ABSTRACT
In the earlier section on CMRR we saw that in a theoretical balanced line, choosing low output impedances and high input impedances would give very good CM rejection even if the resistors were not perfectly matched. Things are a bit more complex (i.e., worse) if we replace the mathematical subtraction with a real opamp. We quickly find that even if perfectly matched resistors everywhere are assumed, the CMRR of the stage is not infinite, because the two opamp inputs are not at exactly the same voltage. The negative feedback error-voltage between the inputs depends on the open-loop gain of the opamp, and that is neither infinite nor flat with frequency into the far ultra-violet. Far from it. There is also the fact that opamps themselves have a common-mode rejection ratio; it is high, but once more it is not infinite.
