[SI-LIST] Re: Chassis GND and Digital GND shorting on daughterboard
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Ignas Mikulevicius <mikulevi@xxxxxxxxx>
- Date: Mon, 28 Jun 2010 19:55:01 -0700
Ignas the topic comes around repeatedly because there is not a single
right answer. You need to understand what is applicable to whatever
design it is that you wish to undertake. Rather than try and cover this
extensive topic, I think you would be best served by spending as much
time as you can afford on Doug Smith 's site: www.emcesd.com. Pay
particular attention to unintended coupling and how it affects isolation
efforts.
Steve.
Ignas Mikulevicius wrote:
> Hello everyone,
> Yes, the topic's back. I know this has been discussed ad nauseum in various
> previous posts, but I felt none of them conclusively established a solution.
> First of all, from previous discussions it seems to me like there are some
> trade-offs between ESD and EMI protection in terms of chassis GND - digital
> GND connections. The way I understand it, for ESD, we ideally want to
> connect chassis ground and digital ground with a large resistor (say, 1
> MOhm), to allow any ESD charge to be bled off to chassis without causing any
> excessive currents and correspondingly dangerous voltages across components.
>
> On the other hand, for EMI, I understand it may be advantageous to short
> digital ground and chassis ground at multiple locations in the vicinity of
> the I/O connector to divert any problematic currents to chassis ground
> before they can radiate out on the cable. So is there any way to reconcile
> the two and get the best of both worlds?
> And how should these recommendations be applied when designing a standard
> PCI or PCI Express add-in card?
> The specific scenario I am interested in is this:
> Say I have a PCI Express daughterboard (add-in card) that plugs into a
> motherboard in a standard PC (let's call this System A). The PCB bracket is
> screwed on to the I/O connector of the PCB and, of course, also contacts the
> computer chassis. The board sends differential signals across a long,
> shielded cable to another system (say, System B).
> First of all, in the case of differential signals inside a shielded cable,
> how much potential for EMI can there be? I would assume the differential
> nature of the signals would greatly minimize radiation.
> The cable shield is shorted to chassis GND. Each differential pair in the
> cable has its own ground wire, and, finally, there is a wire that is called
> "Frame Ground", tied to chassis ground at system B. On the daughterboard,
> there is a "chassis GND" island underneath the connector in the layer below.
> The connector posts as well as the "Frame GND" signal are shorted here. So
> basically we have the chassis GND of system A and system B grounded and this
> also grounds the cable shield. Now clearly I can't leave this floating as it
> would seem that would create an EMI nightmare.
> What would be the ideal way to connect this island and digital GND? A
> capacitor, dead shorting, a resistor?
> It seems whatever method I choose I can find at least one article to endorse
> it.
> Any thoughts or insight would be greatly appreciated. I am not expecting
> someone to remedy my dillemma for me, but would really like to understand
> the theory and thinking behind this once and for all. If someone could
> either elucidate me or point me to some definitive, conclusive articles or
> books on this subject I would be very grateful.
> Thank you very much,
> Ignas M.
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
