# Re: [SI-LIST] : Re: Differential impedance

Nirmal Jain ([email protected])
Thu, 05 Feb 1998 22:32:02 -0500

Hi Michael,

Mike Jenkin's email explains the concept in detail. My assumption was that the TX
lines are symmetrical and Zodd (odd mode impedance ) for symmetrical lines is
given by

= Z11 - Z12, and Zdiff = 2 * ( Z11-Z12 ). Here you see it does take into
account the coupling between the lines.

Regards

Nirmal

Michael E Vrbanac wrote:

> Nirmal,
>
> re: Zdiff = 2 * Zodd (of one of two identical lines?)
>
> Isn't this only true if there is no coupling between the differential
> signal lines and that a true transmission line assumes that there is
> coupling between conductors (+/-, source/return, etc.)?
>
> Regards,
>
> Michael E. Vrbanac
> Cisco Systems, Inc.
> 170 W. Tasman Dr.
> San Jose, CA 95134
> (408)526-4692 (phone)
> (408)526-4184 (fax)
>
> >
> > Mark Nass wrote:
> > >
> > > We are laying out a board that has differential a SCSI bus on it.
> > > The specs say the that the differential impedance should be 110-135 ohms.
> > > What is differential impedance? Is this the impedance between
> > > the 2 traces, or the impedance with respect to the plane?
> >
> > This is actually the impedance between the two traces. If you
> > have a geomtery in mind I could do a quick analysis and give
> > you the differential impedance. In terms of odd mode impedance
> >
> > Zdiff = 2 * Zodd,
> >
> > where Zodd is the odd mode impedance. I think there was lively
> > discussion on differential impedance a while ago on the list.
> >
> > Regards
> >
> > Nirmal
> >
> > >
> > > Thanks for your help,
> > > Mark Nass
> >
> > --
> > Nirmal Jain Ansoft Corporation
> > (412)261-3200 X129 (W) Four Station Sq., Suite 660
> > (412)261-471-9427(FAX) Pittsburgh, PA 15219
> > _________________________________________________________________________
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