Re: [SI-LIST] : Differential impedance

Doug Brooks (doug@eskimo.com)
Thu, 12 Nov 1998 09:11:20 -0800

I wrote a column that (I hope) explained Differential Impedance
in plain english. It was in the August issue of PC Design Magazine,
page 34. A copy of the article is available on our web page at
http://www.ultracad.com

Doug Brooks

At 07:04 PM 11/11/98 -0500, you wrote:
>To calculate the differential impedance of a transmission line pair, I've
>always used
>1) Zdiff = 2 * Zoo, where Zoo is the odd mode impedance of the pair. This
>formula is used by the pc board manufacturers.
>I was speaking to someone last week who has 30+ years in the industry and
>said this formula is wrong, that the real differential impedance should be
>calculated as
>2) Zdiff = square root of (Zoo * Zee), where Zee is the even mode
>impedance.
>
>If I compare 1) to 2), I get quite different values of differential
>impedance. Does anyone have comments on this?
>
>Thanks, Fabrizio Zanella
>EMC corporation
>fzanella@emc.com
>
>
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****************************************************
Doug Brooks, President doug@eskimo.com
UltraCAD Design, Inc. http://www.ultracad.com

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