On the Common Mode Response of Fully Differential Circuits

Differential circuits are often described by their differential gain and common mode rejection ratio (CMRR). This approach, however, neglects the effect that the common mode signal has on the transient response and stability of the circuit. This work shows that the actual behavior of differential circuits in front of common mode voltages is completely described by the common-to-differential mode gain and the commonmode gain. The CMRR is useful to assess common mode errors in the frequency domain, but some circuits that achieve a large CMRR have long transients or are unstable.