RE: [SI-LIST] : Differential Impedance Effects on Diff. Return Cu rrent

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From: Dunbar, Tony (tony_dunbar@mentorg.com)
Date: Wed Jan 03 2001 - 16:23:01 PST


Franz,

In the ideal world, the first statement is valid. It is also usually the
intended operation that the designer is aiming for and assuming when a
(tightly-coupled) diff-pair is implemented. Any signal will always find its
way to a destination via the path of least impedance. This fact is most
often related to when considering the signal's return path. If the return
path through the power/ground planes offers a lower impedance path than any
other path, including the complementary trace of a diff-pair, then that
least-impedance path will be used at any points along the way or way back.

I think a key thing to keep in mind is to consider this in the
electromagnetic realm rather than simply the AC since it is the
electromagnetic coupling from the primary signal conductor to the neigboring
conductive elements within the electromagnetic field that dictates what the
return path elements will be.

I was going to move on and separately address your follow-up question but I
think I may have covered that already with the answer above (??).

Regards,
Tony

-----Original Message-----
From: Dill, Franz @ Celerity [mailto:fdill@celerity.l-3com.com]
Sent: Wednesday, January 03, 2001 4:20 PM
To: 'SI-Mailing List'
Subject: [SI-LIST] : Differential Impedance Effects on Diff. Return
Current

All,

Please excuse my 'newbie-like' questions, my inclusion in this mailing list
is more for curiosity and personal advancement/understanding than as a
profession.

First, is this statement valid?
- In a differential pair, one 'leg' of a signal's return current path is
through the complementary 'leg' of a differential pair and not through the
ground or power planes (Assuming equal trace lengths, Zo=50 single-ended,
Zo=100 diff. impedance - using ECL logic as an example).

Now, assuming the above statement is true:
If the differential impedance is NOT 100 Ohms (Differential traces NOT
routed differentially) how does this effect the return current path? Does
the return current begin to flow through the ground and power planes rather
than through the differential pair?

Thanks SI gurus!

Franz.

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