Re: [IBIS-Users] Extract IBIS data from lab measurement

> 2. For the IV data, I successfully extract it from the measurement but the
> extracted result only show agreement with original IBIS model (provided by
> manufacturer)  for operation region. When out of the operation region, i.e.,
> in diode model, the IV characteristics  I got is quite different from the
> original one.
> For example, the voltage operation region of my chip is [-0.5, 3.8], in this
> range, the measurement value of current is reasonable. However, for the
> current measured beyond this voltage range, such as [-3.3, -0.5], the
> measurement result will distort severely from the original IBIS value. Could
> anyone help me analyze the possible reason for the disagreement? Or is there
> something wrong I may make in measurement setting? (I made the measurement
> setup according to cookbook version4.0)

In theory, you should get good agreement between your measurements and
the IBIS values.

But when you go well into the clamp regions, the current levels can
become excessive, and this can cause difficulties. Some of those
difficulties include heating the die (causing the values to shift),
and damaging or even melting the chip.

When it is impossible to make meaningful, accurate lab measurements at
the extremes, it may be necessary for the IBIS modeler to extrapolate
those values from lower voltages, and/or from SPICE models. Those
extrapolated values might not even be "correct", but they are needed
anyway to help the simulator know what to do when overshoot happens.
In normal use, one shouldn't get a waveform that converges several
volts into the clamp region, so the accuracy of those extreme clamp
currents, is less important than having a value (or a slope) that
pushes the simulator back into the range of realistic values.

Of course you need to add the values from all the IBIS IV tables to
compare with your measurements. How did you do that? Did you run the
IBIS model through a simulation with a DC sweep? There are some cases
where (for example) HSpice simulations don't include the clamp IV
tables, so make sure you are doing that part right.

And it is also possible that the manufacturer's IBIS model is wrong.
Clamp diodes in many SPICE models are grossly wrong, and if the IBIS
model was derived from that SPICE model, this could explain your
discrepancies.

Regards,
Andy

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