What about mixed supply voltage boards?
We have boards with digital (5VDC) and analog (+/-15VDC) sections where the
power planes are split for the two sections. We usually also have a plane
for analog ground and one for digital ground which are tied together at the
ground connector, and two supply planes which are split between the 5VDC
digital supply and the analog supply. Is there a better way to do this on
a 10 layer 0.062 board?
On the other hand I do agree that splitting the ground plane in most cases
is almost as bad as the stupid design we have where they eliminated the
analog plane in one area to route analog input signals.
I have used used cuts to force the return currents to go through one
section of the plane and had it work well. An example of this was
separating the power supply so the power and ground returns went back to
the supply at the same spot at the output cap. The only other places I use
split planes on the ground plane is to provide a barrier to the connectors
with chassis ground on the connector side and digital ground on the board
side.
Rob
"Dennis Saputelli" <[EMAIL PROTECTED]> on 06/09/2001
02:19:07 PM
Please respond to "Protel EDA Forum" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
cc: (bcc: Rob LaMoreaux/DSPT)
Subject: Re: [PEDA] split planes no more?
interesting article in the new 'printed circuit design' mag (june 2001)
by Henry W. Ott (EMI consultant)
title: 'partitioning and layout of a mixed signal pcb'
in it he basically makes the case that split planes are generally a bad
thing
their best use he says is to correct a badly laid out board after the
fact and also in some cases they are needed for safety isolation
but as to localizing noise (preventing polution of low level signals)
and minimizing EMI nothing beats a *properly laid out* (i.e.
'routed/placed') board with a single ground plane possessing a single
net
he also advocates something i learned in a recent job which is much
easier to do than full splits and separate nets, namely ground 'cuts'
these are breaks in the plane so that currents are forced to circulate
locally, then a 'bridge' (plane section) is created over which traces
are passed which connect the sections
i won't go into all that he has covered but as the split planes are so
pervasive and enough of a nuisance to implement i thought i would throw
this out there
Dennis Saputelli
--
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