Page 216 - Troubleshooting Analog Circuits

Appendix G

More on SPICE

Recently I was down in New Orleans at one of the IEEE conferences-the
International Symposium on Circuits and Systems. The keynote speaker, Professor
Ron Rohrer from Carnegie-Mellon University, commented thoughtfully about many
aspects of education for engineers. But what he said that really stunned me was his
observation that “in the era of SPICE, nobody designs on the back of envelopes any
more.” Ouch. It is becoming more and more true that young (or, lazy?) engineers
cannot do much designing without some computers or high-powered calculators.
SPICE just happens to be one of my pet peeves, and I will start gnawing on its ankles
today.
Now, I have always been a friend of analogies, analogues, analogs, similes, models
and metaphors. When I worked at George A. Philbrick Researches, the company’s
motto was, “The analog way is the model way.” In those days we sold some analog
computers, even though that part of the business was shrinking and the popularity of
the op amp was on the rise. But we all tried to follow the party line, that analog compu-
tation was a serious business-and it still is, although as a percentage of the electron-
ics business, it has shrunk to a tiny fraction. Still, there are many times where a little
analog computation is exactly the right thing, and someday I will expound on that . . .
But, SPICE (Simulation Program with Integrated Circuit Emphasis) is a rather
popular and powerful tool these days, and almost everybody finds it useful to some
extent. I remember when my old boss, Tim Isbell. showed me how to use it-and
then we spent half a day horsing around because it said we had a 72-V forward
voltage across a diode, but there was no current through the diode. I will emphasize
today just a few of the basic problems with SPICE.
The first main problem is that people tend to trust its answers, as they trust most
computers, long after the reason to trust it should have evaporated. I have come very
close to fistfights and screaming contests, when a person claims that such-and-such
an answer is obviously right because SPICE gave it to him. Conversely, 1 normally
try to avoid working with SPICE unless I can run a calibration program on it. a sanity
check, so it gives me an answer that makes sense.
This is much like the old days of the slide rule: You couldn’t use the slide rule
unless you already knew approximately what the answer was. It’s not like a calcu-
lator where the decimal place is provided on a platter-you have to provide your own
decimal place. In other words, you are forced to be a pretty good engineer before you
even pick up your slide rule, or your analog computer.
But people who use SPICE are often buffaloed or fooled by any absurd kind of
answer. So, trusting your computer seems to be one of the new trends, which I want
to see quashed. It’s too easy to find (weeks later) that the computer told you a lie.
because the data typed in had a typo error, or a monumental goof. Now. never let it
be said that RAP recommends you use analog computers or breadboards instead of
SPICE because analog computers do not make errors. SPICE lies, but analog com-
puters do not? Oh, please, don’t say that: Analog computers lie, too. and so do bread-

Originally published in Elecfronic Design. November and December 1990.

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