[SI-LIST] Re: On choosing center tap capacitor value for differential termination

  • From: "Tom Cipollone" <tom_cip_11551@xxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 01 Oct 2003 20:21:29 -0000

Michael,

Thank you very much for the post.

I would like to view the articles that you referenced but I was not 
able to find them archived at the PC Design Magazine website. They 
only go back as far as 2002. I go to libraries for IEEE journals and 
such but if you know of somewhere that I can get these articles at 
the "click of a mouse" I would be most appreiciative.

Also, just so that you know, there are a number of printed 
characters in your post that I am sure are unintentional. These are 
the "=3D"s and "=20"s. 

Thank You 
Tom Cipollone




--- In si-list@xxxxxxxxxxxxxxx, "Mirmak, Michael" 
<michael.mirmak@xxxx> wrote:
> Tom,
> 
> Thanks for your message.  Before even getting to the issue of 
capacitor=20
> selection, I would advise caution in assuming that two resistors
> connected=20
> to a common node optimally terminate both common mode and 
differential
> mode=20
> signals.
> 
> As pointed out in several places (including PC Design magazine 
articles
> in
> March of '99 and August of '98), a two-resistor solution may 
terminate a
> 
> coupled trace pair well in differential mode but will likely
> misterminate=20
> in the common mode.  For example, imagine a two-trace system with 
an
> impedance=20
> matrix as follows:
> 
> Z =3D | 60 10 |
>     | 10 60 |
> 
> Here Z11 =3D Z22 =3D 60 ohms; Z12 =3D Z21 =3D 10 ohms
> Zdiff    =3D 2*Zodd    =3D 2*(Z11 - Z12)   =3D 100 ohms
> Zcommon  =3D 0.5*Zeven =3D 0.5*(Z11 + Z12) =3D 35 ohms
> 
> In this case, if you terminate this system with two 50 ohm 
resistors=20
> (one per line) connected to a common node, you will terminate
> differential=20
> mode signals to 100 ohms.  However, common mode signals will see a
> mismatch=20
> -- the termination will be 50 ohms rather than 70 ohms per line 
(35 ohms
> 
> Rcommon is generated by a system with each line terminated to 70 
ohms --
> 
> they add in parallel).  For real systems, you could see mode 
conversion=20
> issues because of this mismatch.
> 
> A PI termination would terminate in both modes and avoid mode 
conversion
> 
> issues.  In this case, each line would be terminated with a 70 ohm
> resistor. =20
> In addition, a third resistor would be placed across/between the
> endpoint=20
> nodes.  The parallel sum of the resistors should be equivalent to 
100
> ohms=20
> to terminate the differential mode signals.  Specifically,
> 
> Rdiff =3D 100 =3D (2*Reven) || X where X is the third resistor
> 2*Reven =3D 140 ohms
> X =3D 350
> 
> So, with a 350 ohm resistor across the endpoint nodes and two 70 
ohm=20
> resistors connected to a common node, the example system should 
be=20
> terminated correctly in all modes. =20
> 
> Having said all that, you are correct that a capacitor at the 
common
> node=20
> should be selected to act an AC terminator to common mode 
signals.  As=20
> an off-the-cuff answer, I believe the capacitance value should be
> equivalent=20
> to twice the cap values required to terminate each line in the 
common
> mode. =20
> The value should be selected based on your interface frequency,
> bandwidth, etc.
> 
> I hope this helps!
> 
> - Michael Mirmak
>   Intel Corporation
> 
> 
> -----Original Message-----
> From: si-list-bounce@xxxx [mailto:si-list-bounce@xxxx]
> On Behalf Of Tom Cipollone
> Sent: Wednesday, October 01, 2003 9:00 AM
> To: si-list@xxxx
> Subject: [SI-LIST] On choosing center tap capacitor value for
> differential termination
> 
> 
> Hi,
> 
> I am trying to find some guidelines concerning choosing the=20
> capacitor that is the center tap of two series resistors that form 
a=20
> differential termination. If my information is correct, the=20
> combination either of the resistors and the center tap 
capacitor,=20
> forms an AC common mode termination.=20
> 
> Is there a difference between choosing a center tap capacitor 
used=20
> in this way and choosing a capacitor for an AC termination?
> 
> This inquiry is directed towards the reduction of radiated=20
> emissions. The signals in questions have a rise time of about a=20
> nanosecond.
> 
> Thank You
> Tom Cipollone
> 
> 
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