Michael, Thank you very much for the post. I would like to view the articles that you referenced but I was not able to find them archived at the PC Design Magazine website. They only go back as far as 2002. I go to libraries for IEEE journals and such but if you know of somewhere that I can get these articles at the "click of a mouse" I would be most appreiciative. Also, just so that you know, there are a number of printed characters in your post that I am sure are unintentional. These are the "=3D"s and "=20"s. 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 > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxx 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@xxxx 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@xxxx 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@xxxx 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