An ideal op-amp amplifies the differential input signal which means that its CMRR tends to infinity. However in real applications common mode signals are also slightly amplified. Suppose you apply a 10V voltage to the non-inverting terminal of the op-amp and a 7V voltage to the inverting one... It is said that the common mode voltage is (10+7)/2 = 8.5V My question is why do we take the average of the inputs when measuring the common mode voltage? In this situation shouldn't 7V be the common signal?