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 > > >------------------------------------------------------------------ >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: =20 > //www.freelists.org/archives/si-list >or at our remote archives: > http://groups.yahoo.com/group/si-list/messages=20 >Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > =20 > >------------------------------------------------------------------ >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 > > > > > ------------------------------------------------------------------ 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