Ignas - I think I see the problem. You first assumption (that: "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.) a little off target. The addition of high resistance values to "bleed off" charge applies to people (in manufacturing) not to product design. There is no conflict between shorting chassis/digi gnd for ESD or emissions. It works fine for both. Best Regards Charles Grasso Desk 303-706-5467 Cell: 303-204-2974 Chamber: 303-706-5144 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Ignas Mikulevicius Sent: Wednesday, June 30, 2010 10:58 AM To: o. laney Cc: shuklas@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Chassis GND and Digital GND shorting on daughterboard Thanks for the reading recommendations, everyone. I will be sure to spend some more time looking through Doug Smith's website and Henry Ott's book and see if I can come up with a solution. On Tue, Jun 29, 2010 at 12:45 AM, o. laney <olaney@xxxxxxxx> wrote: > The reason there are so many seemingly conflicting answers is, as always, > "it depends". If you are hoping for rules of thumb or accepted "best > practices" you will be disappointed. The only adequate way through the > EMI/ESD thicket is a good grasp of the fundamentals. For instance, there > are LF tricks that don't work at HF and vice versa and you need to > understand why. Ground partitioning and current steering are often not > well suited to cookbook solutions. Really, it is impossible to ask > general questions and obtain compact, one size fits all working answers. > You'll have to do the homework on learning the physical reasoning. > Another ad naseum discussion cannot substitute for that because it's a > fool's errand. Try the books on noise control by Henry Ott, and as > previously suggested troll the archives until you understand the > reasoning (both good and bad) behind various recommendations. Good luck, > let us know how you're doing. > > Orin Laney > > > On Tue, 29 Jun 2010 12:39:44 +0530 "Shailendra Shukla" > <shuklas@xxxxxxxxxx> writes: > > facing problem with hyperlynx as attached > > >>> Ignas Mikulevicius <mikulevi@xxxxxxxxx> 6/29/2010 7:20 AM >>> > > 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 > > > ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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