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