Instrumentation amplifier is a type of differential amplifier with input buffer stages. Input buffer stages aids in impedance matching with the previous stage. Instrumentation amplifiers are generally used in industrial and scientific measurements. Instrumentation amplifier has such useful features as low offset voltage,high CMRR,high input impedance, high gain etc. The circuit diagram of an instrumentation amplifier is shown below,
The above circuit produces an output voltage (Vout) proportional to the difference between input voltages((V1-V2).In the circuit diagram two op amps are shown as input buffers. But gain of the input buffers is not unity due to the presence of resistances R1 and Rg. Op amp at the output stage is wired as a standard differential amplifier.R2 is the input resistor and R3 is connected from output of the output op-amp to its inverting input of the op amp.
The voltage gain of the instrumentation amplifier is given by the expression,
Voltage Gain G = (VO/(V1-V2))
= (1+2R1/Rg)(R3/R2)
Instrumentation amplifiers are used where high sensitivity, accuracy and stability are required.High gain accuracy can be achieved by using precision metal film resistors for all the resistances.
As high negative feed back is employed, the instrumentation amplifier has a good linearity. The output impedance is typically in the range of few milli ohms.