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'[EE]: Chassis Ground'
2003\02\14@105125 by Madhu Annapragada

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Your question has the been the subject of quite a few chapters in quite a
few books. The best reference to this topic I found was the High Speed
digital design book by Howard Johnson; another good book is Printed Circuit
Board Design Techniques for EMC Compliance by Mark I. Montrose. If I can
summarize the extensive discussions in a couple of lines it would be this;
and I quote from Montrose's book verbatim: " For systems that are low
frequency" (which is a little fuzzy but I usually get away by defining it
as a maximum system clock of 10MHz which has worked pretty well for me so
far) "a low impedance connection between logic ground and chassis ground
may not only cause electromagnetic interference, but may also prevent
proper functionality of the circuit". For high frequency. multi-point
grounding is mostly a necessity. IF one can tie the logic ground to the
chassis then this needs to be done using RF bonding techniques (secure
bracket, faceplate grounding etc.) You need to have a continuous or at
least numerous bonding points between the logic ground and the chassis. IF
you cannot tie the logic ground to the chassis then you are better off
isolating your logic ground from the chassis and instead have a large
ground plane on a multi-layer PCB. I have found that using safety rated
caps between the logic ground and the chassis when they are isolated from
each other seems to be well accepted at the EMC testing labs I have been
to. To summarize, the standard industrial practice of tying the logic
ground to the chassis at a single point using a 20AWG wire doesn't seem to
make any sense does it? If you are at a low frequency then you are creating
more problems than you are solving. If you are at a high frequency then
that single grounding point is not going to help much. If you are somewhere
in the middle then a nice trick is to use  numerous caps between the PCB
ground and the chassis connection. At low frequencies you do not have any
connection between the PCB and the chassis (and your PCB ground plane will
be enough from an emissions point of view) and as your operating
frequencies increase, the caps would start to form numerous connection
points between the PCB ground and the chassis. I would strongly recommend
the books, lots of nice math to make sense of things
Madhu
{Original Message removed}

2003\02\16@060311 by Biswanath Dutta

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source= www.piclist.com/postbot.asp?id=piclist\2003\02\14\105125a
Thanks Madhu

I found this connection between circuit ground and chassis ground in an AB PLC.
For capacative coupling to chassis ground, what would be the type and value of capacitors

Biswanath




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Biswanath  Dutta
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2003\02\19@154525 by Madhu Annapragada

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You want to pick a value for the capacitor that will start to exhibit a low
impedance to ground at the frequencies you are interested in; typically
above 10MHz.
Madhu
----- Original Message -----
From: "Biswanath Dutta" <spam_OUTbiswanath_duttaTakeThisOuTspamHOTMAIL.COM>

I found this connection between circuit ground and chassis ground in an AB
PLC.
For capacative coupling to chassis ground, what would be the type and value
of capacitors

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