Microsim PSPICE OPTIMIZER User Manual

MicroSim PSpice Optimizer

Analog Performance Optimization Software

User’s Guide

MicroSim Corporation 20 Fairbanks Irvine, California 92618 (714) 770-3022

Printed in the United States of America.

TradeMarks

Referenced herein are the trademarks used by MicroSim Corporation to identify its products. MicroSim Corporation is the exclusive owners of “MicroSim,” “PSpice,” “PLogic,” “PLSyn.”

Additional marks of MicroSim include: “StmEd,” “Stimulus Editor,” “Probe,” “Parts,” “Monte Carlo,” “Analog Behavioral Modeling,” “Device Equations,” “Digital Simulation,” “Digital Files,” “Filter Designer,” “Schematics,” “PLogic,” ”PCBoards,” “PSpice Optimizer,” and “PLSyn” and variations theron (collectively the “Trademarks”) are used in connection with computer programs. MicroSim owns various trademark registrations for these marks in the United States and other countries.

SPECCTRA is a registered trademark of Cooper & Chyan Technology, Inc. Microsoft, MS-DOS, Windows, Windows NT and the Windows logo are either registered trademarks or trademarks

of Microsoft Corporation. Adobe, the Adobe logo, Acrobat, the Acrobat logo, Exchange and PostScript are trademarks of Adobe Systems

Incorporated or its subsidiaries and may be registered in certain jurisdictions. EENET is a trademark of Eckert Enterprises. All other company/product names are trademarks/registered trademarks of their respective holders.

Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of MicroSim Corporation.

As described in the license agreement, you are permitted to run one copy of the MicroSim software on one computer at a time. Unauthorized duplication of the software or documentation is prohibited by law. Corporate Program Licensing and multiple copy discounts are available.

Technical Support

Internet Tech.Support@microsim.com Phone (714) 837-0790

FAX (714) 455-0554 WWW http://www.microsim.com

Customer Service

Internet Sales@MicroSim.com Phone (714) 770-3022

Before You Begin

Welcome to MicroSim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

MicroSim PSpice Optimizer Overview . . . . . . . . . . . . . . . . . . . . . xiv

How to Use this Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

Typographical Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . xv

Related Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvi

If You Have the Evaluation Version . . . . . . . . . . . . . . . . . . . . . . xvii

Things You Need to KnowChapter 1

Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

What is the PSpice Optimizer? . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

Designs that You Can Optimize . . . . . . . . . . . . . . . . . . . . . . . 1-3

Designs that You Cannot Optimize . . . . . . . . . . . . . . . . . . . . . 1-3

Using the PSpice Optimizer with Other MicroSim Programs . . . . . . . . . . 1-4

Terms You Need to Understand . . . . . . . . . . . . . . . . . . . . . . . . . 1-5

Primer: How to Optimize a DesignChapter 2

Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Optimizing a Diode Biasing Circuit—the Objective . . . . . . . . . . . . . . 2-2

Why Use Optimization? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3

Phase One: Developing the Design . . . . . . . . . . . . . . . . . . . . . . . 2-4

The PSpice Optimizer Advantage . . . . . . . . . . . . . . . . . . . . . . 2-5

Phase Two: Setting Up the Optimization . . . . . . . . . . . . . . . . . . . . 2-6

Defining Design Parameters . . . . . . . . . . . . . . . . . . . . . . . . 2-7

Setting Up Goals and Constraints . . . . . . . . . . . . . . . . . . . . . . 2-8

Setting up analyses for each goal and constraint . . . . . . . . . . . . 2-8

Developing performance measures . . . . . . . . . . . . . . . . . . . 2-9

Defining specifications: goals and constraints . . . . . . . . . . . . . 2-10

Phase Three: Running an Optimization . . . . . . . . . . . . . . . . . . . . . 2-11

Running the PSpice Optimizer . . . . . . . . . . . . . . . . . . . . . . . 2-12

Adding a Constraint and Rerunning the PSpice Optimizer . . . . . . . . . 2-14

iv Contents

Changing the Constraint and Rerunning the PSpice Optimizer . . . . . . 2-16

Using Standard Component Values . . . . . . . . . . . . . . . . . . . . 2-18

Producing Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-18

Saving Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-19

Updating the Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . 2-19

Using the PSpice OptimizerChapter 3

Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Activating and Loading the PSpice Optimizer . . . . . . . . . . . . . . . . . . 3-2

Activating the PSpice Optimizer . . . . . . . . . . . . . . . . . . . . . . . 3-2

From Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2

From the Windows 95 Start Menu . . . . . . . . . . . . . . . . . . . 3-3

Changing Activation Options . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Loading a Different Optimization File . . . . . . . . . . . . . . . . . . . . 3-4

The PSpice Optimizer Window . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

Specifications Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

Internal specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

External specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Parameters Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Error Gauge Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Adding and Editing Parameters . . . . . . . . . . . . . . . . . . . . . . . . 3-10

Adding a Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

Selecting a Parameter to Edit . . . . . . . . . . . . . . . . . . . . . . . 3-12

Adding and Editing Specifications . . . . . . . . . . . . . . . . . . . . . . . 3-13

Adding a Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13

Defining an Evaluation for an External Specification . . . . . . . . . . . 3-17

Selecting a Specification to Edit . . . . . . . . . . . . . . . . . . . . . . 3-18

Measuring and Optimizing Performance . . . . . . . . . . . . . . . . . . . . 3-18

Optimizing Your Design . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18

Graphically Monitoring Progress . . . . . . . . . . . . . . . . . . . . . 3-19

Exploring the Effect of Parameter and Specification Changes . . . . . . . . 3-20

Testing Performance when Changing Current Values . . . . . . . . . . . 3-21

Automatically recalculating performance . . . . . . . . . . . . . . . 3-22

Manually recalculating performance . . . . . . . . . . . . . . . . . 3-23

Ensuring reliable results when tweaking values . . . . . . . . . . . . 3-24

Excluding Parameters and Specifications from Optimization . . . . . . . 3-24

Testing Performance when Adding or Changing Parameters or Specifications .

3-25

Saving Intermediate Values . . . . . . . . . . . . . . . . . . . . . . . . 3-26

Viewing Result Summaries . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26

Producing Optimization Reports . . . . . . . . . . . . . . . . . . . . . . 3-26

Contents v

Viewing the Optimization Log . . . . . . . . . . . . . . . . . . . . . . . 3-28

Viewing Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28

Finalizing the Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-29

Using Standard Component Values . . . . . . . . . . . . . . . . . . . . . 3-29

Saving Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-30

Updating the Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-31

Understanding Optimization Principles and OptionsChapter 4

Chapter Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

Goals versus Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

Constrained Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3

Types of Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4

Feasible and Infeasible Points . . . . . . . . . . . . . . . . . . . . . . . . 4-5

Active and Inactive Constraints . . . . . . . . . . . . . . . . . . . . . . . 4-6

Lagrange Multipliers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6

Characteristics of Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7

Global and Local Minima . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

Starting Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

Convergence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9

Parameter Bounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9

Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10

How the PSpice Optimizer Estimates Derivatives . . . . . . . . . . . . . 4-10

Limitations of Derivative Data . . . . . . . . . . . . . . . . . . . . . . . 4-11

Target Value Scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-12

Default Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-13

Controlling Finite Differencing when Calculating

Derivatives (Delta Option) . . . . . . . . . . . . . . . . . . . . . 4-13

Limiting Simulation Iterations (Max. Iterations Option) . . . . . . . . . . 4-15

Specifying a Probe Display (Probe File and Display Options) . . . . . . . 4-16

Advanced Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17

Controlling Cutback (Cutback Option) . . . . . . . . . . . . . . . . . . . 4-17

Controlling Parameter Value Changes Between

Iterations (Threshold Option) . . . . . . . . . . . . . . . . . . . . 4-17

Choosing an Optimization Method for Single Goal

Problems (Least Squares/Minimization Options) . . . . . . . . . 4-19

Tutorial: Optimizing a Design (Passive Terminator)Chapter 5

Tutorial Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

The Passive Terminator Design . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

Loading the Design into Schematics . . . . . . . . . . . . . . . . . . . . . . 5-3

Setting Component Values to Expressions . . . . . . . . . . . . . . . . . . . 5-4

vi Contents

Defining Optimization Parameters . . . . . . . . . . . . . . . . . . . . . . . . 5-5

Defining the Analysis Type . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6

Running an Initial Circuit Analysis . . . . . . . . . . . . . . . . . . . . . . . 5-6

Activating the PSpice Optimizer . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

Viewing the Parameter Description . . . . . . . . . . . . . . . . . . . . . . . 5-8

Defining the Goals and Constraints . . . . . . . . . . . . . . . . . . . . . . . 5-9

Checking that the Design Will Simulate . . . . . . . . . . . . . . . . . . . . 5-11

Starting the Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11

Changing a Goal to a Constraint . . . . . . . . . . . . . . . . . . . . . . . . 5-13

Saving Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13

Tutorial: Exploring Design Tradeoffs (Active Filter)Chapter 6

Tutorial Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

The Active Filter Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

The Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

The Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

Testing Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5

Calculating Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5

Tweaking Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

Tweaking Goals and Constraints . . . . . . . . . . . . . . . . . . . . . . 6-7

Completing Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8

Tutorial: Using Constrained Optimization (MOS Amplifier)Chapter 7

Tutorial Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

The CMOS Amplifier Design . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

The Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3

The Evaluations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4

The Goals and Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6

Setting the Method for a

Single-Goal Optimization . . . . . . . . . . . . . . . . . . . . . . . . 7-7

Performing the Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8

Tutorial: Fitting Model Data (Bipolar Transistor)Chapter 8

Tutorial Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

Using the PSpice Optimizer to Fit Data to Model Parameters . . . . . . . . . . 8-2

The Bipolar Transistor Test Case . . . . . . . . . . . . . . . . . . . . . . . . 8-3

The Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4

The Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-5

The External File of Measured Data . . . . . . . . . . . . . . . . . . . . . 8-5

The Goals and Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6

Monitoring Progress with Probe . . . . . . . . . . . . . . . . . . . . . . . . . 8-8

Contents vii

Fitting the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-10

Error MessagesAppendix A

Appendix Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1

Error Message Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . A-2

File Types Used by the PSpice OptimizerAppendix B

Appendix Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1

File and Program Relationships . . . . . . . . . . . . . . . . . . . . . . . . . B-2

Measuring Performance Using Information in the Circuit File and .prb File B-4

Defining Specification Criteria in the External Data File . . . . . . . . . . B-5

File Type Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-6

Optimizing a Netlist-Based Desi

Appendix Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

Optimizing without a Schematic . . . . . . . . . . . . . . . . . . . . . . . . C-2

Setting Up the Circuit File . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3

Setting Up and Running the PSpice Optimizer . . . . . . . . . . . . . . . . . C-4

Example: Parameterizing the Circuit File . . . . . . . . . . . . . . . . . . . . C-6

nAppendix C

Index

Figures

Figure 1-1 Optimization Design Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4

Figure 2-1 Diode Biasing Design Example . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

Figure 2-2 “Phase One: Developing the Design” Design Flow . . . . . . . . . . . . . . . 2-4

Figure 2-3 “Phase Two: Setting Up the Optimization” Design Flow . . . . . . . . . . . . 2-6

Figure 2-4 “Phase Three: Running an Optimization” Design Phase . . . . . . . . . . . . 2-11

Figure 2-5 PSpice Optimizer Automatic Optimization Process . . . . . . . . . . . . . . . 2-13

Figure 2-6 Optimization Results for the Diode Design Example . . . . . . . . . . . . . . 2-14

Figure 2-7 Results after Adding the Power Constraint . . . . . . . . . . . . . . . . . . . 2-16

Figure 2-8 Results after Changing the Constraint Type . . . . . . . . . . . . . . . . . . . 2-17

Figure 2-9 Report Summary for the Diode Optimization . . . . . . . . . . . . . . . . . . 2-19

Figure 2-10 Updated Diode Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-20

Figure 3-1 The PSpice Optimizer Window . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

Figure 3-2 Example of a Specification Box . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

Figure 3-3 Example of a Parameter Box . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Figure 3-4 Sample Format for an External Specification . . . . . . . . . . . . . . . . . . 3-14

Figure 3-5 Sample Excerpt from a Report . . . . . . . . . . . . . . . . . . . . . . . . . 3-27

Figure 3-6 Sample Excerpt from a Log File . . . . . . . . . . . . . . . . . . . . . . . . . 3-28

Figure 3-7 Sample Derivative Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28

Figure 4-1 Resistive Terminator Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

Figure 4-2 Global and Local Minima of a Function . . . . . . . . . . . . . . . . . . . . . 4-8

Figure 4-3 Hypothetical Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-11

Figure 4-4 Hypothetical Data Glitch . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18

Figure 5-1 Resistive Terminator Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

Figure 5-2 Schematic for the Terminator Example, term.sch . . . . . . . . . . . . . . . . 5-3

Figure 5-3 Optimization Results for the Passive Terminator Example . . . . . . . . . . . 5-12

Figure 6-1 Schematic for the Active Filter Example, bpf.sch . . . . . . . . . . . . . . . . 6-2

Figure 6-2 Optimized Values for the Active Filter Example . . . . . . . . . . . . . . . . 6-8

Figure 7-1 Schematic for CMOS Amplifier Example, m2.sch . . . . . . . . . . . . . . . 7-2

Figure 7-2 Updated Performance Values for the Amplifier Example . . . . . . . . . . . . 7-8

Figure 7-3 Optimized Values for the Amplifier Example . . . . . . . . . . . . . . . . . . 7-9

Figure 8-1 Schematic for the BJT Model Fitting Example . . . . . . . . . . . . . . . . . 8-3

Figure 8-2 Initial Traces for the Ic and Ib Parameters . . . . . . . . . . . . . . . . . . . . 8-9

x Fi

Figure 8-3 Optimization Results for the BJT Model Fitting Example . . . . . . . . . . . 8-11

Figure 8-4 Probe Display after Optimization is Complete . . . . . . . . . . . . . . . . . 8-11

Figure 8-5 MicroSim Program and File Interactions Important to Optimization . . . . . . B-2

Figure B-1 Sample External Data File . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-5

ures

Tables

Table 1-1 Optimization Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5

Table 1-2 Valid Operators and Functions for PSpice Optimizer Expressions . . . . . . . 1-11

Table 3-1 Edit Parameter Dialog Box Controls . . . . . . . . . . . . . . . . . . . . . . 3-11

Table 3-2 Edit Specification Dialog Box Controls . . . . . . . . . . . . . . . . . . . . . 3-14

Table 8-1 Error Message Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . A-2

Table 8-2 Summary of PSpice Optimizer-Related File Types . . . . . . . . . . . . . . . B-6

Before You Begin

Welcome to MicroSim

Welcome to the MicroSim family of products. Whichever programs you have purchased, we are confident that you will find that they meet your circuit design needs. They provide an easy-to-use, integrated environment for creating, simulating, and analyzing your circuit designs from start to finish.

xiv Before You Begin

MicroSim PSpice Optimizer Overview

The MicroSim PSpice Optimizer is a circuit optimization program that improves the performance of analog and mixed analog/digital circuits. The PSpice Optimizer is fully integrated with other MicroSim programs. This means you can design your circuit with MicroSim Schematics, simulate with MicroSim PSpice A/D (or MicroSim PSpice), analyze results with MicroSim Probe and optimize performance within the same environment.

MicroSim PSpice Optimizer

MicroSim Parts

MODEL

+ BF =

symbols packages

models

MicroSim Schematics

MicroSim PSpice A/D

MicroSim Probe

MicroSim PCBoards

SPECCTRA Autorouter

packages footprints padstacks



reports

drill

Gerber

files

How to Use this Guide

This guide is designed so you can quickly find the information you need to use the PSpice Optimizer.

How to Use this Guide xv

This guide assumes that you are familiar with Microsoft Windows (NT or 95), including how to use icons, menus, and dialog boxes. It also assumes you have a basic understanding about how Windows manages applications and files to perform routine tasks, such as starting applications and opening, and saving your work. If you are new to Windows, please review your Microsoft Windows User’s Guide.

Typographical Conventions

Before using the PSpice Optimizer, it is important to understand the terms and typographical conventions used in this documentation.

This guide generally follows the conventions used in the Microsoft Windows 95 User’s Guide. Procedures for performing an operation are generally numbered with the following typographical conventions.

Notation Examples Description

ALL CAPS ANALOG.SLB or

CLIPPER.SCH

Library files and file names.

For UNIX users: All screen captures in this

manual are of Windows 95

boxes and windows.

dialo Most options in these dialo boxes and windows are available in your operatin environment. When certain options are not available to you, or you must do somethin differently than what is primarily outlined, information specific to your platform is provided.

Press C+

Type

... Commands/text entered from

VAC

C+r

monospace font

Notation Examples Description

A specific key or key stroke on the keyboard.

the keyboard.

xvi Before You Begin

If You Have the Evaluation Version

The evaluation version of the PSpice Optimizer has the following requirements and limitations:

• Requires the MicroSim PSpice A/D with Schematics

evaluation package.

• Is limited to one goal, one parameter, and one constraint.

Things You Need to Know

Chapter Overview

This chapter introduces the purpose and function of the PSpice Optimizer, the optimization process, and related terms.

What is the PSpice Optimizer?

capabilities and the criteria designs must meet for successful optimization.

Using the PSpice Optimizer with Other MicroSim Programs page 1-4 presents the high-level design flow for optimization

and how other MicroSim programs are integrated into each design phase.

Terms You Need to Understand

that are important for optimizing designs successfully.

on page 1-2 describes optimizer

on page 1-5 defines the terms

1-2 Things You Need to Know

What is the PSpice Optimizer?

The MicroSim PSpice Optimizer is a circuit optimization program that improves the performance of analog and mixed analog/digital circuits.

Run optimizations

iterative simulations, while adjusting the values of design parameters until performance goals, subject to specified constraints, are nearly or exactly met. Constraints can include simple bounds on parameter values and nonlinear functions. The PSpice Optimizer also computes Lagrange multipliers that provide information on the cost of each constraint on the solution.

Explore performance tradeoffs

values for design parameters, the PSpice Optimizer provides graphical feedback showing performance. You can also tweak goal and constraint values to examine changes to parameter values.

Fit model parameters

set of measured data points, and a good starting point for the parameter values, the PSpice Optimizer fits a more accurate model.

The PSpice Optimizer performs

When you enter new

Given a parameterized model, a

Designs that You Can Optimize

What is the PSpice Optimizer? 1

-3

A design that you can optimize must meet the following criteria:

• It works; that is, it simulates with PSpice to completion and

behaves as intended.

• One or more of its components have a variable value, and

each value that is varied relates to an intended performance goal.

• An algorithm exists to measure its performance as a

function of the variable value.

If you can visualize what factors should be adjusted to improve performance, and how you would manually step through the optimization process (even though the computations might seem unwieldy), then the design is a good candidate for the PSpice Optimizer.

Designs that You Cannot Optimize

You cannot use the PSpice Optimizer to:

Optimization problems are not always solvable by a particular

orithm.

• Create a working design. This especially applies when you

begin with a design that is far from meeting specifications.

• Optimize a design in which the circuit has several states

where a small change in a parameter value causes a change of state.

Example: A flip-flop is on for some parameter value, and off for a slightly different value.

1-4 Things You Need to Know

Using the PSpice Optimizer with Other

Because you can use Schematics, PSpice, and Probe to desi system, subcircuit, or component level, use the PSpice Optimizer to optimize at whatever level is most appropriate.

n and simulate at the

MicroSim Pro

rams

The PSpice Optimizer is fully integrated with other MicroSim programs. This means you can design your circuit with MicroSim Schematics, simulate with MicroSim PSpice A/D (or MicroSim PSpice), analyze results with MicroSim Probe and optimize performance within the same environment. Figure 1-1 illustrates the typical design flow for circuit optimization.

Phase One

MicroSim Schematics

MicroSim PSpice A/D

MicroSim PSpice Optimizer

MicroSim Probe

Develop

the

Design

Phase Two

Set Up

the

Optimization

See Chapter 2, “Primer: How to

Optimize a Design

description of each desi phase.

” for a detailed

ure 1-1

MicroSim PSpice Optimizer

MicroSim PSpice A/D

Run

the

Optimization

Optimization Design Flow

Phase Three

Fit

Model

Parameters

Terms You Need to Understand

Terms You Need to Understand 1

-5

Optimization

Optimization is the process of fine-tuning a design by varying design parameters between successive simulations until performance comes close to (or exactly meets) the ideal performance.

The PSpice Optimizer solves four types of optimization problems as described in Table 1-1.

Table 1-1

Problem Type PSpice Optimizer Action Example

unconstrained minimization reduces the value of a single goal minimize the propagation delay

constrained minimization reduces the value of a single goal while

unconstrained least squares

constrained least squares reduces the sum of squares of the

Optimization Problems

satisfying one or more constraints

reduces the sum of the squares of the individual errors (difference between the ideal and the measured value) for a set of goals

individual errors for a set of goals while satisfying one or more constraints

through a logic cell minimize the propagation delay

through a logic cell while keeping the power consumption of the cell less than a specified value

given a terminator design, minimize the sum of squares of the errors in output voltage and equivalent resistance

minimize the sum of squares of the figures of merit for an amplifier design while keeping the open loop gain equal to a specified value

*. All four cases allow simple bound constraints; that is, lower and upper bounds on all of the parameters. The PSpice Optimizer also handles nonlinear goals and constraints. **. Use unconstrained least squares when fitting model parameters to a set of measurements, or when minimizing more than one goal.

1-6 Things You Need to Know

See Chapter 6,Tutorial:

Exploring Design Tradeoffs (Active Filter) startin

6-1 for a workin

showin values.

For more information, see and

parameterized slider

Constraint on page 1-8.

on page

example

Goal

Parameter

A parameter defines a property of the design for which the PSpice Optimizer attempts to determine the best value within specified limits. A parameter can:

• Represent component values (such as resistance, R, for

a resistor).

• Represent other component attribute values (such as

slider settings in a potentiometer).

• Participate in expressions used to define component

values or other component attribute values.

The PSpice Optimizer can optimize designs with up to eight variable parameters.

Example: A potentiometer symbol in a schematic uses the SET attribute to represent the slider position. You can assign a parameterized expression to this attribute to represent variable slider positions between 1 and 0. During optimization, the PSpice Optimizer varies the parameterized value of the SET attribute.

Specification

A specification describes the ideal behavior

of a design in terms of goals and constraints. Examples: For a given design, the gain shall be 20 dB ±1 dB; for

a given design, the 3 dB bandwidth shall be 1 kHz; for a given design, the rise time must be less than 1 usec.

A design can have up to eight goals and constraints in any combination, but there must always be at least one goal. You can easily change a goal to a constraint and vice-versa.

The PSpice Optimizer accepts specifications in two formats: internal and external.

Internal specifications

An internal specification is composed of goals and constraints defined in terms of target values and ranges, which are entered into the PSpice Optimizer through dialog boxes.

External specifications

An external specification is composed of measurement data, which are defined in an external data file that is read by the PSpice Optimizer.

Terms You Need to Understand 1

-7

Target value

A target value is the ideal operating value for a characteristic of the design as defined by a goal or constraint specification.

Goal

A goal defines the performance level that the design should attempt to meet (e.g., minimum power consumption). A goal specification includes:

• The name of the goal.

• A target value and an acceptable range.

• A circuit file to simulate.

• An evaluation for measuring performance.

• An analysis type used for simulation-based evaluations.

The goal specification can also include:

• The name of the file containing Probe goal function

definitions (.prb file).

• When using an external specification, the name of the file

containing measured data and the columns of data to be used as reference.

Typically, the PSpice

Note

Optimizer measures performance usin

an evaluation that requires a simulation, and therefore, you must specify the circuit file for the simulation. However, when measurin

PSpice Optimizer

usin

performance

expressions which do not require a simulation, you do not need to specify a circuit file.

1-8 Things You Need to Know

Constraint

A constraint defines the performance level that the design must fulfill in which the target value exceeds, falls below, or equals a specified value (e.g., an output voltage that must be greater than a specific level). The constraint specification includes:

• The name of the constraint.

• A target value and an acceptable range.

• A circuit file to simulate. (See note on previous page.)

• An evaluation for measuring performance.

• An analysis type used for simulation-based evaluations.

• An allowed relationship between measured values and the

target value, which can be one of the following: <= measured value must be less than or equal

to the target value = measured value must equal the target value >= measured value must be greater than or

equal to the target value

The constraint specification can also include the name of the file containing Probe goal function definitions (.prb file).

See Optimization on page 1-5 for more on least-squares and minimization al

orithms.

Constraints are often nonlinear functions of the parameters in the design.

Example: Bandwidth can vary as the square root of a bias current and as the reciprocal of a transistor dimension.

Performance

The performance of a design is a measure of how closely its specifications’ calculated values approach their target values for a given set of parameter values. When there are multiple specifications (at least one of which is a goal), the PSpice Optimizer uses the sum of the squares of their deviations from target to measure closeness. For a single specification (goal), the PSpice Optimizer uses either the goal’s value, or the square of its deviation from target.

Each aspect of a design’s performance is found by either:

• first performing the appropriate simulation, then running

Probe to measure characteristics of the resulting waveform(s), or

• evaluating PSpice Optimizer expressions.

In many cases (particularly if there are multiple conflicting specifications), it is possible that the PSpice Optimizer will not meet all of the goals and constraints. In these cases, optimum performance is the best compromise solution—that is, the solution that comes closest to satisfying each of the goals and constraints, even though it may not completely satisfy any single one.

Terms You Need to Understand 1

-9

Evaluation

An evaluation is an algorithm that computes a single numerical value, which is used as the measure of performance with respect to a design specification.

The PSpice Optimizer accepts evaluations in one of these three forms:

• single-point Probe trace function

• Probe goal function

• PSpice Optimizer expression

Given evaluation results, the PSpice Optimizer determines whether or not the changes in parameter values are improving performance, and determines how to select the parameters for the next iteration.

Trace function

A trace function defines how to evaluate a design characteristic when running a single-point analysis (such as a DC sweep with a fixed voltage input of 5 V).

Examples: V(out) to measure the output voltage; I(d1) to measure the current through a component.

Refer to the online MicroSim

PSpice A/D Reference Manual

for the variable formats and mathematical functions you can use to specify a trace function.

1-10 Things You Need to Know

Refer to the Goal Function wizard in Probe user’s how to develop and specify functions.

Here are some quick tips. In Probe:

• To test the value returned by

• To see the waveforms and

See Gain on page 7-5 for an example of the YatX function definition.

uide for information on

a specified select Eval Goal Function from the Trace menu.

marked points used to evaluate a select Display Evaluation in the Probe Options dialo (from the Tools menu, select Options to display this dialo box).

and your PSpice

oal

oal function,

box

oal

Probe goal function

A Probe goal function defines how to evaluate a design characteristic when running any kind of analysis other than a single-point sweep analysis. A goal function computes a single number from a Probe waveform. This can be done by finding a characteristic point (e.g., time of a zero-crossing) or by some other operation (e.g., RMS value of the waveform).

For example, you can use Probe goal functions to:

• Find maxima and minima in a trace.

• Find distance between two characteristic points (such as

peaks).

• Measure slope of a line segment.

• Derive aspects of the circuit’s performance which are

mathematically described (such as 3 dB bandwidth, power consumption, and gain and phase margin).

To write effective goal functions, determine what you are attempting to measure, then define what is mathematically special about that point (or set of points).

Note

Be sure that the goal functions accurately measure what they are intended to measure. Optimization results hi functions behave. Discontinuities in

hly depend on how well the goal

oal functions (i.e., sudden jumps for small parameter chan can cause the optimization process to fail.

es)

PSpice Optimizer expression

A PSpice Optimizer expression defines a design characteristic. The expression is composed of optimizer parameter values, constants, and the operators and functions shown in Table 1-2.

Example: To measure the sum of resistor values for two resistors with parameterized values named R1val and R2val, respectively, use the PSpice Optimizer expression R1val + R2val.

Terms You Need to Understand 1

-11

Table 1-2

Valid Operators and Functions for PSpice

Optimizer Expressions

Operator Meanin

+ addition

- subtraction * multiplication / division ** exponentiation exp e log ln(x) log10 log sin sine cos cosine tan tangent atan arctangent

Note

Unlike trace functions and Probe goal functions,

(x)

PSpice Optimizer expressions are evaluated without usin

a simulation or Probe.

Derivative

A derivative defines mathematically how a specification value changes with a small change in parameter value.

For a given design, the PSpice Optimizer calculates derivatives for each specification with respect to each parameter. Within an applicable range, the optimizer uses the derivatives to estimate new values for the goals and constraints.

See Derivatives on page 4-10 for a detailed discussion.

Primer: How to Optimize a

Desi

Chapter Overview

This chapter guides you through the basic steps needed to setup and run an optimization using a simple diode biasing design.

Optimizing a Diode Biasing Circuit—the Objective

describes the sample circuit and its ideal operating characteristics.

on page 2-2

Why Use Optimization?

your design using the PSpice Optimizer saves time.

Phase One: Developing the Design

through the steps needed to create a working design.

Phase Two: Setting Up the Optimization

through the steps needed to define the parameters, goals, and constraints that describe the optimization.

Phase Three: Running an Optimization

through the steps needed to optimize and finalize the design.

on page 2-3 explains why fine-tuning

on page 2-4 walks you

on page 2-6 walks you

on page 2-11 walks you

2-2 Primer: How to Optimize a Design

Optimizing a Diode Biasin Ob

Assume that you want to design a circuit that drives a current of 1 mA (±5 µA) through a diode (D1N914) using a 5 Vvoltage source and a series resistor to control the current through the diode. A circuit such as this is shown in Figure 2-1.

Your objective is to find a value for resistor R1 so that current through diode D1 falls in the range 0.995 mA to 1.005 mA.

ective

ure 2-1

Circuit—the

Diode Biasing Design Example

Why Use Optimization?

Why Use Optimization? 2

-3

To solve the problem manually, you could assign an arbitrary value to R1, manually calculate the current, then make an educated guess to adjust the values until a satisfactory solution is found. Or, you might use a simulation to sweep the value for R1 with a DC Sweep analysis, carefully analyzing the results to find the best solution.

These manual methods have two major disadvantages:

• Because the diode is a non-linear device, manual

calculations can be time-consuming.

• Sweeping a parameterized value can take a large number of

simulations, depending on the range and increment selected.

The PSpice Optimizer automates these processes by handling calculations for you and intelligently directing the series of simulations. Given results of the previous simulations, the optimizer automatically adjusts the parameterized value of R1 for the next run, thus eliminating unnecessary iterations, which in turn, provides a solution more quickly and with less effort.

Once the PSpice Optimizer settles on the best solution, you can still explore available tradeoffs. When done manually, this iterative process can be difficult and frustrating. With the optimizer, you can tweak the parameter(s) and immediately determine whether the design still meets specifications. You can also change the value of the specification(s) and immediately determine how parameter values change. If you are dissatisfied with the result after any change, you can always return to the last set of values.

When solving complex problems, the manual approach can be too unwieldy to consider. For example:

• When your desi

multiple parameters or complicated parameter interactions, you may find it’s nearly impossible to know which parameters to chan and how best to chan them.

• When solvin

specifications, the solution often depends on the order in which are optimized. This sequential approach can miss possible solutions since it is impractical to repeat the process startin different each time.

oals and constraints

oal or constraint

n has

for multiple

with a

2-4 Primer: How to Optimize a Design

Phase One: Developing

Phase 1 is also the time to investi

• The effects of individual

• Usin

ate:

components by replacin component values with parameters or parameterized expressions.

PSpice to perform a DC, AC, or parametric sweep of each parameterized value.

the Desi

Before optimizing, you must have a working circuit. This means first drawing the schematic, then iteratively simulating with PSpice and adjusting the design until the circuit operates with the intended behavior.

MicroSim Schematics

MicroSim PSpice A/D

Design

Schematic

Simulate

Note

resistor R1, its value is 1k. Later, when you set up the optimization, you will parameterize R1’s value as shown in Fi

When you initially place

ure 2-1.

Flow

ure 2-2

“Phase One: Developing the Design” Design

To draw the schematic for the diode biasing desi

In Schematics, from the Draw menu, use Get New Part to select and place the following symbols on the schematic:

R resistor R1 D1N914 diode D1 VSRC voltage source V1 AGND analog ground 0

+ 131 hidden pages

Microsim PSPICE OPTIMIZER User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

Contents

Figures

Tables

Before You Begin

Welcome to MicroSim

MicroSim PSpice Optimizer Overview

How to Use this Guide

Typographical Conventions

Related Documentation

If You Have the Evaluation Version

Things You Need to Know

Chapter Overview

What is the PSpice Optimizer?

Designs that You Can Optimize

Designs that You Cannot Optimize

Terms You Need to Understand

Chapter Overview

Why Use Optimization?