Page 160 - Troubleshooting Analog Circuits

Lies, Damned Lies and Statistics I47

Unfortunately, this engineer had so much confidence in his statistical program that
he spent a whole week preparing the Beautiful Report. Did he go report to the design
engineer that there were some problems? No. Did he check his data, check the tester?
No. He just kept his computer cranking along, because he knew the computer anal-
ysis was the most important thing.
We did finally get the tester fixed, and we got the product out a little late, but obvi-
ously I was not a fan of that test engineer (nor his statistics) as long as he was at our
company. And that is just one of a number of examples I trot out, when anybody tries
to use statistics when they are inappropriate.
I do like to use scatter plots in two dimensions, to help me look for trends, and to
look for “sports” that run against the trend. I don’t look at a lot of data on good parts
or good runs, but I study the hell out of bad parts and bad runs. And when I work with
other test engineers who have computer programs that facilitate these plots, I support
and encourage those guys to use those programs, and to look at their data, and to
rhink about those data. Anything that facilitates thinking--that I support.
Keep It Cool, Fool.. . .

A couple years ago I was approached by an engineer who was trying to use one of
our good voltage references that had a typical characteristic of about 20 ppm per
loo0 hours long-term stability at +125 “C. He was using it around room temperature.
and he was furious because he expected it to drift about 0.1 ppm per IO00 hours at
room temp, and it was a lot worse than that. Why was our reference no good, he
asked? I pointed out that amplifiers’ drifts and references’ drifts do nor keep
improving by a factor of 2, every time you cool them off another 11 degrees more.
I’m not sure who led him to believe that, but in general, modem electronic compo-
nents are not greatly improved by cooling or the absence of heating. In fact, those of
us who remember the old vacuum-tube days remember that a good scope or volt-
meter had an advantage if you kept it running nice and warm all the time, because all
the resistors and components stayed dry and never got moist under humid conditions.
I won’t say that the electrolytic capacitors might not have liked being a little
cooler. But the mindless effort to improve the reliability by keeping components as
cool as possible has been overdone. I’m sure you can blame a lot of that foolishness
on MIL-HBDK-217 and all its versions. In some businesses you have to conform to
-217, no matter how silly it is, but in the industrial and instrument business, we don’t
really have to follow its every silly quirk and whim. One guy who is arguing strenu-
ously about -217 is Charles Leonard of Boeing, and you may well enjoy his writing
(Ref. 4). So if something is drifting a little and you think you can make a big
improvement by adding a fan and knocking its temperature down from +75 to +55
“C, I’m cautioning you, you’ll probably be disappointed because there’s not usually a
lot of improvement to be had. It is conceivable that if you have a bad thermal pattern
causing lots of gradients and convection, you can cut down that kind of thermal
problem, but in general, there’s not much to be gained unless parts are getting up near
their max rated temperature or above +lo0 degrees. Even plastic parts can be reliable
at +lo0 degrees. The ones I’m familiar with are.

There’s Nothing Like an Analog Meter

Everybody knows that analog meters aren’t as accurate as digital meters. Except.
you can buy DVMs with a 0.8% accuracy; analog meters better than that exist.
Anyway, let’s detail some problems with analog meters.