Common-mode signal

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:

$U_{cm}={\frac {U_{1}+U_{2}}{2}}$

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.

Example: lets take a three phase system having current Ia, Ib, Ic. then common mode current is avg of the sum of the currents.

Icm = (Ia + Ib + Ic)/3;

See also

• Common-mode rejection ratio

References

 This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22.