**From:** Ozgur Misman (*omism@amkor.com*)

**Date:** Wed Nov 22 2000 - 15:22:56 PST

**Next message:**e: "Re: [SI-LIST] : 100Base-TX board SI issues"**Previous message:**Ozgur Misman: "RE: [SI-LIST] : Linpar 2-D field solver experiences?"

Hi everybody,

I have a circuit that I use to design differential pairs. The circuit is based

on an optimization routine that minimizes the reflections ( S11) from a resistor

across the differential pair. Where the relections are zero or close to zero

the value of the resistor would be the differential impedance. Since we are in

the packaging business our trace widths are typically between 60 to 100 micron

range. A higher number would lead to routing inefficiency a lower number would

be very difficult to meet from manufacturing point of view in laminate based

packages.

Typically I set a target differential impedance and optimize the trace width and

spacing to meet target Zdiff. The simulation tool is Microwave design system.

I found out my optimum trace width and spacing greatly depends on the trace

thickness as well! I have been following the discussions very closely, and there

is no reference to trace thickness in the design of differential pairs!

One possibility is that the software might be using rectangular conductor cross

sections and therefore exagerates the effect of trace thickness. (We know a

trapezoid would be a better aproximation)!

Second possibilty is that since the aspect ratio is small in package traces as

opposed to PCB traces, the tighter coupling between the traces results in strong

dependence of Zdiff to trace height.

As an example a two layer stack up would be signal-ground with 100 micron

seperation and 75 micron trace spacing and width. ( Of course this is not 100

Ohms differential impedance)

Well, the physics supports the second option, however I am very interested in

hearing any opinions or perspectives on the importance of trace thickness in

Differential impedance calculations or simulations.

Thanks

Ozgur Misman

Sr. Engineer

Amkor Technology

"Ingraham, Andrew" <Andrew.Ingraham@compaq.com> on 11/22/2000 02:46:37 PM

Please respond to "Ingraham, Andrew" <Andrew.Ingraham@compaq.com>

To: "si-list@silab.eng.sun.com"

<si-list@silab.eng.sun.com>

cc:

Subject: RE: [SI-LIST] : Routing Differential Pairs as 100

differentially Vs individually 50 ohm lines

...

*> Routing them close to each other produces interaction that reduces the
*

*> final amplitude.
*

...

This "interaction" you speak of is the reduction in differential impedance

as the two lines are brought into one another's neighborhood. Simple

physics. You got reduced amplitude because your lines were less than 100

ohms differential, and if you didn't take that into account, you probably

mismatched the line's impedance.

Two widely separated 50 ohm lines give 100 ohm differential impedance.

Two less separated 60 ohm lines give 100 ohm differential impedance.

Two even less separated 70 ohm lines give 100 ohm differential impedance.

And so on.

What's different between these, is the common-mode impedance. Matching the

differential impedance is the first, and most important, step, but you often

want to look at how (or whether) you matched the pair's common-mode

impedance with the terminations at one or both ends, which affects

common-mode noise susceptibility.

There is nothing "wrong" about routing differential pairs close so they

"interact" with one another. After all, that's what a twisted pair cable

is. The wires are close to each other, producing interaction, and a lower

impedance than if the two wires were far apart.

Andy

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