re: [IBIS-Users] Why is it necessary to take R_load/R_fixture, rather than taking only Capacitor(C_load) for V-T tables?

 


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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
 
 
 
 


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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?



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