Re: [IBIS-Users] Capacitance and transition time

In normal usage, the actual load (i.e., traces and other devices) connected
to an output device creates a rather complex, frequency dependent load.
It's highly unlikely that the output device was characterized with that kind
of load.

While there may not be a guarantee, one should expect that an IBIS model
would produce reasonably good results with such complex loads as well as
simple R/C loads (with not unreasonable values), even though they weren't
use to generate the VT curves.

Anyway, most VT curves are established with resistive loads (zero
capacitance), which "never" happens in practice.  The VT curves may be used
only to "calibrate" the simulator's representation of the ouput device, not
to generate actual waveforms.

Regards,
Andy


> A coworker wants to simulate what affect connecting
> different capacitors to the output pin of a device
> would have on its transtion time.  He doesn't have
> access to spice models and he asked me if he could use
> IBIS models.  I told him that since the models are
> characterized under a given load, changing that load
> in simulation may not give him very accurate results.
> Was I correct, or will the simulator use the values of
> the IV curves and test fixture to accurately skew the
> rise and fall times specified in the VT curves?

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