Instrumentation Amplifiers—they’re not op amps but what are they?

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:

  • INA111     FET input, high speed.
  • INA114     Excellent DC accuracy, best choice for very low gains.
  • INA116     Ultra-low input bias current (3fA)  with guard output to drive PCB guard traces.
  • INA118     Low power 350uA, excellent general purpose IA.
  • INA121     FET input, moderate speed, general purpose, lower power, 500uA.
  • INA122     Micropower, 60uA, 36V supply, special 2-op amp topology.
  • INA126     Low cost, general purpose IA, bipolar input.
  • INA128     Precision, general purpose IA. Resistor-programmed gain.
  • INA141     Similar to INA128 but pin-strapped gains for G=10 and G=100.
  • INA163     Very low noise, 1nV/rt-Hz, wide bandwidth.
  • INA326     Zero-drift, special and different rail-to-rail input topology.
  • INA333     Zero-drift, micropower, 5V CMOS IA, 60uA.
  • INA826     Precision bipolar input, excellent common-mode rejection at high frequency.