Been When been When

1. DEPARTMENT OF MECHANICAL ENGINEERING
WACHEMO UNIVERSITY
Instrumentation and Measurement (Meng4211)
chapter 4 –SIGNAL CONDITIONING
By: Mebratu B.

2. Signal Conditioning
The output signal from a sensor usually does not have the suitable characteristics for displaying,
recording, transmission, or processing. Common issues with the sensor output signal:
- Low amplitude
- Contains noise
- Not in the voltage or current form to be directly interpreted by electronics systems
- In analog form, the signal cannot be recorded or processed by digital systems
Signal conditioning is the processing of the sensor signal to adapt the signal to the requirements of the
next stage in a measurement system.
Signal conditioning circuits are used to process the output signal from sensors of a measurement system
to be suitable for the next stage of operation.
Signal conditioning is an electronic circuit that manipulates a signal in a way that prepares it for the next
stage of processing. Many data acquisition applications involve environmental or mechanical
measurement from sensors, such as temperature and vibration.

3. Signal conditioning is the operation performed on the signal to convert it to a form suitable for interfacing
with other elements in the system.
Signal conditioning is a basic component of all measurement devices. It converts incoming measurements
into a form acceptable to digitization hardware. Signal conditioning not only defines what types of signals the
system can accept, but also defines what additional features the system has to offer.
Here are some commonly encountered terms and their definitions in signal conditioning:
Inputs
Signal inputs accepted by signal conditioners include DC voltage and current, AC voltage and current,
frequency and electric charge. Sensor inputs can be accelerometer, thermocouple, thermistor, resistance
thermometer, strain gauge or bridge and Outputs for signal conditioning equipment can be voltage, current,
frequency, timer or counter, relay, resistance or potentiometer.
Processes
Signal conditioning can include amplification, filtering, converting, isolation and any other processes required
to make sensor output suitable for processing after conditioning.

4. Processes
Signal conditioning can include amplification, filtering, converting, isolation and any other
processes required to make sensor output suitable for processing after conditioning.
Input Coupling
Use AC coupling when the signal contains a large DC component. If you enable AC coupling,
you remove the large DC offset for the input amplifier and amplify only the AC component.
This configuration makes effective use of the ADC dynamic range
Filtering
The filtering process blocks unwanted signal frequencies arising from external noise sources
(generators, motors, power lines, etc.) from incoming signals.
Filtering is the most common signal conditioning function, as usually not all the signal
frequency spectrum contains valid data.
Signal filtering consists of processing a signal to remove a certain band of frequencies within it. The
band of frequencies removed can be either at the low-frequency end of the frequency spectrum, at the
high-frequency end, at both ends, or in the middle of the spectrum.

5. Signal filtering consists of selectively passing or rejecting low-, medium- and high frequency signals from the
frequency spectrum of a general signal. The range of frequencies passed by a filter is known as the pass-band,
the range not passed is known as the stop-band, and the boundary between the two ranges is known as the
cut-off frequency.
The graphical representation of output from ideal filter illustrated in figure below.
a) Low pass filter b) High pass filter
c) Band pass filter d) Band stop filter
Figure Output from ideal filter.

6. Excitation
Many common sensors require power to generate a signal( Some sensors require external voltage or current
source of excitation ) and these sensors are called active sensors. These include strain gauges ,temperature
sensors like resistance temperature detector(RTDs),pressure sensor etc.
Excitation provides this power so sensors do not require external power sources.
The stability and precision of the excitation signal directly relates to the sensor accuracy and stability.

7. Amplification
Amplification increases signal amplitude before digitization occurs. Amplification increases the
measurement accuracy of small signals and reduces the effects of surrounding noise sources. For example,
the output of an electronic temperature sensor, which is probably in the millivolts range is probably too low
for an analog-to-digital converter (ADC) to process directly. In this case it is necessary to bring the voltage
level up to that required by the ADC.
Attenuation
Attenuation reduces signal amplitude before digitization occurs.
Attenuation is 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.
Linearization
Often sensors do not have a linear relationship between their signal value and the physical quantity they are
measuring. A thermocouple’s nonlinear temperature-to-voltage relationship is a prime example.
Linearization maps the relationship between a sensor’s signal value and the physical quantity it is
measuring so that an incremental change in the physical quantity corresponds to a similar incremental
change in the signal. It can be implemented in either the hardware or software component of a system.

8. Bridge completion :This specific type of signal conditioning is used with strain gauges. If a given strain
gauge is either quarter-bridge or half-bridge configuration, then the measurement device’s signal
conditioning must provide the necessary completion resistors to make a full Wheatstone bridge.
Shunt calibration :Also used with strain gauges, shunt calibration provides a comparative
signal value for a precisely known strain value (load) that can be used to calibrate the
measurement system.
Switching relays :Both electromechanical and solid-state relays can be used to control whether
external system components or equipment receive power or not. Relays use low voltage (ac or
dc) to control devices that can require much larger voltages and currents to operate than
available in the measurement system. Relays are typically used to control motors, fans, lights,
or even other.

9. Signal conditioning circuits
Signal conditioning circuits are used to process the output signal from sensors of a measurement system to be
suitable for the next stage of operation.
Bridge circuits are used very commonly as a variable conversion element in measurement systems and produce
an output in the form of a voltage level that changes as the measured physical quantity changes.
Both null and deflection types of bridge exist and null types are mainly employed for calibration purposes
and deflection types are used within closed-loop automatic control schemes.
1)Null-type, d.c. bridge (Wheatstone bridge)
A null-type bridge with d.c excitation commonly known as a Wheatstone bridge, shown in Figure below.

10. Figure)Analysis of Wheatstone bridge.
The four arms of the bridge consist of the unknown resistance Ru, two equal value resistors R2 and R3 and
a variable resistor Rv (standard resistance). A d.c. voltage Vi is applied across the points AC and the
resistance Rv is varied until the voltage measured across points BD is zero. This null point is usually
measured with a high sensitivity galvanometer.

11. When the bridge is balanced ,the galvanometer carries zero current and it does not show any deflection.
Thus bridge works on the principle of null deflection.
If a high impedance voltage-measuring instrument is used, the current Im drawn by the measuring
instrument will be very small and can be approximated to zero. If this assumption is made, then, for Im=0.
To have zero current through galvanometer, thus the potential across AD must be same as the potential
across CD.
I1 = I3 and I2 = I4
Looking at path ADC, we have a voltage Vi applied across a resistance Ru +R3 and by Ohm’s law:
And looking at path ABC,we have,
Now we can calculate the voltage drop across AD and AB:

12. By the principle of superposition,
At the null point V0= 0, then

13. b)Deflection-type d.c. bridge
A deflection-type bridge with d.c. excitation is shown in Figure below.
Figure) deflection type dc bridge.

14. This differs from the Wheatstone bridge mainly in that the variable resistance Rv is replaced by a fixed
resistance R1 of the same value as the nominal value of the unknown resistance Ru. As the resistance Ru
changes, so the output voltage V0 varies, and this relationship between V0 and Ru must be calculated
When Ru is at its nominal value, i.e. for Ru =R1, it is clear that V0 = 0 (since R2 = R3). For other values of
Ru, V0 has negative and positive values that vary in a non-linear way with Ru.
Examples
1) A certain type of pressure transducer, designed to measure pressures in the range 0–10 bar, consists of a
diaphragm with a strain gauge cemented to it to detect diaphragm deflections. The strain gauge has a nominal
resistance of 120 Ω and forms one arm of a Wheatstone bridge circuit, with the other three arms each having a
resistance of 120 Ω. The bridge output is measured by an instrument whose input impedance can be assumed
infinite. If, in order to limit heating effects, the maximum permissible gauge current is 30 mA, calculate the
maximum permissible bridge excitation voltage. If the sensitivity of the strain gauge is 338 mΩ/bar and the
maximum bridge excitation voltage is used, calculate the bridge output voltage when measuring a pressure of
10 bar.

15. Figure) deflection type d.c bridge.
Solution
The type of bridge circuit shown in Figure above in which the components have the following values: R1 =
R2 = R3 D=120 Ω

16. Defining I1 to be the current flowing in path ADC of the bridge,
At balance, Ru = 120Ω and the maximum value allowable for I1 is 0.03 A.
Hence,
Thus, the maximum bridge excitation voltage allowable is 7.2 volts. For a pressure of 10 bar applied, the
resistance change is 3.38 Ω, i.e. Ru is then equal to 123.38 Ω.
Applying the equation ,
Thus, if the maximum permissible bridge excitation voltage is used, the output voltage is 50 mV when a pressure
of 10 bar is measured.

17. Case where current drawn by measuring instrument is not negligible
For various reasons, it is not always possible to meet the condition that the impedance of the instrument
measuring the bridge output voltage is sufficiently large for the current drawn by it to be negligible. Wherever
the measurement current is not negligible, an alternative relationship between the bridge input and output must
be derived that takes the current drawn by the measuring instrument into account.
Figure (a) A bridge circuit; (b) equivalent circuit by Thevenin’s theorem; (c) alternative representation; (d)
equivalent circuit for alternative representation.

18. It is apparent from Figure (c) that the equivalent circuit resistance consists of a pair of parallel resistors Ru and
R3 in series with the parallel resistor pair R1 and R2.
Thus, RDB is given by:
The equivalent circuit derived via Thevenin’s theorem with the resistance ´ Rm of the measuring instrument
connected across the output is shown in Figure (d). The open circuit voltage across DB, E0, is the output voltage
calculated earlier, for the case of Rm = 0:
If the current flowing is Im when the measuring instrument of resistance Rm is connected across DB, then, by
Ohm’s law, Im is given by:

19. If Vm is the voltage measured across Rm, then, again by Ohm’s law:
Substituting for E0 and RDB
Simplifying above equation gives,

20. Example
2) A bridge circuit, as shown in Figure below is used to measure the value of the unknown resistance Ru of a
strain gauge of nominal value 500 Ω. The output voltage measured across points DB in the bridge is measured
by a voltmeter. Calculate the measurement sensitivity in volts/ohm change in Ru if,
a) the resistance Rm of the measuring instrument is neglected, and
b) account is taken of the value of Rm
Figure) Bridge circuit.

21. Solution
For Ru =500 Ω, Vm = 0.
To determine sensitivity, calculate Vm for Ru = 501 Ω.
(a) Applying equation ,
Substituting in values,
Thus, if the resistance of the measuring circuit is neglected, the measurement sensitivity is 5.00 mV per ohm
change in Ru.
b) Applying equation,

22. Substituting the given values in above equation,
Thus, if proper account is taken of the 10 k value of the resistance of Rm, the true measurement
sensitivity is shown to be 4.76 mV per ohm change in Ru.