Re: [IBIS-Users] C_Comp

> If the Ccomp value obtaining from Spice model (or from lab measurements) is
> put in the IBIS model and it is found that the IBIS simulation waveforms are
> not correlated to Spice model sim results, should we fine tune the Ccomp
> value until they are correlated?

Since this has gone unanswered ...

If you are the IBIS model (IBIS data sheet) creator, and any value that you
put into the IBIS model (data sheet) results in waveforms that do not
correlate well, then you should fine tune "whatever it takes" to get them to
correlate ... including (but not limited to) the value of C_comp.

C_comp primarily affects received waveforms, for an Input buffer or when a
Bi-directional is tri-stated; but not as much a buffer's driven waveforms.
If the received waveforms look OK but the driven waveforms don't correlate,
I'd probably leave C_comp alone.

But be careful about any such tweaking.  Use a wide variety of test loads,
and if I were you, I would determine WHY the simulated waveforms do not
correlate well.  If you need to alter anything significantly from its
"correct" or previously determined value, it would appear there is something
wrong with your process.

If you got the C_comp value from measurements and it doesn't correlate with
SPICE model simulation results, ask yourself whether you trust the sample
IC(s) more or the SPICE model more.  There are valid reasons for either of
them being more "correct."

If you are not the IBIS data sheet creator, you probably shouldn't be doing
this.  But ... sure, you can do anything you want with your own copy of the
model, exclusively for your own use, and subject to the terms of any agreement
you have with the vendor; but don't complain to the vendor from whom you got
the model when it doesn't work right.  If their model seems wrong, I would
first complain to the person who made the model, before tweaking it on my own.
(Actually, I do like to tweak things on my own, strictly for my own
educational purposes, but I also complain to the vendor once I get a better
idea about what is wrong or how to "fix" it.)

That's my take on it.

Regards,
Andy


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