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

  • From: Doug Brooks <doug@xxxxxxxxxx>
  • To: mkotson@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Thu, 20 Apr 2006 09:58:02 -0700

In my humble opinion, and not counting common mode currents:

During the signal rise and fall times, the return current tends to flow on 
the reference plane, just as signals on single-ended traces do.

During the time that the signal is "stabilized," there is no coupled signal 
on the plane and the loop is around from one trace of the differential pair 
to the other.

It is during this latter phase of the signal that loop area (as in EMI) 
might be an issue. During my signal integrity seminars I show some 
animations that illustrate this pretty clearly.

Doug Brooks


At 09:46 AM 4/20/2006, you wrote:
>Hello Gurus,
>     I have a question regarding the return current for a high speed
>differential signal.  I've seen a couple of sources that state that all
>of the signal return current is handled by the other half of the
>differential pair. It seems more logical to me that there must be some
>amount of return current (not just common mode) on the reference
>plane(s) as well.  What's the truth?
>Thanks in advance,
>
>Mike
>
>Michael Kotson
>Project Engineer
>VT Miltope
>4900 Pearl East Circle Suite 106
>Boulder, CO 80301
>(303) 473-0388 x132
>http://www.miltope.com <http://www.miltope.com/>
>

____________________________________________________________________________-
Check out UltraCAD's differential impedance and skin effect calculators at 
http://www.ultracad.com 

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