In a perfect symmetrical world, differential traces don't need a GND-Plane, but the world isn't perfectly symmetric. And the chips are also not pure differential, they have common modes as well, and that requires a common mode (or single ended) impedance matching. The material is inhomogenious, the traces each have different etching surface, there are pressing tolerances, copper tolerances... And then there are bends and local length differences that will drive common modes. At low speeds, you might get away without a GND plane, below 200ps risetime I wouldn't try it. 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: Edgar-Peter Duening, Torsten Ratzmann, Dr. Alexander Rost -----Ursprüngliche Nachricht----- Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im Auftrag von krishna@xxxxxxxxxxxxxxxxxxx Gesendet: Freitag, 16. März 2012 12:47 An: si-list@xxxxxxxxxxxxx Betreff: [SI-LIST] Differential Traces and GND planes Hi Everyone, Differential traces perfectly cancel out the noise induced on the trace provided the length of the two lines are same and are closely tied up all along with the controlled impedance maintained throughout the routing. But what is not sure is 1. Whether a solid ground plane is needed for a differential pair for getting the full benefit of differential traces? 2. Are there any special considerations in making the choice - to consider providing solid GND plane or get away with it - for say, better EMI/EMC and crosstalk performance? 3. Is there any trade off involved? Would you please help me get things right here if my understanding is wrong? Thank you very much, With Thanks and Best Regards, Krishna. ------------------------------------------------------------------ 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list 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