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

[Mike Brown <bmgman@xxxxxxxxxx>]

As Michael notes, the CT (center tap) capacitor does not guarantee 
optimal termination for common mode voltages - it is correct only if the 
coupling is zero.  However, the absence of a voltage source at the CT of 
the resistors guarantees a CM reflection coefficient of 1.  If the CT is 
present, it is possible to reduce the CM reflection coefficient to 
(near) zero with an appropriate  low-value R between the cap and the CT 
of the differential termination (a Y-topology).  An equivalent pi can be 
derived.  In many cases, the low-value R can be omitted with little 
practical degradation in the signal integrity.  (YMMV)

Regards

Mike

Mirmak, Michael 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@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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