Page 158 - Troubleshooting Analog Circuits

Sugar and Spice and Nothing Nice? I45

My boss tells me that I should not be so negative, that computers
are a big part of our future. When he says that, I am tempted to go out
and buy stock in companies that make ExcedrinTM and antacids, be-
cause that also will be a big part of our future.. .
I have given a couple lectures at major conferences, with com-
ments about SPICE and some of its problems. (Ref. 1 and 2). After
the lecture, engineers from other companies have come up and told
me, “Yeah, we have those kinds of problems, too.. . .” (For additional
comments on SPICE, refer to Appendix G.)
One guy gave me a tip: “Don’t put a 50-LR resistor in your circuit-
put in 50.1 a, and it may converge better.” In other cases, we discov-
ered that a parallel resistor-capacitor combination that was connected
only to ground was helping to give us convergence; when we “*
commented out” the R and C, we could not get convergence any
more. Other people comment that if you change the name of a re-
photo Courtesy AI Neves.
sistor, or its number, or its position on the list of components, conver-
Figure 12. I. I hurled this
computer to its doom from atop gence may be improved-or ruined. So this convergence bird is a
National’s 3-story parking very fragile and flighty thing.
garage. As the dust settled, I My boss reminds me that some versions of SPICE are better than
knew that computer would others at converging, and I shouldn’t just be a complainer. But I am
never lie to me again! just reminding you that all kinds of computer simulation get criti-
cized, and sometimes the criticism is valid-the complainer is not
just imagining things (Ref. 3).
So if the computer persists in lying to you, just tell your boss that the computer has
proved itself incompetent. Junk that digital piece of disaster! !
What I really think you ought to do, instead of using digital simulations, is to make
an analog-computer model-you’ll have a lot less trouble. Be sure to scale all the
transistors’ capacitances at lOOX or lOOOX their normal values, so the time scale is
scaled down by lOOX, which makes the strays negligible. That’s what I do. I have
seen it work when SPICE cannot be beaten into cooperation. You might call it an
“analog computer,” because that’s exactly what it is. I will listen to alternative points
of view but, be forewarned, with frosty skepticism.

Lies, Damned Lies, and Statistics
One thing that doesn’t help me a darned bit is
“statistics,” at least in the sense that mathematicians
use them. I find most statistical analyses worse than
useless. What I do like to use is charts and graphs.
The data I took of the diodes’ VF versus IF back in
Chapter 6 were a little suspicious when I wrote
down the numbers, but after I plotted the data, I
knew there was something wrong. Then I just went
back and took more data until I understood what the
error was-AC current noise crashing into my ex-
periment, causing rectification. If data arise from a
well-behaved phenomenon and conform to a nice
Gaussian distribution, then I don’t care if people use
their statistical analyses-it may not do a lot of
harm. (Personally I think it does harm, because

Photo Courtesy Fran Hoffart