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

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From: Dill, Franz @ Celerity (@)
Date: Wed Jan 03 2001 - 14:20:09 PST


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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