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

["Mirmak, Michael" <michael.mirmak@xxxxxxxxx>]

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@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Tom Cipollone
Sent: Wednesday, October 01, 2003 9:00 AM
To: si-list@xxxxxxxxxxxxx
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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