[SI-LIST] Re: CHGND connection in DVI connector
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>, qantrix <qantrix@xxxxxxxxx>
- Date: Thu, 6 May 2010 07:31:17 -0600
We have several designs that are laid out just like Steve is recommending.
Do not add moats where they are not needed.
Also - I would NOT add the internal shields to the chassis ground at the I/O.
This is an ideal way of letting EMI out and cause immunity problems.
________________________________________
From: si-list-bounce@xxxxxxxxxxxxx [si-list-bounce@xxxxxxxxxxxxx] On Behalf Of
steve weir [weirsi@xxxxxxxxxx]
Sent: Tuesday, May 04, 2010 7:34 AM
To: qantrix
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: CHGND connection in DVI connector
I disagree with the recommended design.
I would connect signal and chassis grounds through a peninsula at the
chassis side of the DVI connector. Depending on what else is going in
and out of the box, bonding at other locations would also be
appropriate. At a minimum you need to do it at the DVI connector. The
basic premise is to reduce antenna efficiency and reduce energy pumped
into the antenna. Big loops make for efficient antennas. Extending the
chassis ground into the main PCB while still moating from signal ground
is likely going to create coupling issues that will bite you, never mind
the SI degradation you will get from crossing the moat.
Much has been written on this area of interest. Doug Smith has a number
of short articles and demonstrations showing how various grounding
schemes work and fail.
Steve.
qantrix wrote:
> Steve,
>
> http://img504.imageshack.us/i/currentdesign.png/
> you can find current design. data(diff.pairs) shield connected to sgnd.
>
> suggested design see here
> http://img266.imageshack.us/i/suggested.png/
>
> As you can see ch_gnd will cover all area of dvi connector and pins of
> data pairs shield connected to ch_gnd.
> diff.pairs will pass under separation between planes.
>
> Idea make it like this seems like bad.
>
> But it's possible to connect pairs shield to ch_gnd without creating
> slot under pairs and cable will have only quite ch_gnd shield for each
> pair and not dirty sgnd. Question is - does this shield carrying
> return currents ? How it will work from EMC point ?
>
>
>
> 2010/5/4 steve weir <weirsi@xxxxxxxxxx>:
>
>> Is there a reason you want to isolate signal common from chassis? Common
>> practice is to join the two near the chassis boundary. If you attempt to
>> isolate them, you will have lots of capacitive sneak paths. If they are
>> joined you will not have a CM discontinuity issue.
>>
>> Steve.
>>
>> qantrix wrote:
>>
>>> I have question about CHGND connection in DVI connector/cable.
>>> There is two kind of shields - one - is shield for data/clock pair;
>>> second - whole cable shield.
>>> http://en.wikipedia.org/wiki/Digital_Visual_Interface
>>>
>>> We had some EMC consultation - was suggested to connect internal pairs
>>> shield to chgnd.
>>> on pcb make polygone chgnd that cover area up to end of dvi connector.
>>> So diff.pairs will run under slot of gnd.separation.
>>>
>>> Question is - how should be connected internal shields in DVI
>>> conenctor - to sgnd or to chgnd?
>>>
>>> diff. pairs running on the slot - seems like bad idea?
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>> --
>> Steve Weir
>> IPBLOX, LLC 150 N. Center St. #211
>> Reno, NV 89501 www.ipblox.com
>>
>> (775) 299-4236 Business
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Steve Weir
IPBLOX, LLC
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Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
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