in theory, for ideal differential transmissions, a differential pair will work without GND-planes. Reality is different though. Think of it this way: For a good signal transmission you need impedance controlled traces. You don't want discontinuities on your line (especially at 3GHz speeds). Looking at one single stand alone differential pair on a PCB (there is nothing else on and around the PCB. Thats the theory I mentioned. in reality you have other traces, components copper pads heat sinks.... Every peace of conductive material in close proximity to your diff. Pair will affect its impedance. You will have dozens of discontinuities, doesn't sound like a transmission line, does it? Metal planes above and underneath the pair helps to control the impedance. You May think using coplanar coupled waveguide routing can do the job too, but thats not true. You will still have the impact of all conductive materials of the other layer, the impact is just a little less than without any GND-plane. Another Reality thing is, that there is no 100% symmetrie of a differential pait, thus there will be common mode currents generating common mode return currents. If you don't provide a GND-return path for these signals, you easily run into SI-trouble. My short answer is: Yes, GND/VCC planes are required above and below the routing layer. PS: si-list doesn't support attachments. You can draw things in ASCII, or upload it to some sharing place. BR Gert -------------------------------------------------------------------------- Absender ist HARTING Electronics GmbH & Co. KG; Sitz der Gesellschaft: Espelkamp; Registergericht: Bad Oeynhausen; Register-Nr.: HRA 5596; persönlich haftende Gesellschafterin: HARTING Electronics Management GmbH; Sitz der Komplementär-GmbH: Espelkamp; Registergericht der Komplementär-GmbH: Bad Oeynhausen; Register-Nr. der Komplementär-GmbH: HRB 8808; Geschäftsführer: Torsten Ratzmann -----Ursprüngliche Nachricht----- Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im Auftrag von chundi srikanth Gesendet: Donnerstag, 29. Oktober 2009 09:06 An: si-list@xxxxxxxxxxxxx Betreff: [SI-LIST] Reg. GND plane requirement Hi Techies, I have a query for you. We are designing a 12-layer mixed signal board. In which High-speed, ADC's are connected to FPGA through LVDS differential pairs. In PCB layout is it really required to have a GND/VCCV plane above and below the plane in which the differential are been routed? If it is not required then What about the cross talk effect? How can we over come that? The LVDS signals are running at 122.88MHz, 245.76MHz and 983.04Mhz and 3GHz speed. And there are few main clocks available i these signals too. Please let me know your views on this. Please see the attachment for my proposed stack up details. Thanks & Regards Srikanth Chundi ------------------------------------------------------------------ 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 or at our remote archives: http://groups.yahoo.com/group/si-list/messages 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 or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu