# RE: [SI-LIST] : return current distribution in diff pairs

Chris Cheng (*[email protected]*)

*Fri, 24 Sep 1999 15:01:04 -0700*

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no need to do fancy fttd. just use a simple free tool like fasthenry

from mit (in fact i think its in one of its example)

and u will notice when differential signals jump reference

plane, the image currents of the signals overlap each other and

travel along the edge of the plane and nearly cancel each other

out. the differential return loop is very small across the

boundary since the both current is force to travel with each other

(i.e. strong coupling and cancellation). i don't think differential

signals crossing boundary is a significant problem.

chris

*>*

*> > -----Original Message-----*

*> > From: [email protected]*

*> > [mailto:[email protected]]On Behalf Of Mellitz, Richard*

*> > Sent: Thursday, September 23, 1999 2:28 PM*

*> > To: '[email protected]'*

*> > Subject: RE: [SI-LIST] : return current distribution in diff pairs*

*> >*

*> >*

*> > Ya know what would be really neat? An FDTD full wave movie*

*> > showing the two*

*> > wave traveling down the differential pair and then encountering*

*> a split on*

*> > the ground reference plane. That way you would see the relative*

*> > attenuation*

*> > of the wave excites in the spit. Then compare it to the same with single*

*> > ended transmission. Any of you EM gurus out there ever do this?*

*> 'Kwon any*

*> > tools that can do this easily?*

*> >*

*> > ...Rich*

*> >*

*> > -----Original Message-----*

*> > From: Eric Bogatin [mailto:[email protected]]*

*> > Sent: Thursday, September 23, 1999 1:17 PM*

*> > To: [email protected]*

*> > Cc: eric*

*> > Subject: [SI-LIST] : return current distribution in*

*> > diff pairs*

*> >*

*> > << File: Current dist.PDF >> Hi guys-*

*> >*

*> > There has been some discussion recently about the current*

*> > distribution of*

*> > the return path in differential pair lines. I think it is a*

*> > common*

*> > misconception that the other line "carries" the return*

*> > current of the first*

*> > line. This may be true when the off diagonal elements of the*

*> > characteristic*

*> > impedance matrix are very small compared to the diagonal*

*> > elements, as in*

*> > shielded twisted pair, but not in typical board geometries.*

*> >*

*> > In the classes I teach, I show an example of the current*

*> > distribution in the*

*> > case of two 50 ohm coupled microstrips, 5 mil line and*

*> > space, coupling of*

*> > about 10%. There is less than 10% overlap of the return*

*> > currents in the*

*> > planes. This is ultimately a "skin depth related" effect. I*

*> > have appended a*

*> > copy of one of my slides showing the current distribution at*

*> > 100 MHz sine*

*> > wave freq for the current in the signal lines and the return*

*> > path in the*

*> > plane below. This was done using the Ansoft Maxwell 2D*

*> > Extractor field*

*> > solver, assuming copper for all the conductors.*

*> > Unfortunately, I can only*

*> > plot the magnitude of the current, not the sign. So, I plot*

*> > in the top*

*> > example, the current in the plane when only one conductor*

*> > has current, +1A,*

*> > showing that most of the return current is directly under*

*> > the signal line.*

*> > Then I plot the current when one has +1A and the other has*

*> > -1A. You can see*

*> > there is clearly a lot of return current in the plane.*

*> >*

*> > The lesson here is to always treat the return currents with*

*> > as much care and*

*> > respect as the signal currents, even in differential pairs,*

*> > unless you know*

*> > for sure the return currents are cancelled in the planes. Of*

*> > course, the*

*> > actual current distribution in the planes will depend on the*

*> > precise cross*

*> > section and spacings.*

*> >*

*> > If the traces go over a split in the return path, the*

*> > currents will probably*

*> > mix, and may go to zero at one spot, but the impedance of*

*> > the two modes will*

*> > be radically changed in this region and you will generate*

*> > common mode*

*> > voltages where there were none before- causing discontinuity*

*> > problems,*

*> > termination problems, switching noise problems and EMI*

*> > problems (did I leave*

*> > any out?). Of course, you need to simulate the magnitude of*

*> > the problem to*

*> > evaluate whether for the given split, the noise is still*

*> > under an acceptable*

*> > limit. But the defensive strategy is treat return paths in*

*> > differential*

*> > pair, like you would for single ended lines.*

*> >*

*> > --eric*

*> >*

*> >*

*> > Eric Bogatin*

*> > BOGATIN ENTERPRISES*

*> > Training for Signal Integrity and Interconnect Design*

*> > 26235 W. 110th Terr.*

*> > Olathe, KS 66061*

*> > v: 913-393-1305*

*> > f: 913-393-1306*

*> > pager: 888-775-1138*

*> > e: [email protected]*

*> > web: www.bogatinenterprises.com*

*> >*

*> >*

*> >*

*> >*

*> > > -----Original Message-----*

*> > > From: [email protected]*

*> > > [mailto:[email protected]]On Behalf Of D. C.*

*> > Sessions*

*> > > Sent: Wednesday, September 22, 1999 7:35 PM*

*> > > To: [email protected].eng.sun.com*

*> > > Subject: Re: [SI-LIST] : Q: Plane-jumping return currents*

*> > >*

*> > >*

*> > > Eric Goodill wrote:*

*> > > >*

*> > > > Mike Jenkins wrote:*

*> > > > >*

*> > > > > Eric,*

*> > > > >*

*> > > > > One line of your question, "My system is running*

*> > pretty fast*

*> > > > > (> 1 Gbps)", caught my eye. At that speed, which I*

*> > assume might*

*> > > > > be Fibre Channel or Gigabit Ethernet, you may well be*

*> > running*

*> > > > > differential. (If not, good luck to you.) But if*

*> > your lines*

*> > > > > are dif'l, they carry their own return current.*

*> > Depending on*

*> > > > > geometry, there is some discontinuity, but MUCH less*

*> > than*

*> > > > > single-ended. If your lines are, in fact,*

*> > differential, and*

*> > > > > if you wish me to elaborate, I will.*

*> > > >*

*> > > > Mike,*

*> > > >*

*> > > > Yes, differential. However, we're using edge-coupled*

*> > pairs, and it's my*

*> > > > understanding, though I've done no analysis, that about*

*> > 10% -*

*> > > 15% is about*

*> > > > as much coupling as you can get between edge-coupled*

*> > lines.*

*> > > Thus, there is*

*> > > > still a strong coupling between the trace and it's*

*> > reference place.*

*> > > > Therefore, I suspect that there's non-ignorable amount*

*> > of*

*> > > return current in*

*> > > > the reference planes. I'd be interested to see a*

*> > > > return-current-distribution plot for a diff pair both in*

*> > the reference*

*> > > > planes and the coupled traces.*

*> > >*

*> > > I don't think so. Sure, there's a fair bit of capacitive*

*> > current between*

*> > > each trace and the adjacent plane, but since they're equal*

*> > and opposite*

*> > > the loop is very small and entirely lateral. Cross a*

*> > plane boundary and*

*> > > there's no need for any current across the break.*

*> > >*

*> > > --*

*> > > D. C. Sessions*

*> > > [email protected]*

*> > >*

*> > > **** To unsubscribe from si-list: send e-mail to*

*> > > [email protected] In the BODY of message put:*

*> > > UNSUBSCRIBE si-list, for more help, put HELP. si-list*

*> > archives*

*> > > are accessible at http://www.qsl.net/wb6tpu/si-list *****

*> > >*

*> >*

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