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

D. C. Sessions (dc.sessions@vlsi.com)
Wed, 22 Sep 1999 17:35:16 -0700

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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