Page 220 - Troubleshooting Analog Circuits

G. More on Spice 207

tics that cannot be absolutely modeled by any computer model. If you happen to
depend on that feature, or the absence of that feature, it is only a matter of time be-
fore you get in trouble.
I will also guarantee that just because you made one breadboard, and it works well.
you cannot put that circuit into production and get lo00 units in a row to work well.
itnless you are a smart engineer and design the circuit “properly” and do your worst-
case design studies, and plan for well-behaved frequency response. etc. And I think
that is true. no matter where you buy your op amps. What’s new? What color is the
king‘s new underwear? Dirty grey, same as everybody else‘s.
I was at an evening session at the IEEE Bipolar Circuits and Technology Meeting
in Minneapolis recently. Several companies that sell CAD tools had done some ce-
rious work to analyze the circuit for a 12-bit A/D converter. Even the ones that had
only a little time to put in were able to show that macro-models were feasible and
effective as a way to do good analysis while saving computing time-that was the
primary objective of the study. But even the ones that put in the nzost time at analyhis
did not recognize (or did not comment about it) that the noise of the reference and the
comparator were rather large, and you could not achieve 12-bit resolution without
slowing down the response a lot more than you would have to do otherwise (for a
circuit where you did not have to consider noise).
If a good designer of A/D converters had these tools. and he knew where to look
for noise, or where to insert lead inductance, or extra substrate capacitances. he might
use some of these CAD tools to help him design a better ADC. But if he just believed
what the computer told him, he would probably be badly fooled.
Once a customer called me up and asked me how to get my LM 108s to stop oscil-
lating in his circuit. He explained it was a simulated LM 108 with some simulated
feedback resistors. and simulated switches and filters. Oh. H’mmmm. I asked if he
had made up a breadboard, and, did it oscillate? He said he had made it and it did not
oscillate. H’mmmmm. I asked him, “If you built up a breadboard and a computer
model, and the real breadboard oscillated, but the computer did not. you wouldn’t be
calling up to complain about the computer, would you?” He stopped and thought
about it. He cogitated for a while. He said “1’11 call you back.” And he hung up. And
he never did call back. I mean, what would you do?