From: abe riazi ([email protected])
Date: Mon Sep 18 2000 - 14:29:59 PDT
Dear Arpad and Others:
I like to add that by reading Ron Kielkowski's book, I did not see any reference to IBIS models. That is the classification, transistor level, macromodel and behavioral macromodel are only a hierarchical definition for SPICE models (i.e. behavioral macromodels defined in this book are not the same as IBIS format models). I am making this point because many times models are broadly classified as FUNCTIONAL (i.e. SPICE models) and BEHAVIORAL (meant to be IBIS models). Similarly the only simulation engine discussed in this book is SPICE.
From: Muranyi, Arpad [SMTP:[email protected]]
Sent: Monday, September 18, 2000 1:15 PM
To: '[email protected]'
Subject: RE: [SI-LIST] : Macromodel Creation
Thanks for your explanation and the quotes from Ron Kielkowski.
However, I have to argue with him then. Not knowing what else
he says in his book (I am not sure whether he spells out some of
his assumptions or not), reading these words as they are I
must say that he is incorrect all the way through. (He may
be correct given some assumptions, but not in general).
1) SPICE transistor level models do NOT "represent devices at
the most basic simulation level possible". I could think of
a future(?) simulator which simulates a transistor by the
electron behavior in the crystal structure of the material(s)
that it is made from. "Most basic" will always be as low level
as our knowledge is about particles, which is currently even
lower than electrons and crystal structures. Is there any mention
in SPICE models how electrons bump into each other in the material?
No. For that reason I don't agree that SPICE represent devices at
the most basic level.
2) Simplification is not necessarily related to accuracy. Think
about a math example: a^2+2ab+b^2 = (a+b)^2 It is probably much
simpler and faster to calculate the result of the right side than
the left side of the above equation. Is one of them less accurate
than the other? Similarly, simplifying a transistor model from its
geometric description (SPICE) to an electrical description (behavioral)
does not have to mean that the latter one is less accurate. It all
depends on how the simplification is made and whether something was
omitted during the simplification process. If things are omitted,
I would rather call the "simplification" compromising. Now, hold
on tightly: Even SPICE transistor level models contain omissions!
Just think how many of the higher order effects are missing in the
earlier MOSFET models (level=3) vs. newer models BSIM4, for example.
So I would be very careful to say that SPICE transistor models are
the king. It all depends on how much effort the model maker puts
into them too, just as it is with behavioral models.
3) The Op-amp example is a perfect one. We know very well how to
characterize them for gain, input and output impedance, frequency
and phase response, distortion, noise, etc... An Op-amp circuit
can be modeled behaviorally very well with a few fairly simple
expressions. Even though I haven't done it (because I work with
digital circuits), I am fairly confident that these expressions can
be formulated so that the response of these expressions will be
right on top of the original transistor level model's response.
Will this behavioral model be faster? Most likely yes. Will it
be less accurate? Not necessarily.
And I agree, there are always exceptions. I could tell you an example
where a behavioral model was slower than the original transistor model
if they wouldn't have to kill me if I did so. And on top of that,
chances are that it still was not as accurate as the original
transistor level model. However, I found out from the guy who
implemented the simulator engine that the reason for that was in
the architecture of the simulator and/or model. Simulators are
optimized for certain things to speed them up. These optimizations
may work in some cases but not in others. I am just guessing, but
I feel that accuracy may also be optimized for certain situations
in simulators. So there are a lot of ifs and buts, but the bottom
line in my opinion is that the relationship that behavioral models
are faster but less accurate then their transistor level equivalents
is not true in general. It all depends on the model maker.
(And remember, IBIS files are not IBIS models, they just contain the
data for the model which resides in the simulation tool. But I don't
want to start IBIS vs. SPICE here, so that's the last I will say on
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