Scott, Tightly coupled pairs (if we can consider 10-20% coupling between traces as tight) have other advantages besides providing a current return path. Keeping the traces close together helps cancel out crosstalk induced by a nearby aggressor net. If the pair is spaced 5 mil edge-to-edge, then it's reasonable to assume the crosstalk magnitude will be approximately equal on each line. If the pair is spaced far apart then the crosstalk on one line will be greater than on the other, as the magnitude falls off with distance, and the noise is no longer common mode; now there is noise on your differential signal. Another benefit of keeping differential pairs close is that they're just easier to route, making it easier to keep the trace lengths equal and reduce skew. (Eric Bogatin's book has an extensive chapter on differential pairs that I've used as a reference in the past, it addresses the coupling issue in some detail.) - B -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Scott.Nixon@xxxxxxxxxxxx Sent: Thursday, April 20, 2006 5:29 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Question regarding return current in a differential pair Tom, If the coupling between a differential pair is only about 20% on a PCB, are the benefits of using differential pairs really that significant? The signal is still 80% coupled to the reference plane. I'm a student and I just want an idea of how this is justified in a practical application. Much Thanks, Scott Nixon ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu