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,
Digital Telecommunications Inc.
From: Laurent BERNARD <firstname.lastname@example.org>
To: 'Signal Integrity LIST' <si-list@silab.Eng.Sun.COM>
Cc: 'Laurent BERNARD' <email@example.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,
>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:firstname.lastname@example.org 92405 COURBEVOIE Cedex
>Web : http://www.microprocess.com FRANCE
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