Page 157 - Troubleshooting Analog Circuits

144 12. Roundup of “Floobydust”

Sugar and Spice and Nothing Nice?

In case you haven’t guessed, I’m not a big fan of digital computers and simulation.
When a computer tries to simulate an analog circuit, sometimes it does a good job;
but when it doesn’t, things get very sticky. Part of the problem is that some people
put excessive confidence and belief in anything a computer says. Fortunately, my
bosses are very skeptical people, and they agree that we must be cautious when a
computer makes outrageous promises. Still, we all agree that computers promise
some real advantages, if only we can overcome their adversities and problems.
In many cases, if you have trouble with the simulation of an analog circuit or
system, you troubleshoot the simulation just as you would the circuit itself. You get
voltage maps at various “times” and “temperatures,” you insert various stimuli,
watch to see what’s happening, and modify or tweak the circuit just like a “real”
circuit. But, just like the Mario brothers, you can encounter problems in
Computerland:
1. You might actually have a bad circuit.
2. You might forget to ask the computer the right question.
3. You might have mistyped a value or instruction or something.
The easiest mistake of this sort is to try to add a 3.3 M resistor into your circuit.
SPICE thinks you mean 3.3 milliohms, not megohms. This problem has hooked
almost everybody I know. I solved it by using “3300 K” (3300 kfl in SPICE), or I
may just type out “33001 11”.
4. You might have a bad “model” for a transistor or device. I’ve seen a typographical
error in the program listing of a transistor’s model tie a project in knots for months.
5. You might have neglected to include strays such as substrate capacitance, PC-board
capacitance, or-something that most people forget-lead inductance.
6. You might get a failure to converge or an excessive run time. Or the computer might
balk because the program is taking too many iterations.
Sometimes problems happen that only a computer expert can address. But when
you ask the computer guru for advice, you might get no advice or-what’s worse-
bad advice. After all, many computer wizards know nothing whatsoever about linear
circuits. If the wizard tells me, “Hey, don’t worry about that,” or, “Just change the
voltage resolution from 0.1 mV to 10 mV,” then I must explain to the wizard that,
although that advice might make some computers happy, it gives me results that are
completely useless. Talking to computer wizards is sometimes difficult.
Even if you do everything right, the computer can lie to you. Then you have to
make a test to prove that you can get the right answer and the computer can’t.
For example, one time we had a circuit with 60 transistors, and a diff-amp
appeared to be oscillating even though it was clearly switched OFF. The computer
experts told us that we had obviously made a mistake. So we disconnected and re-
moved 58 of the transistors-there was nothing left but 2 transistors, and one of them
was biassed off by a full volt. And its collector current was “oscillating” at 100 kHz,
between plus and minus 10 FA, even though nothing in the circuit was moving.
When we confronted the computer experts, they belatedly admitted there was an
“internal timing error,” which they proceeded to fix. But it certainly took us a diffi-
cult week to get them to admit that.