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

Mellitz, Richard (richard.mellitz@intel.com)
Thu, 23 Sep 1999 14:27:45 -0700

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:eric@bogent.com]
Sent: Thursday, September 23, 1999 1:17 PM
To: si-list@silab.eng.sun.com
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: eric@bogent.com
web: www.bogatinenterprises.com

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of D. C.
Sessions
> Sent: Wednesday, September 22, 1999 7:35 PM
> To: si-list@silab.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
> dc.sessions@vlsi.com
>
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