight the added weight is removed and another weight of
smaller value is added. In this manner by adding and removing the
appropriate weight, the weight of the unknown object is determined.

The Successive Approximation Type DVM works on the same principle. Its
basic block diagram is shown in Fig. 5.10. When the start pulse signal
activates the control circuit, the successive approximation register (SAR) is
cleared. The output of the SAR is 00000000. Vout of the D/A converter is 0.
Now, if Vin > Vout the comparator output is positive. During the first clock
pulse, the control circuit sets the D7 to 1, and Vout jumps to the half reference
voltage. The SAR output is 10000000. If Vout is greater than Vin, the comparator
output is negative and the control circuit resets D7. However, if Vin is greater
than Vout, the comparator output is positive and the control circuits keep
D7 set. Similarly the rest of the bits beginning from D7 to D0 are set and tested.
Therefore, the measurement is completed in 8 clock pulses.

At the beginning of the measurement cycle, a start pulse is applied to the

start-stop multivibrator. This sets a 1 in the MSB of the control register and
a 0 in all bits (assuming an 8-bit control) its reading would be 10000000. This
initial setting of the register causes the output of the D/A converter to be
half the reference voltage, i.e. 1/2 V. This converter output is compared to
the unknown input by the comparator. If the input voltage is greater than
the converter reference voltage, the comparator output produces an output
that causes the control register to retain the 1 setting in its MSB and
the converter continues to supply its reference output voltage of 1/2 Vref.

The ring counter then advances one count, shifting a 1 in the second MSB of
the control register and its reading becomes 11000000. This causes the D/A
converter to increase its reference output by 1 increment to 1/4 V, i.e. 1/2 V
+ 1/4 V, and again it is compared with the unknown input. If in this case the
total reference voltage exceeds the unknown voltage,
the comparator produces an output that causes the control register to reset
its second MSB to 0. The converter output then returns to its previous value
of 1/2 V and awaits another input from the SAR. When the ring counter
advances by 1, the third MSB is set to 1 and the converter output rises by the
next increment of 1/2 V + 1/8 V. The measurement cycle thus proceeds
through a series of Successive Approximation Type DVM. Finally, when the
ring counter reaches its final count, the measurement cycle stops and the
digital output of the control register represents the final approximation of
the unknown input voltage.
Microprocessor Based Ramp Type DVM:
A basic block diagram of a Microprocessor Based Ramp Type DVM and its
operating waveform is shown in Fig. 5.17 (a) and (b) respectively. Depending
on the command fed to the control input of the multiplexer by the
microprocessor, input 1 of the comparator can be consecutively connected
to the input 1, 2 or 3 of the multiplexer.

The multiplexer has three inputs — input 1 is connected to ground potential,

input 2 is the unknown input, and input 3 is the reference voltage input.
The comparator has two inputs — input 1 accepts the output signal from the
multiplexer, and input 2 accepts the ramp voltage from the ramp generator.

The microprocessor remains suspended in the resting state until it receives a

command to start conversion. During the resting period, it regularly sends
reset signals to the ramp generator. Each time the ramp generator is reset,
its capacitor discharges. It produces a ramp, i.e. a sawtooth voltage whose
duration, Tr and amplitude, Vm remain constant. The time duration between
the consecutive pulses is sufficiently large enough for the capacitor to get
discharged.

Whenever a conversion command arrives at the microprocessor at a time t1,

the multiplexer first connects input 1 of the comparator to its input 1 (i.e.
ground potential) and brings the former to ground potential.

The microprocessor pauses until another sawtooth pulse begins. When input
2 voltage, arriving from the ramp generator becomes equal to equal to input
1 of the comparator, the comparator sends a signal to the microprocessor,
that ramp voltage is zero. The microprocessor measures this time interval
Δt1 (shown in Fig. 5.17 (b)), by counting the number of clock pulses supplied
by the clock generator during this time interval. Let the count during this
time be N1, which is then stored by the microprocessor.

A command from the microprocessor now causes the comparator input 1 to

be connected to input 2 of the multiplexer. This connects the unknown
voltage, Vx to the input 1 of the comparator. At an instant, when the ramp
voltage equals the unknown voltage, the comparator sends a signal to the
microprocessor the measure the time interval Δt2 (Fig. 5.17 (b)). The count N2,
during this time interval is also stored.

Now, the next command from the microprocessor causes the comparator
input 1 to be connected to the input 3 of the multiplexer, which is the
reference voltage (full scale voltage). The value of the reference voltage sets
the upper limit of measurement, that is, full scale value. At the instant, when
the ramp voltage equals the reference voltage, a pulse is sent to the
microprocessor from the comparator output to measure this time interval,
Δt3 (Fig. 5.17 (b)). The count, N3 during this time interval is also stored.

The microprocessor then computes the unknown voltage Vx by the equation

where C is the coefficient dependent on the characteristics of the instrument

and the units selected to express the result.
In this method of measurement, the zero drift has practically no effect on the
result, because of the variation of slope of the ramp.

Hence from the Fig. 5.17 (b),

Since the clock pulse repetition frequency fc and full scale voltage Vfs are
maintained at a very high level of stability and clock pulses allowed to fall
within all the time intervals come from a common source, the above equation
may be rewritten as

where N1, N2, N3 are the counts representing respectively, the zero drift, the
unknown voltage, and the full scale voltage.

Advantages of Microprocessor Based Ramp Type DVM:

1. Its scale size remains constant due to zero drift correction and
maximum

2. The accuracy of the instrument is not affected by the time and

temperature instabilities of the circuit element values.

3. There is a good repeatability in switching instants in the

presence of noise and interference. This is because the ramp
approaches the point at which the comparator operates always
the same side and always the same rate.
Disadvantages of Microprocessor Based Ramp Type DVM:

1. Noise and interference cannot be suppressed.

General Specifications of a DVM:
Working Principle of Multimeter:
A multimeter is basically a PMMC meter. To measure dc current the meter
acts as an ammeter with a low series resistance. Range changing is
accomplished by shunts in such a way that the current passing through the
meter does not exceed the maximum rated value. A Working Principle of
Multimeter consists of an ammeter, voltmeter and ohmmeter combined, with
a function switch to connect the appropriate circuit to the D’Arsonval
movement.

Figure 4.33 shows a meter consisting of a dc milliammeter, a dc voltmeter,

an ac voltmeter, a microammeter, and an ohmmeter.

Microammeter:

Figure 4.34 shows a circuit of a multimeter used as a microammeter.

DC Ammeter:

Figure 4.35 shows a working principle of multimeter used as a dc ammeter.

DC Voltmeter:

Figure 4.36 shows the dc voltmeter section of a multimeter.

AC Voltmeter:

Figure 4.37 shows the ac voltmeter section of a working principle of

multimeter. To measure ac voltage, the output ac voltage is rectified by a half
wave rectifier before the current passes through the meter. Across the meter,
the other diode serves as protection. The diode conducts when a reverse
voltage appears across the diodes, so that current bypasses the meter in the
reverse direction.

Ohmmeter:

Referring to Fig. 4.38 which shows the ohmmeter section of a multimeter, in

the 10 k range the 102 Ω resistance is connected in parallel with the total
circuit resistance and in the 1 MΩ range the 102 Ω resistance is totally
disconnected from the circuit.
Therefore, on the 1 M range the half scale deflection is 10 k. Since on the 10
k range, the 102 Ω resistance is connected across the total resistance,
therefore, in this range, the half scale deflection is 100 Ω. The measurement
of resistance is done by applying a small voltage installed within the meter.
For the 1 M range, the internal resistance is 10 Ω, i.e. value at mid-scale, as
shown in Fig. 4.39. And for the 10 k range, the internal resistance is 100 Ω,
i.e. value at mid-scale as shown in Fig. 4.40.

The range of an ohmmeter can be changed by connecting the switch to a

suitable shunt resistance. By using different values of shunt resistance,
different ranges can be obtained.

By increasing the battery voltage and using a suitable shunt, the maximum
values which the ohmmeter reads can be changed.

Multimeter Operating Instructions:

The combination volt-ohm-milliammeter is a basic tool in any electronic

laboratory. The proper use of this instrument increases its accuracy and life.
The following precautions should be observed.

1. To prevent meter overloading and possible damage when

checking voltage or current, start with the highest range of the
instrument and move down the range successively.
2. For higher accuracy, the range selected should be such that the
deflection falls in the upper half on the meter scale.
3. For maximum accuracy and minimum loading, choose a
voltmeter range such that the total voltmeter resistance (ohms
per volt x full scale voltage) is at least 100 times the resistance of
the circuit under test.
4. Make all resistance readings in the uncrowded portion on the
meter scale, whenever possible.
5. Take extra precautions when checking high voltages and
checking current in high voltage circuits.
6. Verify the circuit polarity before making a test, particularly when
measuring dc current or voltages.
7. When checking resistance in circuits, be sure power to the circuit
is switched off, otherwise the voltage across the resistance may
damage the meter.
8. Renew ohmmeter batteries frequently to insure accuracy of the
resistance scale.
9. Re-calibrate the instrument at frequent intervals.
10. Protect the instrument from dust, moisture, fumes and heat.