Instrumentation amplifier

Instrumentation amplifiers are used where great accuracy and stability of the circuit both short- and long-term are required.

These are arranged so that there is one op-amp to buffer each input (+, −), and one to produce the desired output with adequate impedance matching for the function.

removed (open-circuited), they are simple unity-gain buffers; the circuit will work in that state, with gain simply equal to

, providing easy changes to the gain of the circuit, without the complexity of having to switch matched pairs of resistors.

In the circuit shown, common-mode gain is caused by mismatch in the resistor ratios

Obtaining very closely matched resistors is a significant difficulty in fabricating these circuits, as is optimizing the common-mode performance.

[3] An instrumentation amplifier can also be built with two op-amps to save on cost, but the gain must be higher than two (+6 dB).

An IC instrumentation amplifier typically contains closely matched laser-trimmed resistors, and therefore offers excellent common-mode rejection.

Instrumentation amplifiers can also be designed using "indirect current-feedback architecture", which extend the operating range of these amplifiers to the negative power supply rail, and in some cases the positive power supply rail.

This can be particularly useful in single-supply systems, where the negative power rail is simply the circuit ground (GND).

Examples of parts utilizing this architecture are MAX4208/MAX4209 and AD8129/AD8130 Archived 11 November 2014 at the Wayback Machine.

Chopper-stabilized (or zero-drift) instrumentation amplifiers such as the LTC2053 use a switching-input frontend to eliminate DC offset errors and drift.