Greg, I have to fess up and admit that I can't give you an answer because I am not a circuit designer or a process engineer. I can only give you some hand waving explanations which you may even be able to do yourself. Consider that the channel and its parasitic capacitance is a parallel RC circuit. If that channel is an ideal switch, it is a short when it is on, and an open when it is off. How much capacitance do you see when you look into this "RC" circuit when it is "driving", or "receiving"? It will be 0 and C, respectively. Of course we don't live in an ideal world, so in reality we would see some non zero values, but I believe this could be part of it in addition to some lower level (quantum?) internal device behaviors inside the transistor itself. Take this a step further, add the predriver and Miller capacitance (and gain) to the circuit. Got the picture?... Anyone else have more info in this? I would be interested in hearing about it too. Regarding the rest of the comments on how to make C_comp more realistic, I would like to mention that I am beginning to be more and more convinced that we should model the buffer's upper and lower halves as two (or more) complex impedances. (I already gave a presentation on this a couple of summits ago, but didn't have time to continue the work yet). In this approach we would have a real part, which is the same as the IV curve today, and is frequency independent (it is only a voltage, and temperature dependent resistance), and an imaginary part, which is frequency (and voltage, and temperature) dependent and describes the parasitic capacitances (series and/or parallel) and of course those resistances which are in series with capacitances. This approach would put the C_comp in order. Any comments? Thanks, Arpad ============================================================= -----Original Message----- From: Gregory R Edlund [mailto:gedlund@xxxxxxxxxx] Sent: Thursday, January 10, 2002 6:09 AM To: si-list@xxxxxxxxxxxxx; arpad.muranyi@xxxxxxxxx Subject: [SI-LIST] Re: IBIS Driver Models, Simulators and CComp Arpad, What are the physical origins of the difference in C_comp between the driving case and the receiving case? Suppose C_comp = 2 pF. How much change could one expect to see? > Regarding the variability of the value of C_comp when the buffer > is driving or receiving, you are also correct in your observation. > It does depend on that as well as voltage, so a single value C_comp > really doesn't cut it. For more precise modeling we would need > multiple curves (C vs. V). Greg Edlund Electronic Packaging & Integration IBM Server Technology Development 3605 Hwy. 52 N, Dept. HDC Rochester, MN 55901 gedlund@xxxxxxxxxx ------------------------------------------------------------------ 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