[SI-LIST] Re: Common Mode vs. Even Mode
- From: Doug Brooks <doug@xxxxxxxxxx>
- To: michael.mirmak@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 16 May 2002 09:20:08 -0700
I think the issue is a simple misunderstanding and confusion in terms.
In a coupled pair of traces, the individual trace termination value is
differential Z11 - Z12
Common Mode Z11 + Z12
In a practical sense, the differential resistors are placed in series, so
the net "differential" impedance is 2X(Z11-Z12)
Others say the common mode resistors can be visualized as being in
parallel, so the net parallel combination result is .5X(Z11+Z12)
So, differential impedance is "approximately" 2 times or 4 times common
mode impedance, depending on your definitions.
Doug Brooks
At 08:50 AM 5/16/2002 -0700, you wrote:
>In the current literature, the equations used to analyze a differential
>system's traces are fairly consistent: differential impedance is equal to
>twice the odd mode impedance, where the odd mode is the difference between
>the single-ended and coupling impedances, etc.
>
>One point of difference is the definition of common mode impedance. Several
>sources, including Douglas Brooks' 1998 Printed Circuit Design article,
>state that the common mode impedance is equal to one-half the even mode
>impedance. Similar statements can be found in industry specifications, such
>as those for Serial ATA.
>
>On the other hand, Eric Bogatin has explicitly stated in his differential
>training materials ("Differential Impedance Finally Made Simple", 2000) that
>common mode impedance is equal to the even mode impedance; yet, on this
>forum, he has recently stated (in an SI-LIST e-mail, Feb. 25, 2002) that
>common mode impedance is one-half of even mode impedance. The "common mode
>Z = even mode Z" definition is also given in a 1999 Printed Circuit Design
>article by Kaufer and Crisafulli.
>
>What is the "correct" definition of common mode impedance? I suspect that
>there is a referencing or perspective discrepancy here, but I would like to
>hear a formal explanation, preferably from the above experts or others on
>the list.
>
>Thanks!
>
>- Michael Mirmak
> Intel Corp.
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
_____________________________________________________________________
UltraCAD Design, Inc. Celebrating 10 years of design excellence!
http://www.ultracad.com
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
