RE: [SI-LIST] : Routing Differential Pairs as 100 differentially Vs individually 50 ohm lines

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Wed Nov 22 2000 - 13:46:37 PST


...
> Routing them close to each other produces interaction that reduces the
> final amplitude.
...

This "interaction" you speak of is the reduction in differential impedance
as the two lines are brought into one another's neighborhood. Simple
physics. You got reduced amplitude because your lines were less than 100
ohms differential, and if you didn't take that into account, you probably
mismatched the line's impedance.

Two widely separated 50 ohm lines give 100 ohm differential impedance.
Two less separated 60 ohm lines give 100 ohm differential impedance.
Two even less separated 70 ohm lines give 100 ohm differential impedance.
And so on.

What's different between these, is the common-mode impedance. Matching the
differential impedance is the first, and most important, step, but you often
want to look at how (or whether) you matched the pair's common-mode
impedance with the terminations at one or both ends, which affects
common-mode noise susceptibility.

There is nothing "wrong" about routing differential pairs close so they
"interact" with one another. After all, that's what a twisted pair cable
is. The wires are close to each other, producing interaction, and a lower
impedance than if the two wires were far apart.

Andy

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