[SI-LIST] Re: chassis ground connections
- From: Cosmin Iorga <ci249534@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx, Daniel Bauer <daniel.bauer16@xxxxxx>
- Date: Tue, 10 Feb 2009 13:46:13 -0800 (PST)
Daniel,
I suggest you to look into the reason why the chassis is connected to the
PCB ground: for example to protect the inside circuits from the outside
electromagnetic field, or to protect the outside circuits from electromagnetic
radiation generated by the inside circuits, or for better ESD protection ...
Intuitively, capacitors may work for ESD transient spikes, but probably not so
well for electromagnetic coupling or radiation. One rule that I always apply
is to connect to the chassis only in a single point. Having two or more
connection points may create ground loops that may couple significant
noise across the entire system.
Regards,
Cosmin Iorga,
NoiseCoupling.com
http://www.noisecoupling.com
--- On Tue, 2/10/09, Daniel Bauer <daniel.bauer16@xxxxxx> wrote:
From: Daniel Bauer <daniel.bauer16@xxxxxx>
Subject: [SI-LIST] chassis ground connections
To: si-list@xxxxxxxxxxxxx
Date: Tuesday, February 10, 2009, 11:57 AM
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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