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. They may be filtered using an inductor where a pair of signalling wires follow the same path through the inductor. Eg in a bifilar winding configuration, or as in the case of Ethernet, terminate in a transformer.

High frequency common mode signals, for example, rf noise from a computing circuit, may be blocked using a ferrite bead clamped to the outside of a cable. These are often observable on laptop computer power supplies near the jack socket, and good quality mouse or printer USB cables and HDMI cables.

Switch mode power supplies include common and differential mode filtering inductors to block the switching signal noise returning into mains wiring.

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.