Subject: Re: Number of Significant Digits
From: Al Davis (aldavis@ieee.org)
Date: Thu Feb 28 2002 - 01:36:37 PST
On Wednesday 27 February 2002 09:57 am, Lewis, Tony L wrote:
> I am wondering if there are any rules or common methods for
> deciding how many significant digits should be used when creating
> an IBIS model?
If you are asking this question, you should also ask about guidelines
for determining how to sample a waveform or I/V table. They are
related. With more digits, you need more data points. With more
data points, you need more significant digits.
The key to picking the proper resolution (in both) is to look at the
derivative. You should use enough digits, and the appropriate
samples, that the derivative looks good. This is more digits than
intuition would suggest, and fewer samples than intuition suggests.
You might need to smooth the data to get a reasonable derivative.
Even though the derivative is not explicitly specified, it is
actually more important than than the values themselves. In
addition, it tends to magnify errors and noise.
You should never have more than 2 adjacent points with the same
value. If you get this, you should drop some points or use more
sigificant digits so they are not identical. If you use more digits,
make sure they are truly sigificant, not just noise. Even 2 adjacent
points with the same value are usually wrong, unless you really mean
to say the derivative is zero.
Speaking of dropping points ... If the derivative is the same on
successive points, you should drop some.
For a I/V table, the derivative is di/dv, or incremental admittance.
The reciprocal is incremental resistance. This is the value that
determines how transmission lines are terminated, so it must be
correct if you want correct modeling of reflections.
For a wave table, the derivative is dv/dt, which enters into the
calculation of current with a capacitive load. Errors in the
derivative cause large errors in load current with reactive loads.
Considering the importance of the derivative, I find it strange that
none of the tools address it. There are tools that show you the
waveform on a scope. It is easy to also show the derivative, which
would be very helpful at revealing model problems.
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