Re: [SI-LIST] : Differential Impedance Effects on Diff. Return Current

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From: Martyn Gaudion (martyn@gaudionm.freeserve.co.uk)
Date: Wed Jan 03 2001 - 15:03:28 PST


Dear Franz,

Yes you are correct, if a differential pair are routed with very close
spacing the return current
is through the other "leg" of the pair.

Move the spacing further apart and if you are between planes you reach a point
where there is no coupling between the pair - and all the return current is
through the ground and power planes. Once you reach this spacing you have lost
many of the benefits of the differential pair.

You can see where the coupling reduces to zero by using a field solver
like Polar CITS25, - put some numbers in for a differential pair then move
the pair further apart until they are uncoupled..

There is further information in the newly revised booklet on controlled
impedance
PCBs which you can download as a PDF from www.polarinstruments.com

Kind regards
Martyn Gaudion
Polar Instruments
Tel + 44 1481 253081
email martyn.gaudion@polarinstruments.com

At 14:20 03/01/01 -0800, you wrote:
>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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