Common Mode

This is the voltage between the analog signal wires and the common of the power supply of a differential signal, or to ground for an isolated signal. It is desirable that all common mode signals are ignored by the circuit, but in practice there is some error introduced
in the data.  Differential circuits also have a maximum common mode voltage specification, usually stated as a maximum voltage with respect to circuit common. Exceeding the common mode voltage rating of differential signals causes large errors in the data conversion.