[SI-LIST] Re: Question regarding return current in a differential pair
- From: steve weir <weirsi@xxxxxxxxxx>
- To: bfields@xxxxxxxx,<Scott.Nixon@xxxxxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Thu, 20 Apr 2006 21:38:50 -0700
Brian, I agree on the value of Eric's book. But I think you are
drawing very different conclusions from it than I do. Consider that
if 20% ( n optimistic number ) of the energy is coupled between the
two lines then that is also the extent of cross talk attenuation the
pair exhibit against an aggressor versus just one or the other
line. I think that debunking the notion that a diff pair on a PCB
markedly improves either radiation or susceptibility is one of Lee's
favorite topics for good reason.
A diff pair that doesn't skew too badly carries its own switching
reference, is DC neutral and has twice the signal energy for a given
constant p-p voltage swing on each of the constituent signals. Those
are each compelling features by themselves. Together, they are hard
to beat. Opinions vary on the value of tight versus loose
coupling. All things considered, if you have the space, loose
coupling is easier to make work right. You still want to keep each
trace in the pair in the same electrical environment. It has been my
experience that tight coupling is primarily born from density
requirements, not performance superiority or ease of implementation.
Steve
At 05:18 PM 4/20/2006, Fields, Brian wrote:
>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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