[SI-LIST] Re: Split planes and ground return wires
- From: James_R_Jones@xxxxxxxx
- To: marc.godbout@xxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Wed, 19 Feb 2003 09:59:08 -0600
Marc,
In my experience with signals of < 25 MHz, you should be able to get away
with just about anything. I tend to think that there is a good possibility
that the system will work fine with one ground wire. That being said, I
would be more comfortable selling a system with ground wires at a 1:10
ratio. I might consider using the same size wire that you are using for all
of the data signals. I don't know of a very scientific approach for finding
the correct wire size, but I might recommend a trial and error method to
determining how many ground wires to use. For example, I might hook the
system up with one ground wire initially, and then measure the data wire
furthest from the ground... note the ringing. Now I might hook up more
ground wires, remeasure. etc. At some point you will have grounding
sufficient to satisfy your design requirements. I haven't seen a lot of
good documentation on the subject, but I have had personal experiences with
this very concern.
James R. Jones
Dell
-----Original Message-----
From: Godbout, Marc [mailto:marc.godbout@xxxxxxxxxxxxxx]
Sent: Wednesday, February 19, 2003 8:57 AM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Split planes and ground return wires
I've got questions concerning ground return wire requirements and hope the
list can help. First the configuration.
Our system as got boards that are physically two separate PCB's, about 6" X
7", sandwiching, and bonded to, a metal plate. Signals need to traverse from
one PCB to another. To accomplish this, we run wires through holes in the
plate. These signals are basically microprocessor-type signals, 5V CMOS. The
bus speeds are 25MHz or lower.
Ground returns are added at about a 10:1 ratio, and placed somewhat near the
groups of signals. The ground planes are also connected to each other
through the backplane connector through 40 low-inductance pins.
So I see this as a not-quite classical split plane problem. The split is
crossed by 40 connector wires and maybe 15 z-axis wires, each 0.200" long.
We've got some mechanical constraints that are making me look into the
minimum requirements of the ground return wires. I feel that the "bigger is
better" theory was used to design them in the first place, but I've got to
cut back on that right now. So the question is, by how much?
So what are the areas or major concerns that I should investigate? How do I
go about calculating a minimum wire size? And are there any texts that would
help me zero in on my analysis methods?
Thanks for any help you could give.
-Marc
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