Signal Conditioning of Thermistor

Introduction

Signal conditioning of thermistor Signal conditioning means manipulating an analogue signal in such a way that it meets the requirements of the next stage for further processing. Operational amplifiers(op-amps) are commonly employed to carry out the amplification of the signal in the signal conditioning stage. The signal conditioning equipment may be required to do linear processes like amplification, attenuation, integration, differentiation, addition and subtraction.

They are also required to do non-linear processes like modulation, demodulation, sampling, filtering, clipping and clamping squaring, linealizing or multiplification by another function etc. the signal conditioning or data acquisition equipment in many a situation be an excitation and amplification system for passive transducer. It may be an amplification system for active transducer. In both the applications, the transducer output is brought up to a sufficient level to make it useful for conversion, processing, indicating and recording.

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Excitation is needed for passive transducers because these transducers do not generate their own voltage or current. Therefore passive transducers like strain gauges, potentiometers, resistance thermometers, inductive and capacitive transducers required excitation from external sources. The active transducers like techno generators, thermocouples, inductive pick ups and piezo-electric crystals. The thermistor constitute one arm or more than one arm of a wheatstone bridge which is excited by an isolated DC source. The bridge can be balanced by a potentiometer and can also be calibrated for unbalanced conditions.

Thermistor is a concentration of the term "Thermal Resistor". It is essentially a semiconductor, which behaves as a resistor with a high negative temperature coefficient of resistance. That is, as the temperature of the thermistor increases, its resistance decreases. The temperature co-efficient is expressed in ohms per unit change in degree Celsius (° C). thermistors with high temperature co-efficient of resistance are more sensitive to temperature change and are therefore well suited to temperature measurement and control.

Wheatstone bridge

Whetstone bridge is the most accurate method available for measuring resistances and is popular for laboratory use. The circuit diagram of typical Wheatstone bridge is given in figure Rx is the unknown resistance to be measured; R, R2 and R^ are resistors of known resistance and the resistance of R2 is adjustable.

If the ratio of the two resistances in the known leg (R2 / R) is equal to the ratio of the two in the unknown leg (Rx / R3), then the voltage between the two midpoints (B and D) will be zero and no current will flow through the galvanometer Vg. R2 is varied until this condition is reached. The direction of the current indicates whether R2 is too high or too low. Detecting zero current can be done to extremely high accuracy (see galvanometer). Therefore, if R, R2 and R3 are known to high precision, then Rx can be measured to high precision.

Very small changes in Rx disrupt the balance and are readily detected. At the point of balance, the ratio of R2 / R = Rx / R3 Therefore, Alternatively, if R, R2, and R3 are known, but R2 is not adjustable, the voltage difference across or current flow through the meter can be used to calculate the value of Rx, using Kirchoff s Circuit laws (also known as Kirchhoff s rules). This setup is frequently used in strain gauge and resistance thermometer measurements, as it is usually faster to read a voltage level off a meter than to adjust a resistance to zero the voltage.

In practical Wheatstone bridge, at least one of the resistance is made adjustable, to permit balancing. When the bridge is balanced, the unknown resistance (normally connected at Rx) may be determined from the setting of the adjustable resistor, which is called a standard resistor because it is a precision device having very small tolerance. Rx= (R2/R1).

A Wheatstone bridge may be used to measure the DC resistance of various types of wire, either for the purpose of quality control of the wire itself or of some assembly in which it is used. For example, the resistance of motor winding, transformers, solenoids, relay coils and resistance of thermistor, RTD also can be measured.

Thermistor

Thermistors are composed of sintered mixture of metallic oxides such as Manganese, Nickel, Cobalt, Copper, Iron and Uranium. They are available in a variety of sizes and shapes. They may be in the form of beads, roads and discs. A thermistor change in electrical resistance due to a corresponding temperature change is evident whether the thermistor's body temperature is changes as result of conduction or radiation from the surrounding environment or due to self heating brought about by power dissipation within the device.

Thermistor is a concentration of the term 'Thermal Resistor". It is essentially a semiconductor which behaves as a resistor with a high negative temperature coefficient of resistance. That is, as the temperature of the thermistor increases, its resistance decreases. The temperature co-efficient is expressed in ohms per unit change in degree celcius (° C). thermistors with high temperature co-efficient of resistance are more sensitive to temperature change and are therefore well suited to temperature measurement and control. Thermistors are available in a wide variety of shapes and sizes.

The word operational indicates that the amplifier can perform mathematical operations like inversion addition, subtraction, multiplication, division, integration and differentiation etc. Properties of ideal operational amplifier are:

• It should have an infinite input impedance.
• It should have zero output impedance.
• It should have an infinite gain (gain of the order of 105 to 109)
• It should have flat response over a wide frequency range.

Some of the important applications of an op-amp are:

• Amplifiers
• Active filters
• Arithmetic circuits
• Log and antilog amplifiers
• Voltage comparators
• Waveform generators
• Precision rectifiers
• Multipliers
• Timers
• Multivibrators
• Regulated power supplies

Operational amplifier characteristics:

1. Input offset voltage : The input offset voltage is defined as the voltage that must be applied to the input terminals to drive the output to zero. This is about 2mV for 741 amplifier. It should be understand thet the offset voltage changes with temperature.
2. Input offset current: just as a voltage offset may be required across the input to make the output voltage zero, so a net current may be required between the inputs to zero the output voltage. This current is called input offset current. This is equal to the difference between the two input currents.
3. Input bias current: It is defined as the mean of the two input currents required to make the output voltage zero.
4. Slew rate: it is the highest rate at which the output can change, it is expressed in terms of v/jiS.
5. Unity gain frequency: in many cases, specifications include the frequency response including unity gain frequency. This is the frequency at which the open loop gain of the amplifier becomes unity. The low frequency gain is about 20,000 and falls to unity at about 1MHz. he amplifier is said to have a 1 MHz gain bandwidth produt.
6. Common mode rejection ratio (CMMR): it is defined as the ratio of differential gain to common mode gain. CMRR is infinite for ideal op-amp. Thus the output voltage corresponding to the common mode noise is zero.

The important features of instrumentation amplifier are as follows.

• Selectable gain with high gain accuracy and gain linearity.
• Differential input capability with high gain common mode rejection.
• High stability of gain with low temperature co-efficient.
• low DC offset and drift errors referred to input.
• low output impedance.

The input amplifiers A[ and A2 act as input buffers with unity gain for common mode signals ecm and with a gain of (1+2R2/Ri) for differential signals. A high input impedance is ensured by the non-inverting configuration in which they operate. The common mode (cm) rejection is achieved by the following stage which is connected as a differential amplifier.

The optimum common mode rejection can be obtained by adjusting R6 or R7 ensuring that Ei - Ei R4 R6 The amplifier A3 can also be made to have some nominal gain for the whole amplifier by an appropriate selection or R4, R5, R6 and R7. The drift errors of the second stage add to the product of the drift errors of the first amplifier and first stage gain.

Hence, it is necessary that the gain in the first stage be enough to prevent the overall drift performance from being significantly affected by the drift in the second stage. The drift problem of instrumentation amplifier can be improved if amplifiers Ai and A2 have offset voltages, which tends to track the temperature. The gain of an instrumentation amplifier can be varied by changing R{ alone. A high gain accuracy can be obtained by using precision metal film resistors for all the resistance. Figure shows a simplified differential instrumentation amplifier using a transducer bridge.

A resistive transducer, thermistor, whose resistance changes as a function of some physical quantity such as temperature is connected in one arm of the bridge and is denoted by (Rr ± A R), where RT is the resistance of the thermistor and delta R is the change in the Generally, resistors Ra, Rb and Rc are selected so that they are equal in value to the transducer resistance RT at some reference condition. The bridge is balanced initially at a desired reference condition. However, as the temperature changes, the resistance of the thermistor also changes causing the bridge to unbalance (Va 4- Vb).

The output voltage of the bridge can be expressed as a function of the change in the resistance of the thermistor. Let the change in resistance of the thermistor be delta R. since Rb and Rc are fixed resistors, the voltage Vb is constant. However, voltage Va varies as a function of the change in thermistor resistance. Therefore The negative sign in this equation indicates that Va<Vb because of the increase in the value of delta R. The output voltage Vab is of the bridge is then applied to the differential instrumentation amplifier composed of three op-amps (see the figure).

The gain of the basic differential amplifier is (-Rf / Rl); therefore the output voltage VO of the circuit Vo=Vab (-Rf / Rj) =AR(V^) . Rf 2 (2R + A R) Rl Generally, the change in the resistance of the thermistor delta R is very small, therefore, we can approximate (2R + delta R) 21 2R. Thus the output voltage V0 =Rf AR- (Vdc) Ri 4R The equation indicates that V0 is directly proportional to the change in the resistance delta R of the thrmistor.

Methodology

1. The wheatstone bridge is balanced initially at a desired reference condition.
2. As the physical quantity to be measured changes, the resistance RT (resistance of thermistor) will also change by A R.
3. Due to change in Rt, the bridge is unbalanced. Hence , Va^Vb
4. The bridge output voltage Vab is then applied to a differential instrumentation amplifier consisting of three op-amps as shown in fig.
5. The expression for the output voltage of the instrumentation amplifier is V()= Vab (-Rf/Rl) Where, -Rf/Rl : gain of the instrumentation amplifier
6. This output is then applied to an indicating meter which indicates the value of the quality being measured.

In industry, the actual process of control panel is situated far away. So output of sensor is sends to the control panel. Due to atmospheric condition, signal at the control panel will not the desired signal. Therefore this type of signal conditioning of thermistor using instrumentation amplifier can be used in industry.

References

1. electrical and electronic measurements : A. K. Sawheny
2. electronic instrumentation : Kalsi
3. electronic instrumentation : Khedkar
4. linear integrated circuits: J. S. Katre
5. www. google. com
6. www. wikipedia. com

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