[SI-LIST] Re: Question regarding return current in a differential pair

  • From: "Fields, Brian" <bfields@xxxxxxxx>
  • To: <Scott.Nixon@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 20 Apr 2006 18:18:50 -0600

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
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