LINEAR AN48 Using LTC Op Amp Macromodels

Application Note 48 November 1991

Using the LTC Op Amp Macromodels Getting the Most from SPICE and the LTC Library

Walt Jung

INTRODUCTION

This application note is an overview discussion of the would not be taking place. However, all is not necessarily Linear Technology SPICE macromodel library. It assumes peaceful bliss for analog designs, and yes, we still need to little if any prior knowledge of this software library or its actually build breadboards to check out circuit designs in history. However, it does assume familiarity with both the the lab. The go-go project managers may say “Simulate it, analog simulation program SPICE (or one of its many we don’t have time to fool with the breadboard and hand- derivatives), and modern day op amps, including bipolar, built prototypes.” Rarely will this ramrod approach be a JFET, and MOSFET amplifier technologies. truly wise move, now or in the future, except in specialized circumstances.

Some Preliminary SPICE Facts of Life

While it is certainly true that we are in the age of comput-

In the past few years, SPICE simulations have really begun ers, and that they really do aid our tasks in many ways, that to capture a high level of attention on the part of analog is simply not enough for all cases. SPICE (or any simula- circuit designers. Perhaps this is due to more affordable tion tool) can only act upon the information fed into it to high performance computers, or perhaps the time for analyze a circuit. Model quality issues set aside for the simulation is now upon us. In any event, the bottom line is moment, can you honestly say that you have fully suffi- that IC vendors are now making macromodels for op amps cient characterization data for every single relevant con- available to their customers. nection point/load that your circuit will ever see? Do you

understand all of the parasitic issues it will face? If you can

For the analog circuit designer, there can be no better fate

say yes to all of these, then maybe all that you need is just

for simulations, viewing this situation in terms of which

a good op amp model, and SPICE. More likely, there will

model to use. Designers no longer need worry about

always be some uncertainties, so breadboarding will re-

whether the third party supplier’s model can really cut it.

main the only advisable choice for relatively complex

Speaking in terms of the ultimate potential, no one can

circuits, particularly those never built before.

know an actual part better than the people who designed and produced it, that is the original source IC vendor. The This then leaves a question of model quality and degree of only possible caveat to this scenario is that the vendor functionality to be answered. Are the presently available supplying an op amp model needs to fully understand not models enough? Just how far can they be trusted for the only the real part, but they must also understand SPICE types of simulations that are to be performed? Hopefully, and modeling issues. Without both types of understand- most of these answers will be more apparent by the end of ing firmly in place, the user can end up with a real part that this note, as it contains many different examples. Never- works well and a model which doesn’t. For such a case, theless, no IC vendor (or other model supplier) is likely to simulation will be of little value; simulation runs to verify ever stand up and say, “We guarantee this model when circuit performance won’t match the actual part’s bench used with simulator ABC, and the simulated performance performance. will be within X% of the actual part connected within a

corresponding circuit.”

Fortunately, this type of problem seems to be diminishing.

If this were not so, the rapid increase in attention to models and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.

AN48-1