Page 218 - Troubleshooting Analog Circuits

G. More on Spice 205

finitely versatile expanded-scale oscilloscope” and if it has a dV/dt that suddenly
changes, well that is quite a surprise.
So, I immediately wrote an open message to all my friends at NSC, warning them
about this potential problem; and now I am writing about this to warn all my friends
everywhere. So, these are just some of the reasons I am not enthusiastic about SPICE.
It’s goofed me up, me and my friends, too many times.
My boss points out that it’s not necessarily true that all kinds of SPICE have such
bad problems with convergence or bad computations or spurious signals. And that
may be so. If somebody who knows all about W-SPICE or J-SPICE wants to write in
and assure me that his SPICE will never do that, well, that is fine by me. But. mean-
while, don’t get me wrong-I don’t hate these digital computers. They hate me; I
despise them.
The other day I was standing out in the rain, talking with a design engineer from
the East Coast. He said all the other engineers at his company ridicule him because
they rely on SPICE, while he depends on the breadboards he builds. There’s just one
hitch: his circuits work the first time. and theirs don’t. To add insult to injury, his
boss forces him to help his colleagues get their circuits working, since he has so much
time left over. I said that sounds pretty good to me, so long as his boss remembers
who is able to get out the circuits, when it comes to doing reviews for all the guys.
This guy gave me a tip: Don’t design a circuit in SPICE with 50 0 resistors. Use
50.1 a. It converges better. H’mmmm. That sounds intriguing.
Right now I am struggling with a SPICE model of a circuit. Not of a new circuit,
but of an old circuit: The band-gap reference of the old LM33 1 which I put into pro-
duction back in ‘77. It’s a good thing I put it into production before we got SPICE.
because if I had first run this through SPICE, I would have been pretty discouraged.
SPICE says this circuit has a rotten temperature coefficient and oscillates like a
politician. I went back and double-checked the actual silicon circuits. They soar like
an angel, have very low TC, and are dead-beat when you bang on them. They have
no tendency to oscillate: they do not even ring. So why does SPICE persist in lying to
me? Doesn’t it realize I will break its back, for the impertinence of lying to the Czar
of Bandgaps?? The SPICE and CAD experts around here tell me, “Oh. you must
have bad models.” I’ve been told that before, when I was right and the experts were
absolutely wrong. (I mean, how can a single FET oscillate at 400 kHz?? With the
help of two resistors. . . ) More on this topic, later.
I’ve already gotten several letters from readers who have asked, “How about all
these new models for op amps? Won’t they lead linear designers in a new direction’?“
My replies to them start out by covering a couple examples of old macro-models of
op amps that have been raising questions for over a dozen years.
A guy calls up and asks me, “What is the maximum DC voltage gain on an
LM108?” I reply, “Well, it’s 40,000 min, but a lot of them run 300,000 or 500,000.
and some of them are as high as 3 or 4 million.” The customer sighs, “Oh. that’s
terrible.. . . ” When I ask why it’s terrible, he explains that when the gain gets high.
the gain bandwidth will get so high that it will be impossible to make a stable loop. if
the gain bandwidth gets up to dozens or hundreds of megahertz.
Sigh. I sit down and explain that there is no correlation between the DC gain and
its spread, compared to the GBW Product and its spread. The guy says, “Oh, I read in
a book somewhere that there’s good correlation, because the first pole is constant.’’ I
tell him to throw out the book, or at least X out those pages, because the first pole is
not at a constant frequency.
These days, I read that several op-amp companies are giving away free SPICE
models. What do I think of these models? Well, on a typical basis, I have read that