RE: [SI-LIST] : Plane Splits Inspection

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Fri Nov 10 2000 - 08:55:17 PST


> Have you tried putting caps not across the split, but from the edge of
> every
> split to the solid plane?
> By putting them across the split you, though eliminating discontinuity,
> defeat the effect of split plane (planes are split not just for the sake
> of
> voltage difference but also for noise reduction, like analog/digital,
> hi-speed/digital, etc), introducing the noise from more noisy plane to the
> quiet one.
 
Sometimes planes are split just because the DC voltages differ, but are the
same AC-wise.

Other times they are split because of analog/digital.

In the latter case, I would suggest that in many cases the WRONG thing to
do, is to put caps from either plane to the solid plane. In fact, there
probably shouldn't even be a solid plane! The split should go uniformly
through all planes in the board layup.

Given the excellent high frequency coupling from both half-planes to the
solid plane, one could argue that the two half-planes are already very well
coupled to one another by way of the solid plane anyway, so why bother
splitting them (except for DC)?

> However, putting caps along edges from edges to Solid plane, you provide
> almost just as good return path without coupling the noise.
 
Sorry, I don't think so. Adding caps to the same solid plane provides a
very good path for noise to get from one to the other.

If you have something like a "quiet" analog +5V and a "noisy" digital +5V,
and both are adjacent to the same ground, they will have nearly the same
noise (or lack thereof, depending on your point of view) as exists on the
ground plane. You might as well have used a solid +5V plane. And you
should NEVER pass signals that switch across a split that is there for noise
isolation, because they will add a lot of noise.

Of course there can be exceptions.

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