# Re: [SI-LIST] : Physcially-small far-end LVDS terminations?

From: Lee Ritchey ([email protected])
Date: Tue Jan 11 2000 - 08:19:12 PST

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

Greim, Michael wrote:

> 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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