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[SI-LIST] Re: Ground plane split widths...

From: "Johnston, Ross" <RJ182000@xxxxxxxxxxxxxxxxxxxxxxxxx>
To: Alan Hilton-Nickel <ahilton@xxxxxxxxxxxxx>
Date: Wed, 7 Aug 2002 07:55:12 +0100

Hi Alan,

The I/O being isolated is standard PC I/O (ie. VGA connector, serial,
parallel, audio). The thinking behind completely isolating the IO ground
from main logic ground is threefold:-
1. To stop noise coming out from the logic ground onto the I/O signals.
2. To stop noise entering into the logic ground from the I/O cabling etc.
3. To provide isolation from ESD and external EMC events.

I am aware that there would be an issue of traces crossing plane splits with
respect to return currents. However, I have stitching capacitors fitted
across the split at the point where traces cross the split (to provide a
path for the return current).

My board is a standard 4 layer motherboard with tracking on the top and
bottom layers and power and ground in between.

CHEERS

ROSS
-----Original Message-----
From: Alan Hilton-Nickel [mailto:ahilton@xxxxxxxxxxxxx]
Sent: 06 August 2002 22:55
To: Johnston, Ross
Subject: Re: [SI-LIST] Ground plane split widths...

Ross,

Unless you have a desire for pain, I'd recommend *reducing* the split
width...to
zero.

I don't know what I/O you are trying to isolate, but the return currents
that
would normally go through the ground planewill just find another route -
probably through your chassis. This opens up a big inductive loop. Even if
it
doesn't result in bandwidth limitation due to signal integrity violations,
it
will cause your board to radiate EMI.

Regards,
Alan Hilton-Nickel
Transmeta Corp
Santa Clara, California

"Johnston, Ross" wrote:
> 
> Hi
> 
> My design is basically a pc motherboard. I have isolated the I/O ground
from
> the logic ground. Currently the ground plane split is 15 thou - I am
> contemplating increasing this width. Should the split be kept as small as
> possible? If so - why? If the ground split was too small, wouldn't noise
be
> coupled across from each plane? What is the general consensus on this?
> 
> CHEERS
> 
> Ross Johnston
> Electronics Design Engineer,
> Core Electronics Group,
> Lifecycle GST,
> NCR FSG (Scotland) Ltd,
> Discovery Centre 2nd Floor West,
> 3 Fulton Road
> Dundee,
> Scotland DD2 4SW
> Tel: +44 (0)1382 592920 Direct Dial
> Fax: +44 (0)1382 591089
> email: ross.johnston@xxxxxxxxxxxxxxxx
> 
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[SI-LIST] Re: Ground plane split widths...
From: Alan Hilton-Nickel

[SI-LIST] Re: Ground plane split widths...

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