It is a theoretical question. I've seen plenty of systems with double caps, and like Steve, never have seen problems related to a charge buildup on the floating node. It doesn't seem enough of an issue that "best" design practices should include a drain resistor when it isn’t possible to guarantee the node won't float. Thanks to all responders. -----Original Message----- From: steve weir [mailto:weirsi@xxxxxxxxxx] Sent: Thursday, March 11, 2010 2:39 PM To: Bowden, Ivor Cc: si-list@xxxxxxxxxxxxx Subject: Re: [SI-LIST] Re: PCI Express AC Coupling All capacitors have some leakage. If you have a really clean PCB and the static builds up faster than the leakage can drain it off you might have an issue. Although I have never heard of a failure actually traced to this mechanism, a 10K - 1M drain or shunt resistor should take care of any such concerns. The biggest impact of such a solution is the additional parasitics. In that respect, I think the drain resistor is easier to implement transparently than the shunt is. Steve. Bowden, Ivor wrote: > I wonder about having 2 series capacitors in a signal line. Could the > floating node be subject to a static voltage buildup due to no DC path to > ground? Enough to damage the capacitor (ESD event for example)? > > -Ivor > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx > [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Michael Rose > Sent: Wednesday, March 10, 2010 2:47 PM > To: Chris Johnson; si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Re: PCI Express AC Coupling > > Chris, I thought PCIe required AC coupling on both ends(?). Generally, I like > to place the coupling caps on the RX end since the channel attenuates the > slew rate making the impedance discontinuity a bit less severe. In some > cases, AC coupling on both ends has the advantage of tolerating a short > circuit through the interconnect without damaging the driver. > > Mike > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx > [mailto:si-list-bounce@xxxxxxxxxxxxx] > On Behalf Of Chris Johnson > Sent: Wednesday, March 10, 2010 4:37 PM > To: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] PCI Express AC Coupling > > Could someone tell me if any of the TX (from motherboard to card), RX (from > card to motherboard), and REFCLK (from motherboard to card) PCI Express > differential pairs are guaranteed to be AC coupled on the motherboard? > > Also, is there significant signal integrity downside to AC coupling on the > plug in card, when the signals are already AC coupled on the motherboard, > assuming that the AC coupling is right at the I/Os to the chip on the plug in > card? > > Thanks, > > Chris > > ------------------------------------------------------------------ > 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 > > > > ______________________________________________________________________ > _ This e-mail and any files transmitted with it are proprietary and > intended solely for the use of the individual or entity to whom they are > addressed. 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Curtiss-Wright Corporation and its > subsidiaries accept no liability for any damage caused by any virus > transmitted by this e-mail. > ------------------------------------------------------------------ > 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 _______________________________________________________________________ This e-mail and any files transmitted with it are proprietary and intended solely for the use of the individual or entity to whom they are addressed. If you have reason to believe that you have received this e-mail in error, please notify the sender and destroy this email and any attached files. Please note that any views or opinions presented in this e-mail are solely those of the author and do not necessarily represent those of the Curtiss-Wright Corporation or any of its subsidiaries. Documents attached hereto may contain technology subject to government export regulations. Recipient is solely responsible for ensuring that any re-export, transfer or disclosure of this information is in accordance with applicable government export regulations. The recipient should check this e-mail and any attachments for the presence of viruses. Curtiss-Wright Corporation and its subsidiaries accept no liability for any damage caused by any virus transmitted by this e-mail. ------------------------------------------------------------------ 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