RE: [SI-LIST] : Macromodel Creation

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From: Aubrey_Sparkman@Dell.com
Date: Mon Sep 18 2000 - 14:25:22 PDT


Richard,

> ... and ultimately based on measurements.

If this has always been true for you, then, you've been lucky!

Aubrey Sparkman
Signal Integrity
Aubrey_Sparkman@Dell.com
(512) 723-3592

> -----Original Message-----
> From: Mellitz, Richard [mailto:richard.mellitz@intel.com]
> Sent: Monday, September 18, 2000 3:50 PM
> To: 'si-list@silab.eng.sun.com'
> Subject: RE: [SI-LIST] : Macromodel Creation
>
>
> All models are behavioral and ultimately based on measurements. One
> behavioral model can be more accurate than another behavioral
> model if you
> put enough work into it. If you put more work in, it can even
> be relatively
> fast. Only time and money ... or is that momentum and position. :-)
>
> Accuracy of a single simulation may be amusing and interesting. The
> application of accuracy is a statistical problem with many
> parameters, only
> one being the accuracy of a single simulation.
>
> Richard Mellitz
> Intel
>
> -----Original Message-----
> From: abe riazi [mailto:ariazi@serverworks.com]
> Sent: Monday, September 18, 2000 3:10 PM
> To: 'si-list@silab.eng.sun.com'
> Cc: 'arpad.muranyi@intel.com'
> 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:arpad.muranyi@intel.com]
> Sent: Monday, September 18, 2000 10:36 AM
> To: 'abe riazi'; 'si-list@silab.eng.sun.com'
> 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:ariazi@serverworks.com]
> Sent: Friday, September 15, 2000 7:17 PM
> To: 'si-list@silab.eng.sun.com'
> 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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