[SI-LIST] chassis ground connections
- From: Daniel Bauer <daniel.bauer16@xxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 10 Feb 2009 21:01:31 +0100
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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