Common-Mode Rejection

In signal processing, the signals of interest are often carried between two points in a long shielded cable.  The zero or reference potential for the signals is the shield that is grounded near the signal source.  The circuits that amplify the signal are associated with a second ground located at the point of amplification.  In analog work the signal of interest can be balanced, such as a signal from a Wheatstone bridge, or single-ended, such as a thermocouple.  It is good practice to avoid using the shield as one of the signal conductors, as surface currents contaminate the signal.  A cable with a single shield can be used to carry many balanced signal pairs or many single-ended signals when they arise in the same ground environment.  A lot depends on signal levels and circuit impedances. To avoid crosstalk separate signal shields are often needed.

The general problem that must be faced is the potential difference between the ground at the receive end of the cable and the ground at the sending end. This potential difference exists because fields in the area cross the loop formed by the cable and nearby conducting structures (grounds).  These fields are often related to utility power, but they can include fields from radio and television transmitters as well as other digital circuitry.  When a field crosses any loop, a voltage is induced in the entire cable.  Signals that are observed between the two grounds at the receive end of a cable will also appear between the source ground and all signal leads in that cable.  These common signals must be rejected or filtered so that the signals of interest (signal differences) can be amplified.  It is not a good idea to connect the two grounds together, as this simply sets up a voltage gradient in the cable that adds to the problem. Grounding the shield cannot get rid of the interfering field. It may, however, change the field pattern.

The ground difference of potential is called a common-mode signal.  For low frequency analog signals, circuits that reject the common signal are called differential amplifiers. This type of linear amplifier conditions (amplifies) the signal of interest (normal-mode signal) and attenuates the common-mode signals. A different form of amplifier is needed to reject the common-mode signal when logic is involved. In analog work, the common-mode signal is attenuated.  In digital work, the common-mode signal is simply rejected. The level of rejected signal is called out in the logic specifications.  A balanced logic signal is referred to as even-mode logic. 

Definition: Common-mode signal: the average interfering signal on a group of conductors measured with respect to a reference conductor. 

Definition: Normal mode signal: the difference of potential between a pair of conductors.  It is also called the signal of interest.