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.
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).
The signal (line and return) may be used to drive the LED in an opto-isolator.