Common-mode signal: Difference between revisions

Revision as of 10:25, 7 May 2013

Revision as of 11:02, 7 May 2013

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* 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.

Common-mode signals may be effectively eliminated by using [[differential amplifier]]s or receivers that respond only to voltage differences, ''e.g.,'' those between the wires that constitute a pair. They may be filtered using an inductor where a pair of signalling wires follow the same path through the inductor. Eg in a bifilar winding configuration, or as in the case of Ethernet, terminate in a transformer.<ref>http://ww2.pulseeng.com/products/datasheets/H325.pdf</ref>

==Methods of eliminating common mode signals==

High frequency common mode signals, for example, 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>

[[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. 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.

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>

Common mode filtering may also be used to prevent egress of noise for electromagnetic compatibility purposes;

High frequency common mode signals, for example, rf[[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>