Vaibhav,
If you could share the resulting waveforms, we might get a better understanding
of the problem.
Here are the answers to your questions.
1. Should we use the load cap while extracting the VT data?
No. In fact, you usually get better model results if the fixture for VT
extraction is purely resistive. Also, extracting VT data with large resistance
values usually gives poor results. You might include this waveform with 2K
pullup and another with 50 or 100 Ohm pullup. The simulator can use the
additional data to make a more accurate internal model.
2. Will the IBIS model take care, the effect of load cap connected
externally (Not included in IV and VT extraction setup) to match the spice
results loaded with the same cap?
This is not an effect of the model, but of the circuit simulator. This is a
basic circuit that any commercial simulator should handle easily.
3. Can we use the IBIS model generated with one R_pullup value, for
simulations with another value of R_pullup?
Yes. The simulators can scale the VT curves correctly for different loads.
Regards,
Weston
From: ibis-bounce@xxxxxxxxxxxxx [mailto:ibis-bounce@xxxxxxxxxxxxx] On Behalf Of ;
Vaibhav R Katkar
Sent: Monday, February 4, 2019 9:00 AM
To: ibis@xxxxxxxxxxxxx; ibis-users@xxxxxxxxxxxxx
Subject: [ibis] Query on C_load
Hi All,
I'm modeling an open drain buffer (I2C) which has a R_pullup of 2KOhms and load
cap is 150pF.
While generating VT data, I am connecting only R_pullup to the VCC and not the
load cap.
But in the application, I am connecting the load cap to the output while
simulating the IBIS model.
Results with this setup are not matching with the design results with the same
setup.
Que:
1. Should we use the load cap while extracting the VT data?
2. Will the IBIS model take care, the effect of load cap connected externally
(Not included in IV and VT extraction setup) to match the spice results loaded
with the same cap?
3. Can we use the IBIS model generated with one R_pullup value, for simulations
with another value of R_pullup?
Thanks in Advance,
Vaibhav
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