[SI-LIST] : Re: Differential Impedance

Mike Jenkins (jenkins@lsil.com)
Thu, 05 Feb 1998 14:35:27 -0800

Mark,

The English answer to your question is that differential impedance
is the impedance of one trace to ground (i.e., "single-ended
impedance") plus the impedance of the other trace to ground minus
something to account for any coupling between the two traces.
If the traces are sufficiently separated (more than about three
times the trace-to-ground distance), then you can forget the
coupling part.

The math answer is a 2x2 matrix equation: ____
I1---> |
| V1 | | Z11 Z12 | | I1 | V1 o----------| black
| | = | | * | | |
| V2 | | Z21 Z22 | | I2 | V2 o----------| box
I2---> |____
"Differential" means I1 = -I2. (Some may
disagree, but this works.) Dif'l impedance is (V1-V2)/I1. If you
plug into the matrix equations, that equals Z11 + Z22 - Z12 - Z21.

For any passive "black box", Z12 = Z21. For a symmetrical system
(I hope yours is), Z11 = Z22. Hence Z(dif'l) = 2 x (Z11-Z12).
if the traces are sufficiently separated, Z12 is negligible,
so Z(dif'l) = 2 x Z(single ended).

Good luck, Mark.

Mike

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