An instrumentation amplifier (an in-amp) is used to measure small differential signals which ride on top of a larger common mode signal. The in-amp is a device that reduces this common mode signal while at the same time amplifying the differential signal. An essential performance specification for an instrumentation amplifier is the common mode rejection ratio (CMRR), which measures the ratio of the differential gain to the common mode attenuation. Using an in-amp is simple once some basic parameters are understood which will be covered in this PTM.