Common-mode signal is the component of an analog signal which is present with one sign on all considered conductors. In telecommunication, common-mode signal on a transmission line is known as longitudinal voltage.
In electronics where the signal is transferred with differential voltage use, the common-mode signal is called a half-sum of voltages:
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.
Common-mode signals may be effectively eliminated by using differential amplifiers or receivers that respond only to voltage differences, e.g., those between the wires that constitute a pair. Induced common-mode signals at low (power-line) frequencies can be greatly reduced by twisting parallel wires to create twisted wire pairs.
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