I wouldn't count much on the EMI help. The vast majority of the return current is still in the return plane and thinking that enough jumps into the diff signals at a gap crossing is generally not a good analysis. jon -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Mohammad Tabatabai Sent: Friday, April 02, 2004 9:55 AM To: jonpowell@xxxxxxxxxxxx; zabinski.patrick@xxxxxxxx; chris_landrum@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: diff signaling Also Routing diff-pairs closely coupled preserves edge rates which will help greatly in reducing ISI jitter (for high-speed data). Another added benefit is as the diff pairs couple more strongly together the impedance matching also becomes a function of their field coupling and not entirely to the return ground below hence providing less susceptibility to impedance mismatch when a diff pair crosses over boundaries which do not have ground plane for return currents not to mention an added benefit for reduction in EMI as well. -Mohammad -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Jon Powell Sent: Friday, April 02, 2004 9:04 AM To: zabinski.patrick@xxxxxxxx; chris_landrum@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: diff signaling In addition, by routing the next to each other they are more likely to receive similar amounts of crosstalk noise (from the same sources) than if you routed them, say, across different parts of the board, even if you did length matching. Also you can get some timing benefits and additional dynamic impedance control from the mutual crosstalk between the adjacent differential signals. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Zabinski, Patrick J. Sent: Friday, April 02, 2004 6:32 AM To: chris_landrum@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: diff signaling Chris, I believe there's a bit a confusion... I agree that the traces/interconnect do not necessarily reject common mode noise any better than single ended traces. If radiated energy is applied to a copper trace (or a pair of traces in the case of differential), there will be energy transferred to the traces. So, a pair of differential traces do not have any special properties that reject common mode noise. However, what's more important is how the receiver electronics react to this noise. In a good differential receiver, the circuit only considers the difference in voltage between the two input nodes and does not care what the common mode voltage is (within the limits of the power rails). Thus, if there is common-mode noise on both traces, then a the receiver will effectively ignore (i.e., reject) the common-mode noise and only consider the difference in voltage between the two traces. Accordingly, I don't believe it's the interconnect that provides good common-mode rejection, but it's the receiver where you get the most benefit. Taking this a step further, the line driver also benefits from differential signals by providing a better, more controlled path for the return signal, where a single ended driver must often rely on decoupling elements to provide the return path. With respect to line length matching, this becomes important from the perspective of common-to-differential mode conversion. If an external source applies common-mode energy to a differential pair (say, through crosstalk), a difference in interconnect lengths between the true and complement signals will effectively convert the common-mode noise to diff-mode noise, and the receiver will not be able to reject this. Accordingly, it's important to consider common-to-diff conversion and line-length matching to maintain the benefits of differential signaling. Hope this helps, Pat > > I have been searching through the archive for information > regarding this but > I have not come up with anything concrete. > Many people mention using differential signaling in an effort > to reduce > common mode noise. Looking through Lee Ritchey's book "Right the First > Time", his discussion implies that this is not the case. For > there to be > common mode noise rejection between two traces, the fields > would have to be > equal and opposite. In a PCB structure with a diff pair > referencing GND > planes (say in a stripline topology) this seems impossible! > Routing signals > next to eachother is convenient for length matching (skew > management) but I > don't see much else.... Any thoughts. > > And I do apologize if this has been discussed in detail before. > 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl List technical documents are available at: http://www.si-list.org 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl List technical documents are available at: http://www.si-list.org 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl List technical documents are available at: http://www.si-list.org 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl List technical documents are available at: http://www.si-list.org 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