Abstract: A circuit for reducing the amplitude of a common mode voltage across the inputs of a differential measuring instrument. The circuit comprises a voltage divider between each of the differential input terminals and the instrument zero reference point on one side, and between the instrument's zero reference point and the common mode voltage reference (usually earth) on the other side. A potentiometer forms the divider junction at the instrument reference point and is used to compensate for resistance value tolerances in the circuit and any imbalance in the input path impedances of the two differential inputs. The value of the voltage divider resistors are selected so as to bring the instrument reference point as close as possible to the level of the common mode voltage without unduly reducing the instrument input impedance.