[SI-LIST] Re: Chassis and Digital grounds connection

Hi All,

Significant ESD currents will flow in the signal ground whether you 
connect it to chassis or not. 2kV/ns*1pf= 2 amps of current! Take a 
look at http://emcesd.com/tt2002/tt050102.htm for some data that 
suggests the more connections the better for at least some 
configurations with respect to ESD.

By far over the last 25 years (since my first ESD problem), I have 
found that connecting signal and chassis grounds at several points 
improves ESD performance on average.

Since it can be difficult to predict where ESD currents flow 
(remember, everthing is connected at high frequencies, so you might as 
well have some control over the connections) I suggest the following 
methodology.

1) The marketing people have probably determined the physical package 
before the first prototype boards are back. So, make a circuit board 
out of a sheet of metal, to simulate a multi-layer board, and mount it 
in the enclosure. Take your best guesses at where to connect the 
"board" to the chassis. Make current (actually di/dt) measurements at 
the chassis connections as per the web article above.

2) Either change the grounding points or move sensitive circuitry so 
that the sensitive circuits are not where the highest density of ESD 
currents flow. For most of us, you cannot trust your intuition on 
this. I am still occasionally surprised when I make measurements on 
systems. There are just too many parasitics (a picofarad here and a 
nanohenry there).

Make the measurements and an informed decision at the early (metal 
board) prototype level. Making measurements to supplement intuition 
and calcualations have made my clients very happy.

It is too simplistic to say that ESD currents only flow where you put 
connections..... My website is full of ways to make these kind of 
measurements.

Doug

BRanjul@xxxxxxxxxxxxxx wrote:
> Dear steve.. 
> Happy new year to all ( wishing "signal integrity and quality " through 
> out the year ! )
> 
> Steve, when you short circuit the signal ground with earth, what is the 
> possibility of encroaching the ESD to the circuit- which normally 
> discharged through the shield ?
> Or in another way, isolated ground and shield or short circuited ground 
> and shield - which will be good for the circuit ; while viewing through 
> the eyes of ESD
> 
> 
> Thanks and Regards,
> 
> Ranjul.B.
> 
> 
> 
> Thanks and Regards,
> 
> Ranjul.B.


-- 
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     ___          _       Doug Smith
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       =========          Los Gatos, CA 95031-1457
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|  q-----( )  |  o  |    Email:   doug@xxxxxxxxxx
  \ _ /    ]    \ _ /     Website: http://www.dsmith.org
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