**From:** Muranyi, Arpad (*[email protected]*)

**Date:** Mon Sep 18 2000 - 13:15:28 PDT

**Next message:**Mellitz, Richard: "RE: [SI-LIST] : Macromodel Creation"**Previous message:**abe riazi: "RE: [SI-LIST] : Macromodel Creation"**Maybe in reply to:**abe riazi: "[SI-LIST] : Macromodel Creation"**Next in thread:**Mellitz, Richard: "RE: [SI-LIST] : Macromodel Creation"

Abe,

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

that).

Sincerely,

Arpad Muranyi

Intel Corporation

================================================================

-----Original Message-----

From: abe riazi [mailto:[email protected]]

Sent: Monday, September 18, 2000 12:10 PM

To: '[email protected]'

Cc: '[email protected]'

Subject: RE: [SI-LIST] : Macromodel Creation

Arpad:

Thanks for your response.

When I wrote that SPICE transistor level models are most accurate but also

most time consuming to simulate, I did not mean that it is "always" true and

there can be exceptions. but it will hold true in many cases. Ron

Kielkowski (Reference 1, PP. 5 - 7) presents good definitions and examples

in support of this point:

1. TRANSISTOR LEVEL MODEL: "represents devices at the most basic simulation

level possible. In many cases, the transistor-level model is the most

accurate model possible for simulation. On the downside though, the

transistor-level model also takes the most time to simulate."

2. MACROMODEL: " A macromodel is a collection of electrical components which

form a simplified representation of the modeled circuit. Many macromodels

contain dependent controlled sources to help simplify the structure of the

model. Being simplified means that the macromodel is often easier to

construct than transistor level model, and the macromodel often simulates

much faster than the transistor level model. But these two elements come at

the expense of a small loss in accuracy."

3. BEHAVIORAL MACROMODEL: "The highest level in modeling hierarchy is the

behavioral macromodel. Behavioral macromodels contain a collection of ideal

electrical or mathematical components. Often behavioral macromodels contain

a collection of ideal electrical or mathematical components which are used

to describe a function of the circuit. Being at the top of the hierarchy

means the behavioral model usually simulates faster than any other type of

model, but often this increased speed comes from a loss in accuracy".

As an example, the transistor level model of an Op Amp can have about 19

transistors (plus some passive components), the macromodel of the Op Amp

consists of only two transistors and four diodes (plus some passive

components and dependent controlled sources). The Op Amp behavioral model

contains much simpler input and output blocks.

Based on above definitions and examples, in many cases the transistor level

models are the most complex (and accurate representation of the device) but

at the price of being most time consuming to simulate.

Best Regards,

Abe

-----Original Message-----

From: Muranyi, Arpad [SMTP:[email protected]]

Sent: Monday, September 18, 2000 10:36 AM

To: 'abe riazi'; '[email protected]'

Subject: RE: [SI-LIST] : Macromodel Creation

Abe,

I would like to comment on the three bullets you listed which put accuracy

and speed into an inverse relationship regarding transistor level and

behavioral models. Simply said this general relationship is NOT TRUE.

You CAN model devices to even a higher level of accuracy behaviorally

than on a transistor (SPICE) level if you like. It all depends on what

parameters you use and what goes into the behavioral model. And this

increased accuracy does not mean that your model will automatically get

slower.

Take a transistor, for example. You can describe it with its geometry,

and properties of the materials that it is made from. A SPICE tool then

converts all that information to electrical characteristics. This takes

a lot of equations and calculations. On the other hand, you can describe

the same transistor's characteristics by providing its node voltage and

current relationships directly (with tables, equations, transfer functions,

etc...) which CAN reduce the number of calculations SPICE has to do, making

it faster.

Now think about the underlying model equations SPICE uses when you do it

the conventional SPICE way. You can have a LEVEL=3 or BSIM4 set of

equations. Which one is more accurate? Most likely the BSIM4, since

it is more recent. However, if your behavioral transistor model DOES

describe something that even BSIM4 cannot, you behavioral model will be

even more accurate. Yet this does not mean that it has to become

automatically slower.

What I wanted to illustrate here is that the accuracy of the model depends

on what goes into it. It's speed, however, depends on how the device is

described. These two are not as strongly related as your three points

suggest.

Arpad Muranyi

Intel Corporation

============================================================================

-----Original Message-----

From: abe riazi [mailto:[email protected]]

Sent: Friday, September 15, 2000 7:17 PM

To: '[email protected]'

Subject: [SI-LIST] : Macromodel Creation

Dear Scholars:

While visiting a Barnes & Noble bookstore in San Jose, I purchased a copy of

the "Spice Practical Device Modeling" , by Ron Kielkowski.

What especially appealed to me about this publication was its high emphasis

on model creation. In this book SPICE models are classified according to a

hierarchy which includes:

1. Transistor-level models ( provide highest accuracy, though most time

consuming to simulate).

2. Macromodels.

3. Behavioral Macromodels (fastest to simulate, but least accurate)

Most attention is devoted to Macromodels, because they offer a practical

level of accuracy (less than 5% rms error over operating range) and can be

created in a reasonable amount of time (less than eight hours).

The procedure recommended by Ron Kielkowski for construction of macromodels

consists of the following steps:

i. Review the datasheet to obtain as much information related to model

creation as possible (although, frequently majority of the information given

in the datasheet has little value towards model generation).

ii. Utilize bench-top measurement equipment to produce I-V, C-V and Z-F

curves.

iii. From above data extract the desired model parameters.

For a resistor, the Macromodel elements consist of a nominal resistance

Rnom and a parallel capacitance Cp; for an inductor, Lnom (nominal

inductance), Rs (coil resistance) and Cp (winding capacitance); and for a

capacitor, Cnom (an ideal capacitor), RL (leakage resistor), Ls (series

inductor) and ESR (electrical series resistance). These macromodels are

illustrated by Figure 1 (attached gif picture).

In this publication (reference 1), the significance of impedance vs.

frequency plots is emphasized, because:

a. Regarding macromodel of a resistor, the |Z| vs. F graphs aid to

ascertain Cp.

b. For inductor Macromodels, they allow determination of the series

resistance frequency (Frs) and self resonating frequency (Fsrf) from which

values of Lnom and Cp can be calculated via simple formulas.

c. Considering capacitor macromodel, several parameters can be extracted

from the impedance vs. frequency curves, such as ESR (RS) , lead inductance

Ls (calculating Ls involves Fsrf which can be obtained from graph) and Cnom

(the nominal capacitance can be also measured by means of a low frequency

capacitance meter).

ESR and |Z| vs. F plots have been explained previously in this forum in

relation to PCB power distribution systems, decoupling and bypass

capacitors. They are also included here due to their significance towards

macromodel generation.

Figure 2 presents two examples of impedance vs. frequency graphs. Such plots

can be created in a number of different ways; here, Microsoft Excel was

employed. In each case the raw data consisted of three columns: current ( I

) , Voltage drop ( V ) and frequency ( F ). The Excel program calculated

another data column (impedance Z = V/I ), and produced the logarithmic

impedance plots. Clearly, ESR strongly influences the shape of |Z| vs. F

curves.

Macromodels can be incorporated into SPICE simulation files as subcircuits;

demonstrated by the example below:

Example 1. Encapsulation of a capacitor macormodel CMACRO, having

parameters Cnom, RL, Ls and Rs (ESR).

In the circuit input file example.cir:

X_MACRO 2 0 CMACRO

.INCLUDE EXAMPLE.MOD

In the model file example.mod:

.SUBCKT CMACRO 10 20

Cnom 10 30 1000uF

Rs 30 40 0.15ohms

Ls 40 20 5nH

RL 10 30 10meg

.ENDS CMACRO

Use of macromodels instead of SPICE primitive models can significantly

enhance the accuracy of a high frequency simulation and yield results in

excellent agreement with physical measurements.

Simulation of certain cases (such as high power circuits) require taking

into consideration effects due to temperature variations. Temperature

dependent macromodels can be readily constructed (reference 1).

To summarize, Macromodels assume an intermediate position in the hierarchy

of SPICE models in the sense that they are below the transistor-level models

in accuracy and rank second to behavioral models in simulation speed. They

are in demand by being practical; i.e., can be created in a reasonable

amount of time with an error margin tolerable in many applications.

Impedance vs. frequency plots play a critical role in creation of

macromodels of passive components. These models can be inserted into SPICE

input files as subcircuits. Simulations utilizing macromodels yield superior

results than using ideal SPICE primitives, particularly in the high

frequency domain.

Reference 1. R. M. Kielkowski, "SPICE Practical Device Modeling",

McGraw-Hill, Inc. 1995.

Thanks for your comments and with best regards,

Abe Riazi

ServerWorks

2251 Lawson Lane

Santa Clara, CA 95054

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**Next message:**Mellitz, Richard: "RE: [SI-LIST] : Macromodel Creation"**Previous message:**abe riazi: "RE: [SI-LIST] : Macromodel Creation"**Maybe in reply to:**abe riazi: "[SI-LIST] : Macromodel Creation"**Next in thread:**Mellitz, Richard: "RE: [SI-LIST] : Macromodel Creation"

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