RE: [SI-LIST] : Bead-choking the power supply--everywhere!

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From: mcrozman ([email protected])
Date: Sun Sep 17 2000 - 10:02:16 PDT

> Subject: RE: [SI-LIST] : Bead-choking the power supply--everywhere!
> I'll 'see' Mike - overkill, and raise him one. Just use the ferrites. The
> caps bypass the noise into the ground planes making them noisy, polluting
> everything else... <grin> and besides, you can't get enough caps anyway.

Maybe someone could correct me, but I thought that if power and ground planes
were used, the idea was to maximise the capacitance between the planes to get
a good decoupling supply and to minimise lead inductance from the IC package
to the planes. Won't adding ferrites between the IC packages and the planes just
act to increase inductance between the package and the supply leading to nasties
within the package itself (ie exhibit symptoms of the supply to the package itself
dipping when switching events happen)

I could be wrong as Im fairly new to this game, but I thought that from the work
that was done at Sun and by others showed that boards with contiguous planes
didn't actually need the traditional one cap per IC, but rather needed a strategy
that was applied to the board as a whole for decoupling purposes. And that the
location of the bypassing caps on the board was not important.

The power system needs to be adequately bypassed at all frequencies up to (and past)
the fastest switching edge on the board. This would mean that the decoupling
system as a WHOLE as frequency increases must remain capacitive and not become
inductive as various caps go through resonance.

Could people comment - is my assumption correct?

Marque Crozman
Innotech Control Systems
Brisbane, Australia
(swamped by the olympics)

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