[SI-LIST] chassis ground connections

Hi, 

could someone of you tell me the best way to connect the chassis ground to the 
supply ground of the PCB? I`ve read a few books and pdfs discussing this 
subject, but the methods are very different. 

The PCB contains two connectors to other PCBs, one ethernet connector and the 
power supply connector. Many manufactorer like Ti, SMCS have their application 
notes describing the way to connect the chassis ground for the ethernet 
connection. They suggest using an capacitor (2kV, approximately 1nF) and no 
direct connection between the supply - and the chassis ground. Micron also 
suggest to use one or two capacitors to connect them between the two grounds 
near by the transformer - for better esd protection. 

Johnson Howard wrote in one of his articles about this subject that each 
connector should have one direct connection (no use of capacitors) to the 
chassis ground in order to avoid antennas. It`s not possible to use both 
suggestion - direct connection / connection via a capacitor. 

Another point is that each capacitor has its impedance; but the impedance of 
the supply ground plane / chassis ground plane is very low. Could you give me 
some more information about the maximal impedance of the capacitor (when using 
an capacitor to connect both grounds on each connector and the power supply 
connector)? Is it maybe better to use an Pi filter (with one ferrite bead and 
one inductor)? 

Last week, I saw a pcb board using an isolated ground (for an rs485 connector) 
with one big resistor (approximately 10MOhm) which connects both ground 
(isolated ground and the normal supply ground). Is this a common way to avoid 
big ground differences between these two grounds? 

best regards
Daniel 


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