**From:** Chan, Michael (*Michael.Chan@compaq.com*)

**Date:** Wed Jan 03 2001 - 15:37:14 PST

**Next message:**Dill, Franz @ Celerity: "RE: [SI-LIST] : Differential Impedance Effects on Diff. Return Cu rrent"**Previous message:**Doug Brooks: "Re: [SI-LIST] : Differential Impedance Effects on Diff. Return Current"**Next in thread:**Dill, Franz @ Celerity: "RE: [SI-LIST] : Differential Impedance Effects on Diff. Return Cu rrent"

Andy:

If you have a tightly coupled pairs but there is also a reference plane

near by

and if in this case the separation between the pair is about the same as the

distance

of the traces to the reference plane then will the image current for each

trace still

shows up on the trace or they will show up mostly on the reference plane?

Regards,

Michael Chan

-----Original Message-----

From: Ingraham, Andrew

Sent: Wednesday, January 03, 2001 5:00 PM

To: 'Dill, Franz @ Celerity'

Cc: 'SI-Mailing List'

Subject: RE: [SI-LIST] : Differential Impedance Effects on Diff. Return

Cu rrent

*> First, is this statement valid?
*

*> - In a differential pair, one 'leg' of a signal's return current path is
*

*> through the complementary 'leg' of a differential pair and not through the
*

*> ground or power planes (Assuming equal trace lengths, Zo=50 single-ended,
*

*> Zo=100 diff. impedance - using ECL logic as an example).
*

The return current flows through any nearby conductors; at high frequencies,

most of the return current is through the closest conductor(s).

If this happens to be a reference plane, that's where it goes. If it is the

complementary signal line, that's where it goes.

You can construct a differential pair out of two separate traces that are

far apart from one another, in which case each one has a return current in

its neighboring plane(s) (which happens to roughly equal the signal current

in the opposite trace). The two currents in the plane are opposite and

would cancel at low frequencies, but at high frequencies the return current

is confined to the space "under" each trace and does not cancel, so in

effect you have a circulating loop of return current in the planes, of

magnitude equal to the signal current.

In the other extreme, you can also construct a differential pair out of two

traces that are tightly coupled to one another (such as broadside coupled)

and further away from reference planes, in which case each line's "return"

current is the other line's signal current and there is no significant

current in the planes.

Differential pairs have both a differential (odd-mode) impedance and a

common-mode (even-mode) impedance. The two examples could have the same

differential impedance but very different even-mode impedances.

If you route two 50 ohm traces over ground planes, you'd get slightly less

than 100 ohms differential impedance. To get closer to 100 ohms, you need

to keep them further apart, in which case most of the return current remains

in the planes. To get more cancellation of the plane's return current, you

need them closer together, which reduces the differential impedance (you

would need to use >50 ohm traces to get 100 ohms differential).

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**Next message:**Dill, Franz @ Celerity: "RE: [SI-LIST] : Differential Impedance Effects on Diff. Return Cu rrent"**Previous message:**Doug Brooks: "Re: [SI-LIST] : Differential Impedance Effects on Diff. Return Current"**Next in thread:**Dill, Franz @ Celerity: "RE: [SI-LIST] : Differential Impedance Effects on Diff. Return Cu rrent"

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