RS-485: Imported from Wikipedia (overwrite)

2026-05-02T17:59:45Z

Imported from Wikipedia (overwrite)

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{{short description|Voltage common to both input terminals of an electrical device}}

In [[electrical engineering]], a '''common-mode signal''' is the identical component of [[voltage]] present at both input [[Terminal (electronics)|terminal]]s of an electrical device. In [[telecommunication]], the common-mode signal on a [[transmission line]] is also known as '''longitudinal voltage'''.

'''Common-mode interference''' (CMI) is a type of common-mode signal. Common-mode interference is interference that appears on both signal leads, or coherent interference that affects two or more elements of a network.

In most [[electrical circuit]]s, desired signals are transferred by a differential voltage between two [[Electrical conductor|conductors]]. If the voltages on these conductors are {{math|''U''<sub>1</sub>}} and {{math|''U''<sub>2</sub>}}, the common-mode signal is the [[Arithmetic mean|average]] of the voltages:

<math display=block>U_\text{cm} = \frac{U_1 + U_2}{2}</math>

When referenced to the local common or [[Ground (electricity)|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 (electronics)|Noise]] induced into a cable, or transmitted from a cable, usually occurs in the common mode, 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 generally designs 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. For [[Sensor|sensors]] with very high [[output impedance]] that require very high [[common-mode rejection ratio]], a differential amplifier is combined with input [[Buffer amplifier|buffers]] to form an [[instrumentation amplifier]].
* An inductor where a pair of signaling wires follow the same path through the inductor, e.g. in a [[bifilar winding]] configuration such as used in Ethernet magnetics.<ref>{{Cite web |url=http://pulseeng.com/products/datasheets/H325.pdf |title=Archived copy |access-date=2013-12-17 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304000938/http://pulseeng.com/products/datasheets/H325.pdf |url-status=dead }}</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>{{Cite web |url=http://pulseeng.com/products/datasheets/H325.pdf |title=Archived copy |access-date=2013-12-17 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304000938/http://pulseeng.com/products/datasheets/H325.pdf |url-status=dead }}</ref>

Common-mode [[Filter (signal processing)|filtering]] may also be used to prevent egress of noise for electromagnetic compatibility purposes:

* High frequency common-mode signals (e.g., [[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 cable|USB cables]] and [[HDMI cables]].<ref>{{Cite web |url=http://www.elmac.co.uk/pdfs/ferrite.pdf |title=Archived copy |access-date=2013-05-07 |archive-url=https://web.archive.org/web/20120706223753/http://www.elmac.co.uk/pdfs/ferrite.pdf |archive-date=2012-07-06 |url-status=dead }}</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 Understanding And Controlling Common-Mode Emissions In High-Power Electronics] {{Webarchive|url=https://web.archive.org/web/20201108100609/http://www.hottconsultants.com/pdf_files/APEC-2002.pdf|date=2020-11-08}}</ref>

[[Common-mode rejection ratio]] is a measure of how well a circuit eliminates common-mode interference.

==References==
{{reflist}}
{{FS1037C}}

[[Category:Electrical parameters]]
[[Category:Telecommunications]]

Common-mode signal - Revision history

RS-485: Imported from Wikipedia (overwrite)