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'''Common-mode signal''' is the component of an [[analog signal]] which is present with one sign on all considered [[electrical conductor|conductor]]s. 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

:<math>U_{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. 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 design electronic circuits to minimise or eliminate common mode effects.

==Methods of eliminating common mode signals==

* [[Differential amplifier]]s or receivers that respond only to voltage differences, ''e.g.,'' those between the wires that constitute a pair. This method is particularly suited for instrumentation where signals are transmitted through DC bias.
* An inductor where a pair of signalling wires follow the same path through the inductor. E.g. in a [[bifilar winding]] configuration such as used in Ethernet magnetics.<ref>http://pulseeng.com/products/datasheets/H325.pdf</ref> Useful for AC and DC signals, but will filter only higher frequency common mode signals.
* A transformer, which is useful for AC signals only, and will filter any form of common mode noise, but may be used in combination with a bifilar wound coil to eliminate capacitive coupling of higher frequency common mode signals across the transformer. Used in twisted pair Ethernet.<ref>http://pulseeng.com/products/datasheets/H325.pdf</ref>

Common mode filtering may also be used to prevent egress of noise for electromagnetic compatibility purposes;
 
High frequency common mode signals, for example, [[Radio frequency|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.<ref>http://www.elmac.co.uk/pdfs/ferrite.pdf</ref>

[[Switched-mode power supply|Switch mode power supplies]]  include common and differential mode filtering inductors to block the switching signal noise returning into mains wiring.<ref>http://www.hottconsultants.com/pdf_files/APEC-2002.pdf</ref>

== See also ==
* [[Common-mode rejection ratio]]

==References==
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{{FS1037C}}

[[Category:Electrical parameters]]
[[Category:Electronics]]