Hi Lee,
Can you provide an example where making multiple ground connections between a
backplane and chassis caused issues, and why? Section 9.9.1 of "Grounds for
Grounding" advocates stitching return planes to the chassis wherever possible.
Can you explain the flaw in their recommendation (which includes substantiating
figures from Bruce Archambeault), or the difference in your situation?
Thanks,
Jeff Loyer
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Behalf Of Lee Ritchey
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Subject: [SI-LIST] Re: An Article on what ground is
I use the sides of the card cage as part of my Faraday cage. This is what is
sometimes called chassis ground.
There are several books on EMI that instruct engineers to connect logic ground
in the backplane to the card cage and then to connect the faceplates to logic
ground. When this is done, the usual complaint is that EMI is "leaking" at the
cracks.
When I get an EMI call with this complaint I know what to do to fix it without
even going on site. But, as you know, if you solve the problem over the phone,
people think that this is free advice and won't pay for it, so I go on site and
solve the problem for a reasonable fee!
Another one of these bad rules is connecting logic ground in a PCB to the case
at every mounting screw.
Sadly, there are two books on EMI published under the IEEE banner that list
both of these methods along with about a dozen other "rules of thumb" that are
technically incorrect.
When I see an EMI book with either of these rules I expect it to have other bad
advice on EMI as well and recommend that my students not buy them..
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Behalf Of Loyer, Jeff (Redacted sender "jwloyer" for DMARC)
Sent: Saturday, January 7, 2017 10:32 AM
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Subject: [SI-LIST] Re: An Article on what ground is
Hi Lee,
Two questions:
1) Do you use a designated Faraday cage that is separate than the
chassis? That seems to be the implication if you distinguish between your
Faraday cage and "chassis ground". My guess is that you use the chassis as
your Faraday cage, but maybe not?
2) Can you provide an example where making multiple connections between
the backplane and the Faraday cage (chassis?) caused issues, and how? I.E.,
why would you insist on only a single connection in this instance? That sounds
very similar to what is recommended when parts manufacturers call for separate
"analog" and "digital" grounds, and is different from my understanding (I would
instead encourage more connections between the backplane ground plane and the
chassis).
Thanks,
Jeff Loyer
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From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Lee Ritchey
Sent: Saturday, January 07, 2017 9:57 AM
To: bbakshan@xxxxxxxxx
Cc: 'si-list' <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: An Article on what ground is
Thanks for the nice feedback.
Aha! You have asked an EMI question! I use the ground plane in a backplane as
one side of my Faraday cage. Where that backplane meets the flanges on the
card cage, I leave the copper on the top layer and connect that with vias to
the ground plane in the backplane. Now, there is the one connection between
logic ground and the Faraday cage. There can be no other connections such as
on face places if one is to avoid "leaking" at the cracks which is a common
complaint when more than one connection is made between logic ground and the
Faraday cage. Notice, I did not use the phrase "chassis ground" in this
discussion.
I have used this method in dozens of systems with very good EMI results.
Hope this helps.
Lee
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From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Boris Bakshan
Sent: Saturday, January 7, 2017 5:24 AM
To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx<mailto:leeritchey@xxxxxxxxxxxxx>>
Cc: si-list <si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>>
Subject: [SI-LIST] Re: An Article on what ground is
Hi Lee,
Very good, comprehensive article, thank you.
What is your recommendation when it comes to backplanes? a backplane is
attached to the chassis with bolts/other conductive mechanical parts. Do you
recommend plating these PCB holes in the backplane? If you do, should these
holes be connected to (any) electrical net?
Thanks Lee.
On Fri, Jan 6, 2017 at 10:43 PM, Lee Ritchey
<leeritchey@xxxxxxxxxxxxx<mailto:leeritchey@xxxxxxxxxxxxx>>
wrote:
I have written an article on ground following the brisk exchange on
this subject last month. I hope it clears up some of the confusion
around this subject.
Here is where it is on the web.
http://ubm.io/2ikEDat
Lee Ritchey
Speeding Edge
P.O. Box 2194
Glen Ellen, CA
95442
707-568-3983
I just used the energy it takes
To get mad and wrote some blues.
Count Basie
Or:
Worry is like a rocking chair,
It keeps you busy but.
It doesn't get you anywhere.
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