Ravinder, Since the drive strength is high, the rise and fall times may be small and this increases the knee frequency (0.5/tR or .5/tF whichever is more, where tR is the rise time and tF is fall time). The trace will be considered distributed if it is longer than L/6 where L is the length of the rising edge =3D tR/D where D is the delay in ps/in. If the rise = time is approximately 1 ns, then traces longer than 1" are considered as distributed. The above formulas (1.1, and 1.3 ) are given in Howard Johnson's book "High Speed Digital Design" and you can do your math. Regards, Prasad -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Ravinder.Ajmani@xxxxxxxxxxxxxx Sent: Tuesday, August 30, 2005 4:11 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Driver strength and trace capacitance I was simulating a 2" long net driven by an ASIC and noticed significant ringing and non-monotonicity. This is a bidirectional net and the driver=20 at the other end is a 50 ohm driver, which does not require any=20 termination. From the IBIS model I deduced the driver impedance to be=20 about 3 ohms, which I thought was quite low. I was told that the ASIC=20 vendor was asked to design the driver to drive a 100 MHz signal in to a 35=20 pF load. The trace capacitance is about 10 pF (including vias), and the receiver capacitance is less than 5 pF (package plus die). To me this=20 appears to be a case of over-design. However, I have one question about the trace capacitance. Can it be considered a lumped load or a=20 distributed load. The Lambda/20 length at 100 MHz is about 3" so I am=20 inclined to treat the total trace capacitance as a lumped load. Regards, Ravinder Server PCB Development Hitachi Global Storage Technologies Email: Ravinder.Ajmani@xxxxxxxxxxxxxx ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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