# Instrumentation Amplifier

## Instrumentation Amplifier

Instrumentation Amplifier

Instrumentation Amplifier

Current flowing in resistor R is I= (V1-V2)/R and it flow through R’ in the direction shown, Voltage at non-inverting terminal op-amp A3 is R2V1’/(R1+R2). By superposition theorem,

• In a number of industrial and consumer applications, one is required to measure and control physical quantities.
• Some typical examples are measurement and control of temperature, humidity, light intensity, water flow etc. these physical quantities are usually measured with help of transducers.
• The output of transducer has to be amplified so that it can drive the indicator or display system. This function is performed by an instrumentation amplifier. The important features of an instrumentation amplifier are
• High gain accuracy
• High CMRR
• High gain stability with low temperature coefficient
• Low output impedance
• There are specially designed op-amps such as µA725 to meet the above stated requirements of a good instrumentation amplifier. Monolithic (single chip) instrumentation amplifier are also available commercially such as AD521, AD524, AD620, AD624 by Analog Devices, LM363.XX (XX -->10,100,500) by National Semiconductor and INA101, 104, 3626, 3629 by Burr Brown.
• In the circuit, source V1 sees an input impedance = R3 + R4 ( = 101K) and the impedance seen by source V2 is only R1 (1K). This low impedance may load the signal source heavily. Therefore, high resistance buffer is used preceding each input to avoid this loading effect
• The op-amp A1 and A2 have differential input voltage as zero. For V1=V2, is common mode condition, the voltage across R will be zero. As no current flows through R and R‘ the non-inverting amplifier.
• A1 acts as voltage follower, so its output V2‘=V2. Similarly op-amp A2 acts as voltage follower having output V1‘=V1. However, if V1≠V2, current flows in R and R‘, and (V2‘-V1‘)> (V2- V1). Therefore, this circuit has differential gain and CMRR more compared to the single op- amp circuit.
• The difference gain of this instrumentation amplifier R, however should never be made zero, as this will make the gain infinity. To avoid such a situation, in a practical circuit, a fixed resistance in series with a potentiometer is used in place of R.