[SI-LIST] Re: Book - (Chassis - signal ground)

Greetings All,
    Many are reticent to delve into "grounding" discussions because 
views on the subject
are varied, and so often defended with religious fervor rather than 
rational reasoning.
The real culprit is the presumption that ground, of any flavor, is a 
equipotential surface.
While this can be true at low frequencies it is absolutely not true at 
VHF and above for
anything but trivial sized systems, as Larry aptly noted.  Furthermore 
chassis ground and
logic ground are nearly always tied together anyway at the power supply 
end and the I/O
end of the system (because of I/O connector design).  I think it is 
instructive to avoid
making a rule about connecting, or not connecting, the two grounds 
together but rather
consider the resulting impact on signal coupling.  If using the chassis 
as a return path
for a high speed signal achieves the smallest loop area (I can't imagine 
it, but maybe)
then this is probably the way to go.  Alternatively if the high speed 
signal return path
is provided more intimately (read "smaller loop") in the PCB structure 
then connecting
the two grounds together increases your EMI liability.  In the limit 
case, where all
signal routes are perfectly coupled to their respective return paths, it 
actually doesn't
matter whether you tie the two grounds together or not.  My view is that 
the whole
issue can be readily resolved if approached from the perspective of  
promoting the
coupling paths that you want and avoiding the large RF loops that you 
don't want.
Think of "ground" as just another trace, with admittedly lower impedance. 
    Just my $0.0195 worth, after rebate.

John Howard
EMC Consultant


Larry Miller wrote:

> This is impossible in any but the smallest systems.
> 
> We have quite a number of EMI experts on this reflector; I am surprised that
> we are not hearing from them....
> 
> The problem is much more complex than the posts to this thread indicate.
> 
> Larry Miller
> 
> -----Original Message-----
> From: Ashok Babu K [mailto:k.ashokbabu@xxxxxxxxxxxxx]
> Sent: Thursday, June 21, 2001 11:55 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Book - (Chassis - signal ground)
> 
> 
> 
> I feel that if signal ground is to be connected to the chassis ground, then
> that connection should be done at one single point only. If connections are
> done at multiple points, that would eventually cause ground loops. Such
> ground loops will cause EMI radiations, ground bounce etc.
> 
> More discussions are welcome.
> 
> Regards,
> Ashok.
> 
> 
> ----- Original Message -----
> From: "Paul Hamilton" <paulh@xxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Friday, June 22, 2001 12:06 PM
> Subject: [SI-LIST] Re: Book - (Chassis - signal ground)
> 
> 
>> An issue to consider might be ESD (EMC electrostatic discharge)
>> susceptibility. A complex system could present several current paths for
> 
> ESD
> 
>> strikes which may be difficult to manage. Isolating signal ground from
>> chassis (and also ensuring that signal ground cannot take an ESD strike)
>> would be one way to guard against ESD hassles. I've had grief with a
> 
> simple
> 
>> clam style enclosure about the size of a telephone - and chassis was
>> isolated from signal ground except at one point.
>> 
>> Paul Hamilton.
>> 
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> 
> On
> 
>> Behalf Of Alex Horvath
>> Sent: Friday, 22 June 2001 9:34
>> To: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Re: Book
>> 
>> 
>> Someone recommended Henry Ott's book "noise reduction techniques in
>> Electronic Systems" and I have this book and didn't find it very useful
> 
> for
> 
>> chassis grounding (it's very old). He actually recommends separating
> 
> chassis
> 
>> and digital ground. I think this is based on how systems in the past often
>> had shielded cables going from one chassis to another. Often the ground
>> potentials are different in those systems and large currents can flow in
> 
> the
> 
>> shield and thus in the chassis. The large currents in the chassis can
> 
> cause
> 
>> noise and or the ground reference to change if the digital ground is part
> 
> of
> 
>> the chassis. Most telecomm systems I work with today don't have any
> 
> shielded
> 
>> cables or for that matter any ground connections between chassis. Also,
> 
> all
> 
>> of the telecom companies I have worked at have tied digital ground to the
>> chassis (also PC's are made this way).
>> 
>> snip ..
>> 
>> 
>> _____________________________________________________
>> Thorlock International Ltd.    enquiries@xxxxxxxxxxxx
>> +61 8 9351 9200 (tel)          +61 8 9351 9522 (fax)
>> 
>> 
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> 
> 
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