SLOS358C September 2011 – April 2020 THS4531
The input common-model voltage of a fully differential op amp is the voltage at the + and – input pins of the op amp.
It is important to not violate the input common-mode voltage range (VICR) of the op amp. Assuming the op amp is in linear operation the voltage across the input pins is only a few millivolts at most. So finding the voltage at one input pin will determine the input common-mode voltage of the op amp.
Treating the negative input as a summing node, the voltage is given by:
To determine the VICR of the op amp, the voltage at the negative input is evaluated at the extremes of VOUT+.
As the gain of the op amp increases, the input common-mode voltage becomes closer and closer to the input common-mode voltage of the source.