_____ From: Lynne D. Green [mailto:lgreen22@mindspring.com] Sent: Wednesday, February 14, 2007 12:33 PM To: 'bhavneet aujla' Subject: RE: [IBIS-Users] Why is it necessary to take R_load/R_fixture, rather than taking only Capacitor(C_load) for V-T tables? Hello, Bhavneet, Having worked on both ICs and PCBs, here is what I have learned of the history: PCBs are more sensitive to transmission line effects, because the traces are long compared to an edge delay. There is usually a reference plane (ground/power) in the stackup, making the resistor in IBIS is a good model for this transmission line. In addition, SI algorithms can have simulation problems if the V-t load is not resistive. The lumped Cload in CMOS modeling is used to generate "cell" timing tables. HOWEVER, above about 300 MHz, longer on-chip traces need to be modeled as LC circuits with capacitive coupling, rather than just a lumped Cload. Modeling traces as full transmission lines has been a hot topic for about five years; the challenge is modeling without a ground plane (the CMOS substrate being a highly-lossy plane). Hope this helps. Lynne "IBIS training when you need it, where you need it." Dr. Lynne Green Green Streak Programs http://www.greenstreakprograms.com 425-788-0412 lgreen22@mindspring.com _____ From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf Of bhavneet aujla Sent: Wednesday, February 14, 2007 8:55 AM To: ibis@eda-stds.org; ibis-users@eda.org Subject: [IBIS-Users] Why is it necessary to take R_load/R_fixture, rather than taking only Capacitor(C_load) for V-T tables? Hello, For the realisation of V-T table/Ramp [Rising Waveforms and Falling Waveforms] at the output we require a Resistance [R_fixture/ R_load], but in CMOS, Capacitor acts as load so why it is'nt neccessary to take Capacitor at output for extraction of V-T. Since i know R-fixture specifies the impedance of the system transmission lines the buffer will drive, but still confused that, for CMOS load is capacitor, and finally output via transmission line has to drive CMOS (which is modeled as capacitor). Regards Bhavneet Singh Sub: Why is it necessary to take R_load/R_fixture, rather than taking only Capacitor(C_load) for V-T tables? _____ -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean. -------------------------------------------------------------------- |For help or to subscribe/unsubscribe, e-mail majordomo@eda-stds.org |with the appropriate command message(s) in the body: | | help | subscribe ibis <optional e-mail address, if different> | subscribe ibis-users <optional e-mail address, if different> | unsubscribe ibis <optional e-mail address, if different> | unsubscribe ibis-users <optional e-mail address, if different> | |or e-mail a request to ibis-request@eda-stds.org. | |IBIS reflector archives exist under: | | http://www.eda-stds.org/pub/ibis/email_archive/ Recent | http://www.eda-stds.org/pub/ibis/users_archive/ Recent | http://www.eda-stds.org/pub/ibis/email/ E-mail since 1993Received on Thu Feb 15 13:34:27 2007
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