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

si_fan, the first issue of ground is safety.  If you choose to join the 
digital and chassis grounds together, which is very common, then you need 
to ensure that the chassis does not become a return power conductor that 
bypasses the intended return current wiring.  If you are building a system 
with isolated DC-DC converters, this tends to be easily met by joining the 
power return to chassis at only the power source, and keeping that isolated 
through the wiring and up to the DC-DC converters.

If you DC isolate the chassis and signal ground, more common when there is 
a big low-voltage supply feeding multiple assemblies, then your EMI 
suppression is going to be a lot trickier and you will need to provide for 
lots of AC bypass between the two grounds.  You are placed in a position of 
needing to find a way to suppress any common mode on your signal ground 
before it gets out on those copper cables.

Mark Montrose has covered the issue in his book "EMC and the Printed 
Circuit Board", as have other authors.

Steve.

At 12:06 AM 12/31/2003 -0800, si_fan_1@xxxxxxxxxxx wrote:
>Hi all,
>
>I'm beginning a new project and discussing the ground scheme.
>I'm talking here about a multi board design that will fit into a few U
>height chassis. There are dozens of ports out of the box (2.5Gb/s each)
>over copper cable.
>
>One of the concepts is connecting all the grounds together (both chassis
>and digital of the boards and cable connectors).
>The other method is separation of the chassis and digital grounds with
>one connection between them (close to the power supply).
>I'm aware that both methods are used but would like to better understand
>what are the reasons to choose one way on top of the other? what are the
>pros and cons of each in terms of EMI?
>
>Thanks,
>
>--
>http://www.fastmail.fm - I mean, what is it about a decent email service?
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