Signal Conditioning for Thermistors

In the last article we have seen some signal conditioning circuits for RTD. Today we are going to learn signal conditioning for thermistors. We know that thermistor is temperature measuring sensor made up of semiconducting material. The resistance of thermistor normally decreases as the temperature increases hence it is has negative temperature coefficient (NTC).

Change in resistance of thermistor due to change in temperature is given by the following equation,

Where

RT is the temperature at T(K),

R0 is the resistance at T0 normally at 298K,

? is the characteristic temperature constant of thermistor,

The characteristics of thermistor resistance vs. temperature is non linear therefore linearization circuit is also included along with amplifier in the signal conditioning circuits for thermistor.

Signal Conditioning for Thermistors

Signal conditioning of thermistor includes bridge amplifier and linearization circuit. These circuits are explained as follows.

Bridge amplifier

Following circuit shows a bridge amplifier used for amplifying the output of thermistor. As the output range of thermistor is quite low and it not good to use such short range of output for getting good accuracy for operating any field devices.

Bridge amplifier for thermistor
Bridge amplifier for thermistor

Bridge amplifier consist of wheatstone bridge in which inverting amplifier with thermistor as feedback resistor is used in one of the arm as shown in the diagram. This operational amplifier produces output voltage proportional to the change in the resistance of the thermistor.

Linearization of thermistor

For linearization of thermistor characteristics there are several methods available.

  • Using parallel resistor:

In this method a parallel resistor is connected with thermistor. This method increases linearity but also decreases the sensitivity of the circuit.

Parallel Resistor with thermistor
Parallel Resistor with thermistor

the value of the equivalent reistance is given by,

parallel resistor output
parallel resistor output

 

where Rp is value of parallel resistor,

Rtm is thermistor temperature at mid scale temperature,

Tm is mid scale of temperature variation,

? is characteristic temperature constant.

  • Using serial resistor:

In this method a series resitance is used with thermistor. It reduces nonlinearity of conductance vs. temperature characteristics of thermistor.

Series resistor with thermistor
Series resistor with thermistor

The conductance Gs is given as,

series output
series output

Where Gtm is the conductance of thermistor at mid scale temperature Tm

  • Using op amp:

The third method for linearising the thermistor output is by using op amp.

Following circuit shows the linearization circuit for thermistor. Here we have used a thermistor along with series resistor connected to the inverting terminal of the op amp. An adjustable supply voltage is used to adjust the gain of the amplifier.

Thermistor linearization using op amp
Thermistor linearization using op amp

 

In this way we have seen different signal conditioning circuits for thermistor.

See also: Signal Conditioning for RTD

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Signal Conditioning Circuits

We know that signal conditioning is a process in which signals from different sensors are transferred into a form necessary to interface with other modules of system.

For example, we know that thermocouple produces very low output voltage and this voltage is not sufficient to operate the other controlling modules. Therefore there is need to amplify such signals. For this purpose we use different signal conditioning circuits. In case of thermocouple, we have to use amplifier, linearization circuits, etc. the purpose of using linearization circuits is that, thermocouple has non linear characteristics but in most of the cases we need linear controlling action.

Signal conditioning circuit
Measurement System Block Diagram

Signal Conditioning Circuits:

There are different types of signal conditioning operations such as amplification, filtering, isolation, linearization, excitation, etc. we will discuss all these operation one by one.

Amplification

We know that most of the sensors produce output in the form of change in resistance, voltage or current. All these parameters are having very low strength i.e. very small voltage in case of thermocouple, small change in resistance in case of RTD, etc. Therefore we have use current or voltage amplifiers in case of sensors which produces output in the form of current or voltage.

If the sensor produces output in the form of change in resistance (such as resistance thermometer) we have to use bridge amplifiers. We can make use of operational amplifiers to amplify the signal.

Filtering

Another important signal conditioning circuit is filter. As mentioned earlier most of the sensor produces very low output and therefore electromagnetic noise may get added in the original output. To remove the electromagnetic noise from sensor output we have to use different filter circuits. Filter circuits eliminates noise i.e. undesired frequency components from original signal without affecting it.

Active filters, passive filters, bypass filters are the common types of filter circuits.

Isolation

Isolation circuits are required to differentiate signals from unwanted common mode voltages. Another advantage of isolation circuit is that, it protects measuring devices (sensors) if high voltage is applied to other circuit. It also breaks ground loops.

Linearization

There are many sensors which produces non linear output such as thermocouple, thermistor, etc. linearization circuits are used to convert non linear signal into linear one. It can be achieved by varying the gain of an amplifier as a function of input signal.

Excitation

Another signal conditioning operation is current or voltage excitation. Signal conditioning circuits provide the required voltage or current excitation to some passive sensors such as strain gauge, RTD, etc.

In the upcoming posts we will see signal conditioning of RTD, thermistor and thermocouple.

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