Common-mode signal: Difference between revisions

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'''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, thedesired signalsignals isare 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:

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

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* 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 {{BareUnderstanding URLAnd PDFControlling 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=March 20222020-11-08}}</ref>

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