Instrumentation amplifiers (IAs) are complete closed loop amplifiers with feedback components. They amplify differential input signals while rejecting common-mode signals. Though internal designs can vary, the commonly used 3-op-amp topology (figure 1) is representative. Many types allow gain selection from G=1 to G=1000 or greater with a single external resistor, RG. Some types have internal RG resistors for very accurate fixed gains.
The input stage is comprised of a two op amps in adjustable gain and provides very high input impedance on both the inverting and non-inverting inputs. The output stage consists of a difference amplifier—four accurately matched resistors around a single op amp that rejects common-mode voltage and noise. The amplified difference voltage is referred to an output ground (Ref) or reference voltage.
In comparison, a simple op amp amplifier circuit (figure 2) shares a common input and output reference ground and cannot reject a common-mode voltage or noise. It has only one high impedance input.
A modern integrated circuit IA has a great advantage over one built with discrete op amps and resistors. The internal resistors in the difference amplifier are matched, laser trimmed and temperature tracking to achieve excellent common-mode rejection—better than can be achieved with commonly available discrete components. The three op amp circuit shown in figure 1 is a simplification of an optimized signal path that can achieve DC accuracy, low noise and dynamic performance very difficult to achieve with discrete op amps and resistors.
A few noteworthy example products:
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