Re: [SI-LIST] : IBIS Subcommittee Formed to Review Spice to IBIS

Al Davis (aldavis@ieee.org)
Wed, 09 Jun 1999 07:10:12 +0000

jonp@pacbell.net wrote:
> I have actually been faced with the problem that even with software to remove
> extra points I cannot
> create IBIS models to the accuracy that I desire with the 100 point limitation.

I don't believe even 10000 points would provide the accuracy you desire,
because the limits to accuracy are usually elsewhere.

For example ....
100 points chosen to be uniformly spaced in voltage, modeling a 5 volt
rising waveform, provides a data point every .2 volts. Assuming that
the interpolation is completely bogus, this leads to a worst case error
of .1 volts. In reality, interpolation is much better than that, and
even this choice of data points is far from optimal, so the accuracy is
much better than this. Remember, this isn't DC, but the dynamically
changing voltage during a high speed transition.

Some questions ....

Is your Spice model that accurate? (I don't think so.)

If you measure one real unit with perfect accuracy, and compare to the
next one, are they this close to each other. (I don't think so.)

Are your instruments this accurate? (Don't forget that you change the
reading when you attach the probe.)

What if the real load is different from the test load? Since the
driving impedance is nonlinear, and not specified anywhere, even with
perfectly accurate and complete data for the nominal load, all bets are
off for anything else.

Sure, the voltage is right, into a resistor, but what about reflections
and noise?

How accurate is the rest of the simulation?

There are other ways to improve the accuracy. One obvious one is to
carefully select the data points (as Kellee said). Another is to add
another VT table with a different load. (As Kellee also said) The
third table does much more to improve accuracy than more points will
do. Tying the load resistor to a middle voltage does a good job at
modeling the overlap.

I wouldn't object to removing the limit entirely, because whatever the
limit is, most files will use it, without any valid reason. With no
limit at all, it is up to the modeler to decide what trade-offs to use.
Maybe with nothing in the spec saying how many points to use, we might
actually get some good 20 point models, that are more accurate than the
256 (or whatever) point models that we will get if the limit is raised,
and the capability is there for those rare cases when you really do need
the extra points. So, maybe the answer is to remove the hard limit, but
add a comment that tables with more than 50 points are rarely worth the
extra space.