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

Michael E Vrbanac (mvrbanac@cisco.com)
Thu, 5 Feb 1998 14:11:55 -0800 (PST)

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.
> Let me know if you need more info.
>
> 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
> _________________________________________________________________________
> Check out Ansoft's WWW site at http://www.ansoft.com for product
> information, technical support, employment opportunities, much more...
> -------------------------------------------------------------------------
>