Common-mode interference

In telecommunication, the term common-mode interference has the following meanings:

Interference that appears on both signal leads (signal and circuit return), or the terminals of a measuring circuit, and ground.
A form of coherent interference that affects two or more elements of a network in a similar manner (i.e., highly coupled) as distinct from locally generated noise or interference that is statistically independent between pairs of network elements.

Techniques for dealing with common-mode interference

Common mode noise may be isolated from the desired signal by various means:

Common mode noise may sensed and fed back negatively into object providing the signals (see Driven Right Leg).
Both signal and signal return may be applied to the primary of a transformer, with the signal taken from the secondary. As common mode interferance will not cause voltage to be induced in the primary, no signal from this source will be seen in the secondary, while differential signals on the primary will cause current in the primary and so cause induced voltage in the secondary.
A signal transformer may have a center-tapped primary to ground, with the signal and signal return operating as a ballanced line (push-pull technique). This is adventagious its resistantce to signals raised on ground due to ground loop induction and ground circuit resistance.
The signal (line and return) may be used to drive the LED in an opto-isolator.