Common-mode signal: Difference between revisions

Revision as of 13:19, 8 August 2020

Revision as of 16:53, 30 March 2021

Line 12:

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.

* 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>{{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</ref>

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