# RE: [SI-LIST] : Broadside Coupled Traces

Chan, Michael ([email protected])
Thu, 22 Apr 1999 17:07:11 -0500

Go to the Si-List Archive and look for a response from Eric Wheatley to Eric
Bracken's mathematical explanation of differential impedance. That is the
best I have seen so far.

-----Original Message-----
From: Eric Goodill [mailto:[email protected]]
Sent: Thursday, April 22, 1999 3:49 PM
To: [email protected]
Subject: Re: [SI-LIST] : Broadside Coupled Traces

Fred,

I'm still having some trouble getting my head around differential pairs and
impedance. Would you recommend any particular sources for good coverage of
the topic?

-Eric

Fred Balistreri wrote:

> Somebody is confused about differential signals. The whole idea of
> having differential signaling involves tight coupling. If you
> seperate the traces such that they are not coupled then they are not
> differential signals. Now there are papers that conclude differential
> impedance is not needed for digital signals running on pcb boards.
> And this may indeed be the case. However if the application involves
> differential inputs and outputs its best to have the pair closely
> coupled, impedance aside. In that regard broadside is electrically
> better if it can be built. Manufactures however tell us otherwise.
> Seperating the traces so they are not coupled and then measuring the
> impedance across them yields 2*ZO and is easier to calculate. However in
> the strick sense of the definition this is NOT differential impedance
> because all of the return currents are found in the adjacent planes.
> By definition at least some of the return currents need to be in the
> trace pair in order to have any kind of differential signaling to
> be effective.
>
> The primary reason on a PCB board to have differential signals is the
> reduction of EMI. However there are other benefits as well. If the
> traces are tightly coupled the pair offers much greater noise emmunity
> than single ended traces. Lee Ritchie's paper in PCB Design does a
> great job of describing differential IC's functionalities.
> Unfortunately his conclusions do not apply to all applications. Low
> voltage high frequency signaling tends to lend itself well to
> differential routing. Differential signaling works especially well when
> other higher voltages that can disturb the signals are around. This
> means mixed parts such as LVDS with traditional 5V CMOS. One should not
> confuse the requirement of impedance matching with differential
> signaling. Unfortunately in the digital world it seems as if they
> are the same.
>
> Best Regards,
> --
> Fred Balistreri
> [email protected]
>
> http://www.apsimtech.com
>
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Eric Goodill                        Cisco Systems  M/S SJ-N2
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