The common mode rejection ratio (CMRR) of a differential amplifier (DA) using a single operational amplifier and an instrumentation amplifier (IA) using three operational amplifiers is analyzed, and the complete equations are derived for the case when op amps have finite differential and common mode gains. Amplitude and phase measurements support the theoretical predictions. It is concluded that, at low frequencies, for the single-op-amp DA the use of a trimming potentiometer is better than relying on low-tolerance resistors, because of the higher CMRR achieved. The DA yields a fixed 90 degrees phase shift for the CMRR at frequencies above 1 kHz. For the three-op-amp 1A, it is important that the input buffers are coupled and that they are built from a matched op amp pair. The best CMRR is obtained when the differential gain is concentrated in the input stage, but in any case it decreases at frequencies above 1 kHz because of the reduced CMRR for the differential stage at these frequencies. >