COMMON-MODE REJECTION RATIO IN DIFFERENTIAL-AMPLIFIERS

We have analyzed the common mode rejection ratio (CMRR) for a differential amplifier (DA) using a single operational amplifier and for an instrumentation amplifier (IA) using three operational amplifiers, and we have derived the complete equations for the case when op amps have finite differential and common mode gains. Amplitude and phase measurements support our theoretical predictions. We conclude 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, giving a clear advantage if synchronous demodulation is used for further processing of amplitude-modulated signals. For the three-op-amp IA, it is extremely important for input buffers to be "coupled" and to be 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.