'''Common-mode signal''' is the componentvoltage ofcommon anto [[analogboth signal]]input whichterminals isof presentan with one sign on all considered [[electrical conductor|conductor]]sdevice. In [[telecommunication]], the common-mode signal on a [[transmission line]] is also known as '''longitudinal voltage'''.
In most [[electronicselectronic]] wherecircuits the signal is transferred by a differential voltage between two conductors. If the voltages on these conductors are U<sub>1</sub> and U<sub>2</sub>, the common-mode signal is athe half-sum of the voltages:
:<math>U_\text{cm} = \frac{U_1 + U_2}{2}</math>
When referenced to the local common or ground, a common-mode signal appears on both lines of a two-wire cable, in- phase and with equal amplitudes. Technically, a common-mode voltage is one-half the vector sum of the voltages from each conductor of a balanced circuit to local ground or common. Such signals can arise from one or more of the following sources:
* Radiated signals coupled equally to both lines,
* An offset from signal common created in the driver circuit, or
* A ground differential between the transmitting and receiving locations.
Noise induced into a cable, or transmitted from a cable, usually occurs in the common mode;, i.e.as the same signal tends to be picked up by both conductors in a two-wire cable. Likewise, RF noise transmitted from a cable tends to emanate from both conductors. Elimination of common-mode signals on cables entering or leaving electronic equipment is important to ensure [[electromagnetic compatibility]]. Unless the intention is to transmit or receive radio signals, an electronic designer will generally designdesigns electronic circuits to minimise or eliminate common-mode effects.
