[SI-LIST] Re: Shield ground isolation
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxx>
- To: <paulnets@xxxxxxxxxxxxxx>, <Anand.Kuriakose@xxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Mon, 24 Feb 2003 09:32:01 -0700
Paul, et.al,
I think the I/O shield isolation from digital ground concept is an outdated
EMC concept used for analog, low speed systems. Multi-point grounding
works. Provide board grounds with the proper aspect ratio relative to the
primary frequency of operation (lambda/20), use short, fat, standoffs (as
much surface area as you can get)to the chassis, and use a bulkhead type
connection scheme. External common mode ferrites can be used on the cables
in question if you bulkhead mount is not at the board, or you can get a
connector with the ferrite wafer integral to the bulkhead connector. The
amount of mating surface area between board and chassis is key.
Having said all that, provide for the ability to lift one end of the shield,
just in case. I've never had to do it, but this is EMC after all. Other
than the promises of a politician, there is nothing more undefined than what
can happen during certification testing.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Paul Young
Sent: Sunday, February 23, 2003 5:52 PM
To: Anand.Kuriakose@xxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Shield ground isolation
Hi,
Same to me.
Can anyone out there give the answer?
Paul
----- Original Message -----
From: "Kuriakose, Anand" <Anand.Kuriakose@xxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Sunday, February 23, 2003 1:07 AM
Subject: [SI-LIST] Shield ground isolation
>
> Hi All,
>
> On high speed boards with I/O connections running to outside world, the
> shield ground of the connectors should be isolated from the digital ground
> if the board has to meet the requirements of EMI/EMC compliance standards.
>
> Are there any side-effects of isolating the shield ground from the digital
> ground?
>
>
> I have seen on some boards, that the isolation is achieved by using
ferrite
> beads. On few others i have seen capacitors being used b/n shield and
> digital ground.
>
> How are capacitors useful in acheiving isolation? Or are these caps used
for
> some other purpose, which i am not able to understand?
>
> What is the best method to achieve this isolation?
>
> Thanks in advance.
>
> Regards,
> Anand.
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