Re: [SI-LIST] : connector model for differential signal

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From: Dima Smolyansky (dima@tdasystems.com)
Date: Wed Oct 25 2000 - 23:56:45 PDT


connector model for differential signalJohn:

This is a question I have asked myself more than once. However, it appears that the definitions for Zeven have been set and spec'ed by the industry in such a way that Zeven = Zcommon / 2. Some older Tek and Agilent appnotes, as you mention, refer to that definition.

I am sure Eric Bogatin's definition is correct and does not contradict the definition above, probably just a matter of terminology, how the Zs are defined by each of us.

As for your second question, if you are trying to propagate differential signals, Zdiff should be your main priority. Zcommon (or more precisely, the ratio of Zdiff and Zcommon) should give you a qualitative picture of how well your common mode noise is rejected by your differential pair. Guys, anybody has more comments on the subject?

Thanks,

===================
Dima Smolyansky
TDA Systems, Inc.
11140 SW Barbur Blvd., Suite 100
Portland, OR 97219
(503) 246-2272
(503) 246-2282 (fax)
(503) 804-7171 (mobile)
http://www.tdasystems.com
The Interconnect Modeling Company(TM)
  ----- Original Message -----
  From: John Lin (L·)
  To: Dima Smolyansky
  Cc: eric@bogent.com ; si-list@silab.eng.sun.com
  Sent: Wednesday, October 25, 2000 6:42 PM
  Subject: RE: [SI-LIST] : connector model for differential signal

  Dear Dima,

  Thank you for your response. After downloading the application note for the web site, I find
  it is exactly what I am looking for.

  However, I got a little confusion about common mode impedance in the paper "Characterization of Differential Interconnects from TDR Measurement" ,DIFF-1199.pdf.

  On the page two, equation 3, the common mode impedance is Z_common=Zeven / 2.

  Checking the article "Differential impedance ... finally made simple" (CAPGLTIF.pdf) by Eric Bogatin, from www.BogatinEnterprises.com, Slide 26, I find the Z_common=Zeven=Z11+Z12.

  Checking the HP54754A TDR user's manual, I find a description with measurment waveform shown
  "The common mode impedance of two 50 ohm uncoupled lines is 25 ohms". It match what you described Zcommon.

  Shall the Z_common be Zeven/2 or Zeven?

  To meet Zdiff is always mentioned when doing PCB impedance control. Usually, I totally forget Zcommon
  when try to meet Zdiff. What circumstance shall I maintain Zcomm , instead of Zdiff , or both?

  Thank you for your inputs.

  Best Regards,
  John Lin
  SI Engineer, ARD4
  Quanta Computer Inc.,Taiwan, R.O.C.
  Email: John@quantatw.com
  Tel: 886+3+3979000 ext. 5183

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