Unit V

Signal Conditioning and DAQ systems

CONTENT
Amplification
Filtering
Sample and hold circuit
Data Acquisition: Single and multiple data Acquisition
Data Logging
Applications: Automobile, Aerospace, Home Appliances,
Manufacturing, Environmental monitoring
 Amplification
The operational amplifier manufactured with integrated circuit
technology contains transistors, diodes, resistors and capacitors is an
extremely versatile device that does countless jobs in many
electronics circuits.
 Non - Inverting Mode
 There is a single external input signal v1 = v+ that is applied to
the +ve pin OPAMP. A signal is also made to appear at the –ve input
terminal, but this is derived from resistors R1 and Rf.
Next we introduce characteristic information about OPAMP. The
input voltage is a finite quantity and is often less than 15V. The open
loop gain Avol is very large and may be taken as infinite.
Inverting Mode
 when the OPAMP is used in inverting mode, the input resistance is
given by Ri = R1.
Note that the input resistance for the inverting mode is
considerably less than the very high value encountered in the non-
inverting mode of OPAMP.
 Applying kirchoff’s current law at 2 in figure
In case impedances are used in place of resistances, closed loop gain
is

The output impedance of an ideal OPAMP is zero and therefore the output impedance of the
full circuit is zero
 SAMPLE AND HOLD CIRCUITS
Sample and hold circuits are the devices that store along
information and reduce the aperture time of an A/D converter. A
sample hold is simply a voltage - memory device in which an input
voltage is acquired and then stored on a high quality capacitor.
A1 is an input buffer amplification with a high input impedance so
that the source, which may be an analog multiplexor, is not loaded.
The output of A1 must be capable of driving the hold capacitor
with stability and enough drive current to charge it rapidly.
S1 is an electronic switch, generally an FET, which is rapidly
switched on or OFF by a driver circuit that interfaces with TTL
inputs.
A2 is the output amplifier that butters the voltage on the hold
capacitor. It must therefore have extremely 20 W input bias current
and for this reason an FET input amplifier is required.
 C is a capacitor with low leakage and low dielectric absorption
characteristics; it is a polystyrene, polycarbonate, polypropylens,
or teflon type. In the case of hybrid sample holds the mos-type
capacitor is frequently used.
 2. Extrinsic type sensor

Gain and Phase Components of Zero - Order Hold Transfer Function

A common application for sample - hold circuits is data recovery on signal reconstruction
filters.
The problem is to reconstruct a train of analog samples into the original signal; when
used as a recovery filter, the sample-hold is known as a zero - order hold.
It is useful filter because it fills in the space between samples, providing data smoothing.
 By an analysis based on the impulse response of a sample - hold
and use of the laplace transform, the transfer function is found to
be

Where; fs is the sampling frequency

Function contains the familiar term plus a phase term, both of which
are plotted.
•The sample - hold is a low pass filter with a cut off frequency slightly than
and a linear phase response that results in a constant delay time of ,
where; T is the time between samples.
•Notice that the gain function also has significant response lobes beyond fs for
this reason, a sample – hold reconstruction filter infrequently followed by
another conventional low-pass filter.
Other Sample - Hold Circuits

Fig: Closed Loop Sample - Hold Circuits

 Sample Hold Characteristics
A number of parameters are important in characterizing sample –
hold performance. Probably most important of these is acquisition
time. The definition is similar to that of settling time for an amplifier.
 The most critical phase of sample - hold operation is the transition
from the sample mode to the hold mode. Several important
parameters charactize this transition.
 Hold - mode droop is the output voltage change per unit time
when the sample switch is open. This droop is caused by the
leakage currents of the capacitor and switch and the output
amplifier bias current.
 Sample to hold offset (or step) error is the change in output
voltage from the sample mode to the hold mode, with a
constant input voltage. It is caused by the switch transferring
change on to the hold capacitor as it turns off
 Aperture delay is the time elapsed from the hold command
when the switch actually opens; it is generally much less than
a microsecond.
 FILTERING
 The signal originating from a transducer is fed to the signal
conditioning equipment. In order to measure the output signal
of the transducer originating on account of variations of a
physical change, it is desirable that the output signal be
reproduced faithfully.
• For faithful reproduction of signal it becomes necessary to
eliminate any kind of spurious or unwanted signals which may
get introduced into the system either at the transduction stage
or at the signal conditioning stage.
• The filters are thus designed to pass the signals of unwanted
frequencies and to reject the signals of unwanted frequencies
which may be unwanted harmonics and noise. The harmonics
or noise may be due to some form of distortion.
 FILTERING
The signal originating from a transducer is fed to the signal
conditioning equipment. In order to measure the output signal of the
transducer originating on account of variations of a physical change,
it is desirable that the output signal be reproduced faithfully.
Types of Filters
Filters may be of any physical form. They may be electrical,
mechanical, pneumatic, hydraulic, acoustical, etc., The most
commonly used filters are electrical inform.
1. Passive filters 2. Active Filters
Passive filters only use passive circuit elements like resistors,
capacitors and inductors while active filters use active elements like
operational amplifiers in addition to passive elements like resistance,
inductance and capacitance.
 Both active and passive filters may be classified further as,
Low pass filters High pass filters
Band pass filters Band stop filters

Basic Configuration of an Electric Filter

 Passive Filters
Low Pass Filters

 It is R - C network.
 At low frequencies, the capacitive reactance is very high and
therefore the capacitor circuit can be considered as an open circuit.
 Under these condition eo = ei or the voltage gain is equal to unity.
At very high frequencies, the capacitive reactance is very low and
therefore the output voltage eo is very small as compared with the
input voltage ei.
 ԏ
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High Pass Filter
 Band Pass Filters
• In passband, with a lower cut - off frequency fc1, and upper
cut off frequency fc2. In the passband, the circuit behaves like
a voltage divider network
In this pass band, the circuit behaves almost as a resistive network and
therefore the gain in the passband is
 Band Rejection (Band Stop) Filters
• At very low and very high frequencies, the gain is almost
unity. Somewhere in between their is a frequency where the
gain becomes zero.
• The output frequency fo is equal to zero. This type of filter is
often called a notch filter because it completely rejects a
particular frequency and attenuates a range of frequencies
greatly while passing the frequencies below and above the
notch. Frequency fo is called notch frequency.
Active Filters using Non - Inverting OPAMPS

Low Pass filter

Low - Pass Filter Response

Low Pass Filter
c) High Pass Filter

d) High - Pass Filter Response

 Voltage is amplified in passing through the non – inverting
configuration.

The amplification provided in the low frequency region is

 The cut off frequency in the high frequency region is

The output voltage of high pass filter,

The lower cut off frequency

Combining the actions of the low and high pass filters, a band
pass filter is obtained.
The output voltage is given by the product of results
Active Band Pass Filter and Its Characteristics
The variation of magnitude
Methods of Manufacturing of Nano Sensors
 DATA ACQUISITION
 Data acquisition system via analog signals are used in
communications, electronic and medical applications.
 conversion to digitized system is widely used today because
complex circuits are low cost, accurate, simple to implement.
 In addition there is rapid growth of microprocessors and
microcomputers to perform difficult measurement and control
functions.
 The demands for more and faster information from physical
systems and devices used for space and nuclear research and
development and for industrial automation have product many new
and effective instrumentation systems for data acquisition and
reduction.
Data acquisition systems are used to measure and record analog
signals in basically two different ways,
• i) Signals which originate from direct measurement of
electrical quantities. These signals may be DC or AC voltages,
frequency or resistance, etc.,
• ii) Signals which originates from use of transducers
Type of Instrumentation Systems
Classified into two distinct categories,
1. Analog Systems -Systems deal with information in analog form.
Defined as continuous function such as a plot of voltage versus time
or displacement versus force.
2. Digital Systems-consist of a number of discrete or discontinuous pulses
whose time relationship contains information about the magnitude and the
nature of the quantity under measurement.
 Components of an Analog Data Acquisition System
• Consists of the following elements
a) Transducers
b) Signal Conditioning Equipment
c) Multiplexer
d) Calibrating Equipment
e) Integrating Equipment
f) Visual Display Devices
g) Analog Recorders
h) Analog Computers
i) High Speed Cameras and TV Equipment
Generalized diagram of digital data acquisition system

The essential functional operations of a digital data acquisition systems are

Handling of analog signals
 Making the measurement
Converting the data to digital form and handling it and
Internal Programming and Controls
 Single Channel Data Acquisition System

 The analog to digital converter performs the conversions

repetitively at a free running internally determined rate.
 The most popular example of the single channel data acquisition
system is the digital panel meter (DPM).
 A/D converters based on dual slope techniques are mostly used
for the conversion of low frequency data, generally from
thermocouples.
 The successive approximation technique is most widely used
because it gives high resolution and high speed at moderate cost.
Multichannel Data Acquisition System
• Basically there are 3 types of multiplexed
systems. They are as follows.
Multichannel Analog Multiplexed System
Multiplexing Outputs of Sample/Hold Circuit
• When more number of channels are to be monitored at
the same time, the multiplexing at the outputs of sample
and hold circuit is done.
• In this case, each sample and hold circuit attached to
each channel.
• They are synchronously updated by timing circuit. The
sample and hold circuit are first multiplexed and then
they are connected to analog to digital converter. This
gives the sequential readout of the signals.
• The advantages of this multiplexing technique is that
this is moderately faster than earlier.
 Multiplexing After A/D Conversion
• In this technique, each sample and hold circuit and A/D
converter is assigned to the individual channel.
• The conversion speed is as per the requirement. This technique
is exhibiting parallel conversion.
 In individual systems, the number of strain gaugers,
thermocouples, LVDTS are distributed over large
 As the analog signal is digitized at the centre, the data
transmission provides great immunity against line frequency
and other interferences.
 The data in digital form performs logical operations.
 Based on relative speed at which increased or decreased.
These are also called fast data acquisition systems.
 DATA LOGGING
 The data logger is the application of a data acquisition
system.
• The basic function of data logger is to automatically
make the record for the readings of the various
instruments located at different parts of the plant.
• The data logger measures and records data very quickly
and accurately without any efforts.
• Measurement errors are eliminated completely.
 Advantage of Data Logger
• It can measure the output from almost all type of
transducer and can log the value automatically.
• It can detect the outputs going beyond the limits
specified.
• It can take the corrective action.
• To record all or selected readings on variety of output
devices, data loggers are used.
• It can also pass the data to a computer for further
processing.
• The data logger takes care of the measurement range and
speed, operation of recorders for different blocks of
different channel or single channel.
The logging sequence can be started manually just by
pressing single push button or by giving on clock pulse.
The clock will start logging sequences at intervals of one
second intermediately on any channel.
CHARACTERISTICS OF DATA LOGGERS
1. Can be expanded considering future requirements. This cannot
interrupt the existing work system. The expansion of the data
loggers is very simple and efficient.
2. It has good reliability. It is designed to operate continuously
without any interruption even under worst industrial conditions.
3. The required or specified accuracy is maintained throughout the
period used.
4. This interfaces with the operator with very easy, logical but simple
manner. Hence, it is very easy to operate, understand and expand.
 Basic Blocks of the Data Logger System
1. Input Scanner
2. Signal conditioner
3. Analog to digital convertor
4. Recording system
5. Programmer
Block Diagram of a Data Logger
• The data logger can measure any electrical output from almost all
the transducer.
• The input signals those are fed to the input scanner are high level
pressure transducer signal, low level thermocouple signal,
pneumatic signals from pneumatic transducers on - off signals
from relays or switches, AC Signals and Digital quantities.
• The low dc level signals are first amplified, then conditioned and
then fed to analog to digital converter. The high level signals are
directly fed to the A/D converter. The pneumatic and ac signals are
converted to dc voltage level, conditioned and then converted to
digital form.
• All types of input signals are converted to suitable form that can be
handled by the data logger.
• The purpose of conditioner is to provide similarly between the
signals from various transducers which may not have linear
characteristics.
• The filter circuits are also used to remove noise signal and ripple voltage
suppression.
• The input scanner is an automatic switch which can select each signal in
turn. The scanner is selecting only one input signal at a time, the data
logger needs only one signal amplifier and conditioner also one A/D
converter and signal digital recorder.
• The signal amplifier amplifies the low level signals and
maintain all the input signals to the 5V level.
• The signal if varies non – linearly with the parameter measured, the
linearization of signal is done by the signal conditioner.
• The analog signals are then converted to the digital form which are
suitable to drive the digital recorders.
• The programmer controls the sequence operation of the various items
of the logger. It commands the scanner, converter and recorder.
• The real time clock is used to automate the system. The clock
commands the programmer to sequence the measurements at the
intervals selected by the user.
 Environmental Monitoring Sensors
• On accounts of health and normal survival due to hazards arising
out of biological, chemical and radiation effects on the
environment which not only work locally but are also likely to
affect around the globe through transportation.
• Hazards are critical / serious environmental problems and to assess
the extent to which they can affect human and other living entities.
• Measurement of certain selective parameters are needed.
Environmental monitoring is not possible to be done in a simple
way by measuring temperature of a hot body in fact, a few steps
are involved in the process of monitoring.
• As environment is affected by pollution, the pollutants are to be
identified.
• The quantity/concentration of pollutants in specific collected
‘sample’ need be determined.
• The hazard occurring at a place is not endemic to that place alone
and it is spread.
• The three main ways that cause this spread are
 Atmosphere
 Surface water and
 Ground Water
The simple chart of hazards affect human/living being
Monitoring the environment pollution again involves three steps, namely
• Collection of sample representative enough of the environmental pollution
content.
• Pre - treatment of the sample using extraction, separation and so on.
• Analysis for identification and quantification of analytic pollutant is sample and
expressing it in proper level of concentration.
• Sampling is the major player in the three stage process. Depending on the
time and situation, the sampling techniques vary. Similarly analysis techniques
• differ depending pending on the type of sample.
1. Pollution Hazards
Urine, stool, exhaled breath and so on.
• Example, maximum level for lead in blood is 0.5 mg/t and maximum
concentration of creatinine in urine is 0.15 mg/g.
• Hazards evolve from various sources like
(i) radiation - both ionizing and non ionizing (ii) biological (iii) chemical.
 Ionizing radiation includes α-particles, β-particles, X-rays, neutrons, gamma -
rays which are capable of biological mutation. Non - ionizing radiation includes
IR, UV, radiowave and microwave and extremely low frequency (ELF) (within
300 Hz) radiation.
• While IR radiation cause injury to ophthalmic organs such as cornea, retina (λ <
320 nm), and to a certain extent skin (760nm ≤ λ ≤ 1400 nm).
• UV radiation is known to damage skin (cancerous, sunburn cerythema)and
occular organs. Radio and microwave exposure may lead to cardiovascular
nervous and haematopoitic functions.
2. Sensing Environmental Pollution
• For ionizing radiation, different sensors are used depending on the
characteristics of ionization.
• For low penetration, α-particles (He++) ionization and proportional counters
are recommended, scintillation detectors using Zn s as scintillators are also
used.
• Semi conductor detectors have also begun to be used lately. For b - particle (e+,
e–) which are slightly counters such as proportional counters.
• Scinitllation counters with solid and liquid scintillators are used besides
semiconductor detectors.
• - rays and x - rays are even more penetrating radiations. These are sensed
by Na I and Bi4 Ge3 O12 scintillators. Gas filled detectors using 3 He and BF3
are also used for detecting X and Y rays.
 Home Appliance Sensors
• Nowadays several home appliances have been automated for users
comfort such as washing machines, microwave ovens, refrigerators,
air conditioners, rice cookers etc.,
• When the sensors available in those household appliances are
coupled with micro processors, controlling of those appliances can
be automated.
• The expected outcomes from those smart household appliances
are listed as follows,
1. Low cost
2. Compact size
3. Lesser weight
4. Easy to handle
5. Reliability
6. Highly efficiency
7. Easy to maintain
They have been developed by considering three categories such as
i) Over all control of the house
ii) Energy control and optimization
iii) Overall security of the house.
 Manufacturing Sensor

 Manufacturing, basically is a controlled process or system is the key to the control being
the sensors used in automated manufacturing
• In a plant increased interconnection has been employed because of the invasion of the
computer in manufacturing and special software based on process data available from
signals delivered by the sensors at various stages and level and the concept is known as
computer - integrated manufacturing (CIM).
 Sensors

Specific sensors are applied in production engineering.

 AEROSPACE SENSORS
• It is well known that the density of the air changes from a free
molecule to normal state.
• The temperature of the fluid used in space will vary from 20° -
350°K.
• They are also subjected to varying acceleration, rainy
condition and other environmental conditions.
• Specially developed sensors are used to monitor and measure
the aerospace parameter like pressure, temperature, velocity,
direction of air, air speed, etc.,
Aerospace Static Pressure Sensors
• It is essential to have the monitoring of vertical speed static
pressure for altitude and time rate of change of altitude
conditions. Pitot tubes are preferred for this purpose, and its
need accurate alignment.
 It shows an industrial type pitot tube which consists of a
cylindrical probe inserted into the air flow stream. At the upstream
side of the probe, the flow velocity is reduced to zero.
 Velocity head is converted into impact pressure, which is sensed
through a small hole available at the upstream section of the
probe.
 One small hole is available in the side of the probe which senses
the static pressure. It can be determined as the square of the air
flow velocity in the area of the impact pressure sensing hole.
Aerospace Temperature Sensors
• In order to measure the aerospace temperature, most used
temperature measuring elements are RTD and thermocouple
• To measure high temperatures, thermocouple is the primarily
used element.
• Especially, type K thermocouple is used for the temperature
ranging upto 850°C and other types such as B and S are used
for the temperature ranging upto 1250°C.
Aerospace Velocity Sensors
• There are two types of velocity that is taken into account: local
linear velocity and bulk velocity or mass flow rate.
• Pitot tube, laser doppler anemometer (LDA) and hot wire
anemometer are used to measure the local linear velocity.
• Especially, LDA can be more preferably used to measure the
aerospace velocity.
• Mass flow rate can be measured by gyroscopic, coriolis and
angular momentum type sensors.
 Aerospace Air Flow Direction Sensors

• There are two holes on the inclined surface placed diametrically

opposite to each other.
• Based on the direction of the air-flow, there will be pressure
difference which can be measured from the pressure tapping
connected in the two holes in the inclined surface of the core.
• It is also proportional to the angle between the axis of the probe and
the direction of the air - flow. there are different designs available
for the probe and hole positions
Measurement of Aerospace Air - Speed

The aerospace air - speed can be determined from the measurement of certain
parameter like total pressure (Pt), static pressure (Ps) and temperature (T) with
reference to the equations of an ideal gas