Re: [SI-LIST] : USB differential lines.

Fred Balistreri (fred@apsimtech.com)
Wed, 30 Sep 1998 09:06:39 -0700

Norm Ebsary wrote:
>
> Laurent,
>
> The differential characteristic impedance is dependent on the center to
> center distance of the two conductor pair, with single ended characteristic
> impedance on the distance from the single trace to ground. While it is a
> common misnomer in the industry to route two conductors as close as possible
> in a differential pair, this is generally incorrect where the differential
> characteristic impedance , Zo = 120/Er * ln(h/w + ((h/w)**2 -1)**.5) - Note
> that h is the center to center separation of the two traces with w equal to
> the trace width. For example the distance between conductors of a
> differential pair for a w = .005" wide trace with a separation of .005"
> (h=.010" center to center) and an Er = 3.5 is 45.12 Ohms. So in this case,
> keeping two traces as close together as possible with .005"/.005" design
> rules would yield a 45 Ohm differential characteristic impedance. For a
> differential characteristic impedance of 90 Ohms, the separation of .005"
> traces for a differential pair should be approximately .035" center to
> center or for .005" trace width a separation of .030".
>
> Hope this helps,
> Regards,
> Norm Ebsary
> Digital Telecommunications Inc.
>
> -----Original Message-----
> From: Laurent BERNARD <l.s.bernard@usa.net>
> To: 'Signal Integrity LIST' <si-list@silab.Eng.Sun.COM>
> Cc: 'Laurent BERNARD' <lsbernard@chez.com>
> Date: Wednesday, September 30, 1998 8:49 AM
> Subject: [SI-LIST] : USB differential lines.
>
> >Hello SI experts,
> >
> >I am designing a PCB which should use differential lines for USB.
> >
> >I am a little bit confused by Intel data on the USB. The Single Ended
> >impedance should be 45 Ohms, the Differential impedance should be
> >90 Ohms to match the cable impedance if I have well understund.
> >
> >>From what I have read in the SI-LIST archives
> >Zdifferential = 2 * Zsingle ended in the case where there is no coupling
> >between the traces : the lines have to be separeted by a large
> >distance, which is stupid for a differential pair where both wire should
> >be near to each other.
> >
> >Any comment on this strange requirement is welcome.
> >
> >Thank you very much for your help,
> >
> >Best Regards,
> >
> >Laurent BERNARD. MicroProcess Ingenierie
> >Ingenieur d'etudes / Design Engineer.
> >
> >Tel : 33 (0)1 47 68 80 80 B.P. 87
> >Fax : 33 (0)1 47 88 97 85 97 bis, rue de Colombes
> >Email : mailto:lsbernard@chez.com 92405 COURBEVOIE Cedex
> >Web : http://www.microprocess.com FRANCE
> >
> >
> >
> >
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It is not incorrect to route differential signals close to each other.
It is preferable. The caveat being that the required ZO be met. It is
incorrect to call traces that are not coupled to each other differential
pairs. One can certainly measure an impedance across such a pair and
call that an impedance measured differentially. However the pair is NOT
a electrical differential system. The degree to which a pair can be
called differential will depend on the coupling. This effects where
the return current is located and the magnetic flux cancellation. In an
uncoupled system the return current is not located on the other trace
but rather somewhere else (hopefully a nearby plane). And there
certainly is no magnetic flux cancellation between the two. Your
definition of differential systems seems to be predicated on Zo only.

Best Regards,

-- 
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com

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