Subject: Re: [IBIS] IV curve question...
From: Al Davis (aldavis@ieee.org)
Date: Sun Nov 03 2002 - 00:43:13 PST
On Friday 01 November 2002 12:23 pm, Silva, Benjamin P wrote:
> The simulator should do that automagically.
Don't count on it!!
Actually, it usually works because the elements are effectively
placed in parallel, which results in summing of the currents.
This is ok as long as ground_clamp_ref is the same as
pulldown_ref, and power_clamp_ref is the same as pullup_ref,
and if you are simulating them as fixed DC voltages.
It isn't ok if you are doing power plane analysis. The
voltages move and can be different. The current can follow two
different paths. This is why there are separate tables like
this.
In all cases where it doesn't matter, the new improved C_comp
doesn't matter either. It just puts them all in parallel. Yet
the committee approved the standard enhancement for that.
Truth is it does matter in some cases, particularly the same
ones that accurate separation of the clamp currents matters.
To make a model work in this type of analysis, you must be
careful with the subtraction, and look at both to see that they
make sense separately, and the derivatives make sense.
Most of the time, curves that are non-monotonic in this region
means the data is bad.
In the clamp region, the clamp current is large, and the
pullup/pulldown current is small. Most of the time what you
get when you subtract curves is numerical noise. If all you
have is numerical noise, you are better off just extending an
arbitrary straight line.
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