**From:** Greim, Michael (*[email protected]*)

**Date:** Mon Jan 10 2000 - 11:58:50 PST

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Joel Said:

BTW, WRT the guy who said to route two 50 ohm lines instead of a 100

ohm

differential pair... two 50 ohm lines as close together as

differential

pairs usually are is going to produce something more along the lines

of

90

ohms of differential impedance, I believe.

Hi Joel,

I believe it was Lee Ritchey who brought up this topic. He

has also written a paper on this topic (differential signaling

doesn't require differential impedance). The quickest way

to describe this is as follows: Where the following comes from

is through simplifications of the matrix equations:

Z diff = Z(11) Z(12)

Z(21) Z(22)

The first simplification occurs is that since the pair are routed

with the same routing rules in the same stack up, the equation

simplifies to the following.

Z diff = 2 ( Z(11) - Z(12))

As you stated below, if the lines are close together, the mutual

coupling reduces the impedance of the line. However......

As the separation between the two traces becomes large in

comparison to the height above the plane. the mutual coupling

approaches zero. This is sometimes refered to weak coupling

model. Therefore the equation simplifies to:

Z diff = 2*Z(11) or 2 * Zo

For traces routed with a 50 ohm impedance this effectively yields

a 100 ohm differential impedance.

Best Regards,

Michael

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**Next message:**Ray Anderson: "[SI-LIST] : message foul-up"**Previous message:**[email protected]: "Re: [SI-LIST] : Clamp diodes in models (was Input switchingthreshold & CPCI)"**Next in thread:**Lee Ritchey: "Re: [SI-LIST] : Physcially-small far-end LVDS terminations?"**Reply:**Lee Ritchey: "Re: [SI-LIST] : Physcially-small far-end LVDS terminations?"

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