# Re: [SI-LIST] : Differential impedance

Danwei Xue ([email protected])
Wed, 11 Nov 1998 18:23:41 -0800

1) Suppose a transmission line pair consists of two coupled traces and a ground plane. The odd mode impedance
Zoo of the coupled traces is the impedance from the positive voltage trace to the negative trace with the exist of
the ground plane. Therefore, the odd mode impedance IS the differential impedance, i.e.

Zdiff == Zoo.

where I assume it is the definition.

2) The even mode impedance Zee is defined as the impedance from the traces to the ground when traces are both
with the same positive voltage.

3) When one of the trace is grounded, then the impedance of the other trace, say Z1, is
Z1 = square root of (Zoo * Zee).

4) If the traces are not coupled, i.e. they are farther apart or they are at the opposite side of the ground plane, then
Zoo = 2 * Z1.
Zee = 0.5 * Z1.

Please let me know if I am wrong.

Danwei

fabrizio zanella wrote:

> To calculate the differential impedance of a transmission line pair, I've
> always used
> 1) Zdiff = 2 * Zoo, where Zoo is the odd mode impedance of the pair. This
> formula is used by the pc board manufacturers.
> I was speaking to someone last week who has 30+ years in the industry and
> said this formula is wrong, that the real differential impedance should be
> calculated as
> 2) Zdiff = square root of (Zoo * Zee), where Zee is the even mode
> impedance.
>
> If I compare 1) to 2), I get quite different values of differential
> impedance. Does anyone have comments on this?
>
> Thanks, Fabrizio Zanella
> EMC corporation
> [email protected]
>
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