3-Signal Conditioning Circuits

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Kuwait University Electrical

Engineering Department

Measurement & Instrumentation Laboratory

Signal Conditioning Circuits

Objectives:

Design an analog signal conditioning circuit

Design an analog signal conditioning circuit to provide a range of to provide a range of desired output voltages indesired output voltages in respond to a certain range

respond to a certain range of input voltages.of input voltages.

Theory:

The signal conditioning circuit is an electronic circuit that converts signals provided by a sensor The signal conditioning circuit is an electronic circuit that converts signals provided by a sensor to useful electric signals. These electric signals must meet specific criteria so that they are to useful electric signals. These electric signals must meet specific criteria so that they are corre

correctly intectly interpretrpreted and processed by the rest of the system’ed and processed by the rest of the system’s circuits circuitry. ry. The use of Op-ampsThe use of Op-amps allows signal conditioning circuits to be more compact

allows signal conditioning circuits to be more compact and precise in their implementations.and precise in their implementations.

Figure 1. Signal conditioning is important in a system of process control.

Signal conditioning carries out one or several of the following actions:

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1. Change voltage levelsChange voltage levels so that they are compatible with the following circuitry.so that they are compatible with the following circuitry. 2.

2. ConvConvert ert curcurrent rent to to voltvoltageage. . SomSome sensore sensors, such as the NTC (Negas, such as the NTC (Negativtive e TemTemperperatuaturere Coefficient) and the PTC (Positive Temperature Coefficient) RTD (Resistance Temperature Coefficient) and the PTC (Positive Temperature Coefficient) RTD (Resistance Temperature Depende

Dependent) convert the varint) convert the variationations of the s of the procesprocess to control resiss to control resistance variatance variationstions. . SignaSignall conditioning circuits provide the necessary current that converts a resistance variation to an conditioning circuits provide the necessary current that converts a resistance variation to an appropriate voltage.

appropriate voltage. 3.

3. Convert the analog signal to digital signal.Convert the analog signal to digital signal. The signal conditioning circuit ensures theThe signal conditioning circuit ensures the analog signal is at

analog signal is at levellevels s that are compatible with the that are compatible with the analog to analog to digitdigital al converconversion circuitsion circuitry.ry. Aft

After er havihaving ng tratransfnsformormed ed the the anaanalog log sigsignalnals s intinto o digdigitital, al, we we can can stostore re thetheir ir numnumeriericalcal representations on a memory, process them with an application program, display them on a representations on a memory, process them with an application program, display them on a monitor, send them through the Internet to ano

monitor, send them through the Internet to another place, or print them.ther place, or print them. 4.

4. Convert the analog signal to current signal.Convert the analog signal to current signal. It is an industry standard that the controlIt is an industry standard that the control range is normalized from 4mA to 20mA dc. The minimum value of 4mA is defined as

range is normalized from 4mA to 20mA dc. The minimum value of 4mA is defined as "zero"zero

active"

active" bec becausause e it offerit offers s the advantthe advantage of age of beibeing ng ablable e to to detdetect an ect an intinterrerruptuption of ion of thethe con

connectnection betweion between the en the sensensor and the sor and the sigsignal condinal conditiotioninning g circircuicuit. t. WheWhen n the signthe signalal conditioning provides 0 mA at its output, it will be an indication that the sensor is defective conditioning provides 0 mA at its output, it will be an indication that the sensor is defective or some other faulty

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5. Isolate the sensor. The signal conditioning circuit should isolate the sensor electrically when the sensed signal contains high voltage pulses that can affect the measurements and the subsequent circuitry of the system.

Problem Statement

Build a circuit that converts an input voltage of – 1 V to + 2.2 V (which will be the output of any sensor) to an output voltage of 0 to 5 V. We have a dual supply of ± 12 V available and we need to drive a 10 k Ω load. Test your designs under different input voltages and verify that your output voltages.

Solution

The input voltage (– 1 V to + 2.2 V) is the independent variable while the output voltage (0 to 5V) is the dependent variable.

By plotting the independent variable (x axis) and the dependent variable (y axis), we have:

Connect the intersection points, we find that the graph is a straight line, then:

Vout = mVin + Vo (1) equation of the straight line.

From the point you can find that

Vout = 1.56Vin + 1.56 equation of the straight line or

Vout = 1.56Vin + 1.56 (2)

Then: Vout = Av (Vin) + Voffset Av= 1.56 V/V and Voffset = 1.56V

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Replacing values in equation (4), we have: Vout = Av(Vin) + Voffset (3) Vout = 1.56 (Vin) + 1.56

Vout = 1.56 (Vin + 1) (4)

Equation (4) provides a second possible solution; the block diagram is the following:

The third possible solution comes from the previous circuit whose simplification is:

Note: In all the solutions, it is recommended to use an op-amp voltage follower between Vin and the input of the circuit. The purpose is to maintain impedance matching and prevent excessive loading of previous circuit stages.

The design of the problem statement will be accomplished by using solution 2. Figure 2 shows the implementation of equation 4.

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Figure -2

Experimentally

:

1- Connect the designed circuit of Figure 2. 2- Adjust the potentiometer R1 to set Vx=1 V.

3- Adjust the potentiometer R2 to set Vin= -1.1 V. Record the value of output voltage. 4- Adjust the potentiometer R2 to set Vin= 2.2 V. Record the value of output voltage.

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Discussion

:

1- Using PSPICE, simulate the designed circuit in fig.2 for two cases, when Vin =-1.1 V and Vin =2.2 V. show the output voltages.

2- Compare between the experimental and PSPICE results.

3- Your manager has given you the task of designing a circuit that can take an unconditioned output signal from sensor and convert it into a reliable signal that can be analyzed by any software like LabVIEW. The output signal must be interfaced to a data acquisition card that can only accommodate 0 to 5V signal inputs.

a. Design using sum amplifier.

b. Design using differential amplifier.

Given:

• We have a dual supply of ± 12 V available and we need to drive a 10 k Ω load. • The sensor signal output is in the range of – 1V to +1V.

• The sensor’s output impedance low.

• The signal conditioning circuit output must be 0V to 5 V (because the data acquisition card requires this input signal level to function properly)