Common Mode Rejection Ratio CMRR

A real op-amp amplifies not only the differential voltage at its inputs, but also the common mode voltage. What is this common mode voltage? It's a voltage that is common to both inputs. For example if the input signals are riding on a fixed DC voltage, then that DC voltage is the common mode voltage. The common mode voltage can also be an AC voltage - the input signals are riding on a common carrier signal.

The gain applied to the differential signal and common mode signal is different. The ratio of the gains is the common mode rejection ratio. The larger the CMRR, the better the op-amp rejects the common mode signal.

CMRR = differential gain / common mode gain

The effect of CMRR can be modeled as an offset voltage = Vcm/CMRR , where Vcm is the common mode voltage.