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