Re: [SI-LIST] : Q: Plane-jumping return currents

Douglas C. Smith (doug@dsmith.org)
Wed, 22 Sep 1999 18:55:21 -0700

Hi "D.C."

I just wanted to restate my observation that it is difficult to obtain
a balance coefficent more than 20 dB. That will leave about 10% of the
differential current flowing in the ground plane as a common mode
current. From an EMC point of view, this could be a problem.

Doug

"D. C. Sessions" wrote:
>
> Eric Goodill wrote:
> >
> > Mike Jenkins wrote:
> > >
> > > Eric,
> > >
> > > One line of your question, "My system is running pretty fast
> > > (> 1 Gbps)", caught my eye. At that speed, which I assume might
> > > be Fibre Channel or Gigabit Ethernet, you may well be running
> > > differential. (If not, good luck to you.) But if your lines
> > > are dif'l, they carry their own return current. Depending on
> > > geometry, there is some discontinuity, but MUCH less than
> > > single-ended. If your lines are, in fact, differential, and
> > > if you wish me to elaborate, I will.
> >
> > Mike,
> >
> > Yes, differential. However, we're using edge-coupled pairs, and it's my
> > understanding, though I've done no analysis, that about 10% - 15% is about
> > as much coupling as you can get between edge-coupled lines. Thus, there is
> > still a strong coupling between the trace and it's reference place.
> > Therefore, I suspect that there's non-ignorable amount of return current in
> > the reference planes. I'd be interested to see a
> > return-current-distribution plot for a diff pair both in the reference
> > planes and the coupled traces.
>
> I don't think so. Sure, there's a fair bit of capacitive current between
> each trace and the adjacent plane, but since they're equal and opposite
> the loop is very small and entirely lateral. Cross a plane boundary and
> there's no need for any current across the break.
>
> --
> D. C. Sessions
> dc.sessions@vlsi.com
>
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