Re: [SI-LIST] : Differential Signals

Mike Jenkins (jenkins@lsil.com)
Wed, 21 Oct 1998 15:37:12 -0700

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Susan,

Since dif'l signals carry their own return current, they survive
the trip across split planes much better than single-ended signals.
What ground currents are induced (depending on the distance between
the dif'l pair lines and their spacing to the ground plane) loop
back on themselves when a split in the ground plane is encountered,
and another such "racetrack of ground current" forms on the far
side of the split.

To look at it another way, the dif'l mode impedance goes up from
the case of the pair's cross section WITH the ground plane to the
case of the pair's cross section WITHOUT a groundplane. This
increase is usually small and applies only for the distance across
the split. The common mode impedance, of course has the same
large increase, EMI problems, etc., as single-ended signals do
when they encounter a split ground plane.

Others may have a less sanguine view of the consequences, but
that's my opinion based on a number of applications.

Regards,
Mike

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