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

  • From: "Tom Cipollone" <tom_cip_11551@xxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 02 Oct 2003 22:03:31 -0000

Michael,

Thank you for the most informative post. However, I have a few 
questions. 

In describing the pi network, for the example given, there is a 350 
Ohm resistor placed between the end nodes, and two 70 Ohm resistors, 
one placed from each end node to a common node. Call that common 
node ground, if we are dealing with signals which are symmetrical 
about ground. The parallel combination of all these resistors is 100 
Ohms.

Would I be correct in thinking that for the differential signals, 
there would be a virtual ground (in this case, at ground) "in the 
middle" of the 350 Ohm resistor? If that is so, then "half" of the 
350 Ohm resistor in parallel with 70 Ohms is pricisly 50. Could the 
pi network, with 3 resistors be replaced with two resistors, 50 Ohms 
each, one from each end node to ground? 


Also, further down, you write:
> Having said all that, you are correct that a capacitor at the 
common
> node
> should be selected to act an AC terminator to common mode signals.

In the pi network that you describe, where is the common node?


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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