Common-mode signal: Difference between revisions

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

'''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 [[electrical engineering]], a '''Commoncommon-mode signal''' is the identical component of an [[analog signalvoltage]] which is present withat one sign onboth all consideredinput [[electricalTerminal conductor(electronics)|conductorterminal]]s of an electrical device. In [[telecommunication]], the common-mode signal on a [[transmission line]] is also 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

'''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.

:<math>U_{cm} = \frac{U_1 + U_2}{2}</math>

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''₁}} and {{math|''U''₂}}, the common-mode signal is the [[Arithmetic mean|average]] of the 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.

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

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.

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:

==Methods of eliminating common mode signals==

* 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;, i.e.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 will generally designdesigns electronic circuits to minimise or eliminate common -mode effects.

[[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.

==Methods of eliminating common -mode signals==

An inductor where a pair of signalling wires follow the same path through the inductor. Eg in a [[bifilar winding]] configuration such as used in Ethernet magnetics.<ref>http://ww2.pulseeng.com/products/datasheets/H325.pdf</ref> Useful for AC and DC signals, but will filter only higher frequency 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]].

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://ww2.pulseeng.com/products/datasheets/H325.pdf</ref>

* An inductor where a pair of signallingsignaling wires follow the same path through the inductor., Ege.g. in a [[bifilar winding]] configuration such as used in Ethernet magnetics.<ref>{{Cite web |url=http://ww2pulseeng.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://ww2pulseeng.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, for example(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.

== See also ==

* [[Common-mode rejection ratio]]

==References==

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[[Category:Electrical parameters]]

[[Category:ElectronicsTelecommunications]]