Page 219 - Troubleshooting Analog Circuits

206 G. More on Spice

some of these models are pretty good, as in several typical situations, they slew and
settle (and ring just a little, as real op amps do) and have as good accuracy as a real
typical op amp and its feedback resistors. Maybe in a few years the slow ones will be
trustworthy. But I don’t think you can get very good results from the fast ones. Why?
PC layout strays. Enough said.
And besides, how good are those models if you ask their makers? Are the models
guaranteed to give such a good representation of reality that if SPICE gives good
results, the op amps are guaranteed to work? Well .......... no, not exactly. In fact, from
what I have read, none of the op-amp models are guaranteed for anything. The only
thing they can do, “guaranteeably,” is to give a customer something when he begs for
SPICE models. It’s guaranteed to make the customer go away happy and to keep him
busy for a while. But it’s not guaranteed to make him happy in the long run. Because
the performance of high-speed op amps and precision circuits depends so critically
on the layout, and on the resistors and capacitors, that the model itself is almost
irrelevant.
Now some people might say, “How does Pease dare to say that?”
It’s easy. I haven’t got any SPICE models of my op amps to give away. Not at this
time. And if I did, or when I do, I won’t be able to guarantee them either. At best, I
may be able to say, “If you are a good engineer and use these models as a tool to
pioneer some experiments that are inconvenient to test on the breadboard, you may
find these models are helpful, so long as you then check it out on your breadboard to
confirm the circuit. For example, you can use SPICE to ‘measure’ some voltages or
currents that are so small and delicate that you really could not measure them with a
scope or buffered probe or current probe, not in the real world. But if you try to rely
solely on these models, without breadboarding, they will not tell you the whole story,
and your crutches will collapse, sooner or later, and you can’t say I didn’t warn you.”
I showed this to Bettina Briz in Amplifier Marketing, and she said, “Bob, you
can’t say that.” I said, “Oh, tell me where I have said anything that is untrue, and I
will fix it.” She admitted that what I had said probably was.. . . quite true. Then I
said, “Well, why try to soft-pedal the truth, and pretend that you can trust computers
all the time? Wouldn’t that be a disservice to our customers?” And Bettina replied,
“When we have models, we’ll educate our users-we’ll point out when you can trust
the models, and when you shouldn’t. So-after that-are we in disagreement?”
Well-maybe we did agree after all.
At present, we have a small library of op-amp models released with Analogy
(Beaverton, Oregon 97075).’ They are only level I models (low precision), and while
we have made some progress on good-precision ones (level 11), they are not released
yet. These are “behavioral models” rather than SPICE models, and we think they
have several advantages over SPICE models. There are some min/typ/max specifica-
tions that pretty much correspond to data sheet limits. If you use them wisely, they
may be helpful-subject to the conditions I listed in the previous paragraph. These
models are not free. They are not even cheap. But we think they are worth what you
pay for them. Still, they are not guaranteed.
Now, seriously, where can you get a model of a transistor that is guaranteed? To
run under all conditions? I don’t think you can beg or steal or borrow or buy a model
of a transistor that is guaranteed.* Or of a capacitor. Or even of a resistor.
But I can guarantee that every op amp you can buy or make has some characteris-

I. Analogy Inc., P.O. Box 1669, Beaverton, Oregon 97075, (503) 626-9700.
2. For information on guaranteed models for CMOS transistors, inquire with James Smith,
Semiconductor Physics, Inc., 639 Meadow Grove Place, Escondido, California 92027-4236
(619) 741-3360.