Common mode voltage rejection circuit

In an electrical apparatus for measuring minute differential voltage levels between a first low-source-impedance signal applied to the apparatus positive input terminal and a second low-source-impedance signal applied to the apparatus negative input terminal in the presence of substantial common-mode voltages in reference to earth, wherein said apparatus has balanced high input impedances between each of said terminals and the apparatus zero-reference point, and wherein said zero-reference point is isolated from earth,

• an improvement for limiting the effect of any unbalance between the input-path impedances of said signals which comprises:
• a low-impedance shunt across each of the input terminals and the apparatus zero-reference point;
• and
• a high-impedance shunt between the apparatus zero-reference point and earth;
• wherein the value of said low-impedance shunt in relation to the high-impedance shunt is selected so as to attenuate the common-mode voltages without reducing the terminal input impedances in relation to the signal source-impedances to a point where the measurement error due to the signal source impedance is likely to be equal or greater than the measurement error due to the effect of the common-mode voltages in the absence of said low-impedance and high-impedance shunts.