When working with real-world signals, designers often face the significant challenge of picking out a small signal from a large and varying common mode voltage. An ideal op amp would reject the common mode voltage and amplify only the differential signal. However, real-world op amps introduce offset voltages related to the common mode voltage. This limits the accuracy of the amplified signal. The amount of offset or error induced by common mode can be calculated, using CMRR(dB) = 20log(CMRR(V/V)).
Linear Technology, op amp, common mode voltage