IC designers should pin out pairs close together in order to make it easier to maintain length matching between members of the pair. Otherwise, it doesn't matter. Lee > [Original Message] > From: Dunbar, Tony <tony_dunbar@xxxxxxxxxxx> > To: si-list@xxxxxxxxxxxxx <si-list@xxxxxxxxxxxxx> > Date: 10/9/2003 4:01:42 PM > Subject: [SI-LIST] Re: Diff.Pairs > > Lee, > > Should we, then, not be concerned if the Si or package vendor chooses to > pin-out the device with differential signal pairs all over the package, > rather than together? > > Regards, > Tony > > -----Original Message----- > From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] > Sent: Thursday, October 09, 2003 12:57 PM > To: Knighten, Jim L; Doug Brooks; si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Re: Diff.Pairs > > > Why do people try so hard to make the coupling between the two members of a > pair so important. These are two independent signals that travel over > planes independently unless they are placed close enough together that > there is some interaction. This interaction is not beneficial. > > Lee > > > > [Original Message] > > From: Knighten, Jim L <JK100005@xxxxxxxxxxxxxxxx> > > To: <leeritchey@xxxxxxxxxxxxx>; Doug Brooks <doug@xxxxxxxxxx>; > <si-list@xxxxxxxxxxxxx> > > Date: 10/9/2003 10:40:29 AM > > Subject: RE: [SI-LIST] Re: Diff.Pairs > > > > Lee, > > > > Your post is interesting! > > > > Differential signaling is usually implemented with coupled transmission > > lines. The mutual coupling between the traces affect the two modes that > are > > always present (even and odd modes). In the traditional configuration, the > > two traces are parallel and of the same width and thickness and located > > adjacent to a plane. The degree of coupling between the traces is usually > > described as "loosely coupled" or "tightly coupled." In either case, if > the > > signal and signal traces are perfectly differential (i.e., no imbalance, > > perfectly symmetrical), then there is always current in the adjacent > ground > > plane, but the net current in the longitudinal direction (the direction of > > the traces) is zero. The currents that exist in the adjacent plane are > > circulating currents that reflect the distributed coupling between the > > traces down the length of the transmission line. > > > > So, what if the two coupled traces are not co-planar, i.e., not in the > same > > plane? Well, you still have two coupled transmission lines, but the > mutual > > capacitance and inductance between them may be different than if they were > > co-planar, hence the even and odd mode impedances may be different. These > > non-co-planar coupled lines can still carry differential signals, though. > > > > > What if the two coupled lines were not co-planar and actually had the > ground > > plane between them? This is just a special case of the "loosely coupled" > > case, in that the lines are now not coupled at all. Still, the lines can > > support differential signaling, but the relationships between even and odd > > modes are not quite the same as when they were coupled. (Perhaps even mode > > and odd mode impedances are equal?) > > > > So, how about current in the ground plane? For perfect differential > > signaling, the net current in the plane is zero. When you introduce > > imbalance, either in the signal source, or in the signal path, you create > > net longitudinal current in the ground plane. This is the even mode > signal, > > which has no bearing on your intended differential signal (the odd mode) > and > > represents an EMI source on the ground plane. > > > > If you route differential signals on different layers, it may be more > > difficult to maintain balance (symmetry) in the traces than if the traces > > were co-planar. If this is true, you have more potential for EMI issues. > > > > ...My thoughts > > > > Jim > > > > ________________________ > > James L. Knighten, Ph.D. > > Teradata, a division of NCR http://www.ncr.com > > 17095 Via del Campo > > San Diego, CA 92127 > > tel: 858-485-2537 > > fax: 858-485-3788 > > > > > > -----Original Message----- > > From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] > > Sent: Thursday, October 09, 2003 9:30 AM > > To: Doug Brooks; si-list@xxxxxxxxxxxxx > > Subject: [SI-LIST] Re: Diff.Pairs > > > > If this discussion is about differential pairs travelling over the planes > > of a PCB, the return current for each member of the pair travels on the > > plane over which it travels, not on the other wire. If they are very > > tightly coupled to each other, perhaps 5% of the current from one travels > > in the other. It is coincidental that the two currents are equal in > > magnitude and opposite. They don't have to be. Their "return currents" > > still travel on the plane, not on the other wire. > > > > As far as EMI is concerned, it has been demonstrated many times, once in > > the paper done by Doug Brooks with the staff at UMR, that traces traveling > > over planes are not a detectable source of EMI. Therefore, constraining > > the routing of differential pairs to prevent them from creating EMI is not > > appropriate or necessary. > > > > It is still true that the two members of a differential pair are two > > independent signals traveling on two independent transmission lines. All > > they have in common is that the have equal amplitudes and are 180 degrees > > out of phase with each other. If the protocol is LVDS, each member of the > > pair should be parallel terminated in an impedance equal to Zo for that > > line to Vref (about 1.25V) which is half way between the two logic levels. > > > > As long as the two signals switch at the same time, the current flowing > out > > of Vref into one line is the same magnitude an opposite in polarity to > that > > flowing into the other. The net current into and out of the Vref terminal > > is zero, so we can omit the connection. When we do this, we have two > > resistors, each of value Zo across the ends of the two transmission > lines. > > For convenience, we use one resistor of value 2 X Zo. This is not a > > differential impedance of 100 ohms, but two parallel terminations of value > > Zo terminating two transmission lines each of impedance Zo. > > > > As long as the two edges switch at the same time, there is no current > > imbalance and all is well. Soon as one edge switches before the other, > > there is a need for a momentary current spike to flow into or out of the > > Vref terminal. If there is no connection to Vref for the current flow, > the > > result is the edges are degraded. To avoid this degradation, a very small > > capacitor is often connected between the two resistors and ground. This > is > > a very common termination for 2.4 GB/S signal links. > > > > It is time to stop representing differential signals as needing to be > > tightly coupled to each other in order to operate properly. It is simply > > not so. I have routed thousands of differential signal where each member > > of the pair is on a different layer. If this were not possible, 1 mm > pitch > > BGAs with differential signals would be un routable. There are tens of > > thousands of such parts being shipped every month on PCBs where they are > > routed apart from each other. > > > > This is all described in my recently published book, "Right the First > Time, > > A Practical Handbook on High Speed PCB and System Design". It is also > > covered in Howard Johnson's new book whose title escapes me at the > moment.. > > > > Lee > > > > > > > [Original Message] > > > From: Doug Brooks <doug@xxxxxxxxxx> > > > To: <si-list@xxxxxxxxxxxxx> > > > Date: 10/3/2003 1:02:25 PM > > > Subject: [SI-LIST] Re: Diff.Pairs > > > > > > Tight may be a relative word. But a differential pair constitutes a > > "loop" > > > in EMI terms. That is, the loop is the area encompassed by the signal > and > > > its return. Smaller loop areas perform better than larger loop areas > when > > > EMI is a concern. The closer the differential pair, the smaller is the > > > loop. If we are NOT concerned about EMI, then this is not an issue. If > we > > > ARE, then we might want to pay attention to this and keep the loop small > > by > > > routing the traces close together. > > > > > > The equal spacing "requirement" comes from the control of reflections > (ie > > > transmission line termination issues.) IF we are concerned about > > > reflections, THEN we need a constant impedance everywhere along the > > trace. > > > IF the (differential) traces are close together (for EMI reasons) THEN > > they > > > will interact (a very special case of crosstalk, which in this > particular > > > case [signals --- being equal and opposite --- are exactly correlated > > with > > > each other] is not a problem.) IF we want to keep a constant impedance > > > along the traces, THEN we must keep a "constant" spacing between them, > > > because the coupling between them, and therefore the differential > > > impedance, will vary if we don't. > > > > > > There is a further design rule you sometimes hear, that being that the > > > differential traces must be equal length. This is NOT for timing > reasons, > > > but for common mode reasons. A strong assumption we make about > > differential > > > signals is that they are equal and opposite, and therefore there is no > > > return signal through the ground system. Even if the signals are > perfect, > > > if the traces are different length, then the signal will not arrive at > > the > > > far end at exactly the same time and the signals will not be "equal and > > > opposite" at the receiver. Just a couple of degrees phase shift can make > > a > > > surprising difference between the signals when we are talking about > > > (square-wave) clock signals. If the signals are not exactly equal and > > > opposite, then there MUST be a net current flowing somewhere else. This > > > will quite likely be a common mode noise current that might cause an EMI > > issue. > > > > > > None of the differential signal trace design rules are necessary taken > by > > > themselves. This is important to recognize. But if are concerned about > > > certain SI issues, they might lead to some design considerations which > > THEN > > > might cascade (like a domino effect) into other areas. > > > > > > This is in my book, too............... > > > > > > Doug Brooks > > > > > > > > > > > > > > > At 11:41 AM 10/3/2003 -0700, Lee Ritchey wrote: > > > >More than that, it does not have any benefit. Tight coupling of > > > >differential pairs forces the traces to be narrower increasing the skin > > > >effect losses. Also, this tight coupling is going to result in good > old > > > >cross talk that actually degrades the edges. > > > > > > > >How the notion of tight coupling of differential pairs as beneficial > got > > > >started is a mystery to me. There are several references that show > that > > > >tight coupling is not beneficial, one of them is Howard Johnson's > latest > > > >book, at least one column he has written and my recently released book. > > > > > > > >Lee Ritchey > > > > > > > > > > > > > [Original Message] > > > > > From: Duane Takahashi <duanet@xxxxxxxxxxxxxxxxxxxxxx> > > > > > To: <si-list@xxxxxxxxxxxxx> > > > > > Date: 10/2/2003 3:58:59 PM > > > > > Subject: [SI-LIST] Re: Diff.Pairs > > > > > > > > > > Hi Juergen: > > > > > > > > > > Aligning the stack up for the broadside coupled diff lines is > > expensive. > > > > > You can do this, but it drives up the cost of the board. > > > > > > > > > > Duane > > > > > > > > > > > Hi Juergen, > > > > > > You can find lots of application notes > > > > > > especially with respect to process variation > > > > > > on differential pairs here: > > > > > > > > > > > > > > > > > > www.polarinstruments.com/support/cits/cits_index.html > > > > > > > > > > > > In particular this one may be of interest: > > > > > > > > > > > > > > > > > > How measured impedance may vary from field solver calculations > when > > > > > > using woven glass reinforced > > > > > > <http://www.polarinstruments.com/support/cits/AP139.html>laminates > > > > > > > > > > > > www.polarinstruments.com/support/cits/AP139.html > > > > > > > > > > > > > > > > > > And this note: > > > > > > > > > > > > Copper thickness, edge coupled lines and > > > > > > characteristic > > > > > > <http://www.polarinstruments.com/support/cits/AP151.html>impedance > > > > > > > > > > > > > > > > > > www.polarinstruments.com/support/cits/AP151.html > > > > > > > > > > > > > > > > > > > > > > > > Hope this helps.... > > > > > > > > > > > > > > > > > > Kind regards > > > > > > Martyn Gaudion > > > > > > www.polarinstruments.com > > > > > > T: +44 1481 253081 > > > > > > F: +44 1481 252476 > > > > > > M: +44 7710 522748 > > > > > > E: martyn@xxxxxxxxxxxxxxxxxxxx > > > > > > > > > > > > ============================================ > > > > > > Controlled Impedance & Signal integrity tools > > > > > > for the Printed circuit fabrication industry > > > > > > ============================================ > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > At 19:00 02/10/2003, you wrote: > > > > > > > > > > > >>I am seeking help in finding enlightenment regarding electrical > > > > > >>performance pros and cons and how manufacturing tolerances play a > > role > > > > > >>when comparing side by side and tandem differential pairs. I'd > > > >appreciate > > > > > >>your opinion, experience, analysis, pointers to papers and > articels, > > > >etc. > > > > > >> > > > > > >>In return, I would offer to share a summary of the > > finding/discoveries > > > > > >>with interested parties. > > > > > >> > > > > > >>Thanks > > > > > >> > > > > > >>Juergen > > > > > >> > > > > > >> > > > > > >> > > > > > >>------------------------------------------------------------------ > > > > > >>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 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 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 > > > > > > > > > > > > > > > > -- > > > > > Duane Takahashi phone: 408-720-4200 > > > > > Greenfield Networks fax: 408-720-4210 > > > > > 255 Santa Ana Court email: duanet@xxxxxxxxxxxxxxxxxxxxxx > > > > > Sunnyvale, CA 94085 > > > > > > > > > > * MOVING! Please note new numbers and address * > > > > > > > > > > ------------------------------------------------------------------ > > > > > 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 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 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 > > > > > > > > > > Doug Brooks' new book, "Signal Integrity Issues and Printed Circuit > Board > > > Design" has just been released by Prentice Hall. See details and > ordering > > > info at www.ultracad.com > > > > > > ____________________________________________________________________________ > > __ > > > > > > > > > ------------------------------------------------------------------ > > > 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 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 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 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 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 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