**From:** Lee Ritchey (*leeritchey@earthlink.net*)

**Date:** Tue Jan 11 2000 - 08:19:12 PST

**Next message:**Hans Mellberg: "[SI-LIST] : Re:Santa Clara Valley EMC Meeting Notice 11Jan2000"**Previous message:**Lee Ritchey: "Re: [SI-LIST] : What's your favorite Screwy SI Concept?"**In reply to:**Greim, Michael: "RE: [SI-LIST] : Physcially-small far-end LVDS terminations?"**Next in thread:**Tom Dagostino: "RE: [SI-LIST] : Physcially-small far-end LVDS terminations?"

The explaination by Michael below is the basis of my termination rule in the paper cited.

Greim, Michael wrote:

*> Joel Said:
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*>
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*> BTW, WRT the guy who said to route two 50 ohm lines instead of a 100
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*> ohm
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*> differential pair... two 50 ohm lines as close together as
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*> differential
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*> pairs usually are is going to produce something more along the lines
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*> of
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*> 90
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*> ohms of differential impedance, I believe.
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*>
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*> Hi Joel,
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*>
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*> I believe it was Lee Ritchey who brought up this topic. He
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*> has also written a paper on this topic (differential signaling
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*> doesn't require differential impedance). The quickest way
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*> to describe this is as follows: Where the following comes from
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*> is through simplifications of the matrix equations:
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*>
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*> Z diff = Z(11) Z(12)
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*> Z(21) Z(22)
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*>
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*> The first simplification occurs is that since the pair are routed
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*> with the same routing rules in the same stack up, the equation
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*> simplifies to the following.
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*>
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*> Z diff = 2 ( Z(11) - Z(12))
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*>
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*> As you stated below, if the lines are close together, the mutual
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*> coupling reduces the impedance of the line. However......
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*>
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*> As the separation between the two traces becomes large in
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*> comparison to the height above the plane. the mutual coupling
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*> approaches zero. This is sometimes refered to weak coupling
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*> model. Therefore the equation simplifies to:
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*>
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*> Z diff = 2*Z(11) or 2 * Zo
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*>
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*> For traces routed with a 50 ohm impedance this effectively yields
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*> a 100 ohm differential impedance.
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*>
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*> Best Regards,
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*>
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*> Michael
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*>
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**Next message:**Hans Mellberg: "[SI-LIST] : Re:Santa Clara Valley EMC Meeting Notice 11Jan2000"**Previous message:**Lee Ritchey: "Re: [SI-LIST] : What's your favorite Screwy SI Concept?"**In reply to:**Greim, Michael: "RE: [SI-LIST] : Physcially-small far-end LVDS terminations?"**Next in thread:**Tom Dagostino: "RE: [SI-LIST] : Physcially-small far-end LVDS terminations?"

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