From: Doug Brooks ([email protected])
Date: Wed Jan 03 2001 - 15:12:09 PST
At 02:20 PM 1/3/01 -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?
First, change the question slightly to: If the SINGLE ENDED TRACE
IMPEDANCES ARE NOT EXACTLY MATCHED (it doesn't particularly matter what the
values are, it matters whether they are matched) then the currents down
them won't be exactly equal (and opposite) and therefore the difference in
current will flow through (through the device and to) another return path
(presumably ground).
Doug Brooks
.
************************************************************
Doug Brooks' book "Electrical Engineering for the Non-Degreed
Engineer" is now available. See our web site for details.
.
Doug Brooks, President [email protected]
UltraCAD Design, Inc. http://www.ultracad.com
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