Subject: Re: [IBIS-Users] damped oscillations in V-T curve
From: Ingraham, Andrew (a.ingraham@ieee.org)
Date: Tue Jul 01 2003 - 09:08:19 PDT
Depending on the internal feedback, feedback could screw up a simulation when using an IBIS model, because IBIS does not handle feedback very well if at all. If the feedback just reduces the output impedance or establishes a more stable operating point or something similar, then it might be OK. But the positive feedback that is used to make a "bus-hold" circuit, for example, is not handled by conventional IBIS models at all.
Feedback should not be a problem with a SPICE-like simulator and structural (transistor-level) models.
Truncating a waveform table does not sound good. On the other hand, what the simulator does with a waveform table is implementation dependent, and I believe that many simulators make little use of them ... just as a guide for how the simulator should ramp its drivers between Low and High states. (In which case the simulator might ignore everything after the waveform first reaches 100% anyway.) Most of the characteristics of the driver come from the I-V curves, and the waveform tables are just a second-order tweak in some cases. But again, I stress the "implementation dependent" part. You might find that your model works well with one simulator but horribly with another.
The reason I brought up the nonlinearity, is that (as I recall) Spectre is a frequency-domain simulator, and (P)ECL drivers are extremely nonlinear at the cutoff point, where they abruptly go from ~5 ohm drive strength to open-circuit, in a matter of millivolts. Representing such behavior with reasonable accuracy might require a very large number of harmonics, or else you could get ringing as an artifact.
Regards,
Andy
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