What is an instrumentation amplifier? It is a device that measures small signals in the presence of a noisy environment. Now how does it do this? In many applications, the signal of interest is a small differential signal, riding on top of a large common mode signal. The instrumentation amp is a device that gets rid of this common mode signal while at the same time amplifying the differential signal. This is why an essential figure of merit for an instrumentation amplifier is the common mode rejection ratio (CMRR) which measures the ratio of the differential gain to the common mode attenuation. The instrumentation amplifier converts the differential input into a single ended output for further processing. The output is referred to a reference pin on the instumentation amp that sets the output offset. Like most amplifiers, an instrumentation amplifier has high input impedance, so it can accurately measure high resistance sensors. It also has a low output impedance which allows it to drive later stages that may have limited input impedance. A word about frequency in case any high speed designers are listening. Instrumentation amplifiers typically operate between DC and about 1 MHz. At high speeds, input capacitance becomes more an issue than input resistance, so these applications are generally addressed with a class of parts called differential amplifiers. These have lower input resistances than in amps, but can be much faster.