Page 161 - Troubleshooting Analog Circuits

I 48 12. Roundup of “Floobydust”

Even if an analog meter is accurately calibrated at full scale, it may be less accu-
rate at smaller signals because of nonlinearity arising from the meter’s inherent im-
perfections in its magnetic “circuits.” You can beat that problem by making your own
scale to correct for those nonlinearities. Then there’s the problem of friction and
hysteresis. The better meters have a “taut-band” suspension, which has negligible
friction-but most cheap meters don’t. Now, as we have all learned, you can neu-
tralize most of the effects of friction by gently rapping on, tapping at, or vibrating the
meter. It’s a pain in the neck, but when you’re desperate, it’s good to know.
Even if you don’t shake, rattle, or roll your meters, you should be aware that they
are position-sensitive and can give a different reading if flat or upright or turned
sideways. The worst part about analog meters is that if you drop them, any of these
imperfections may greatly increase until the meter is nearly useless or dead. This is
“position sensitivity” carried to an extreme. Ideally, you would use digital meters for
every purpose. But analog meters have advantages, for example, when you have to
look at a trend or watch for a derivative or an amplitude peak-especially in the
presence of noise, which may clutter up the readings of a digital voltmeter. So,
analog meters will be with us for a long time, especially in view of their need for no
extra power supply, their isolation, and their low cost.
But, beware of the impedance of meter movements. They look like a stalled
motor-a few hundred millihenries-at high frequencies. However if the needle
starts swinging, you’ll get an inductive kick of many henries. So, if you put an analog
meter in the feedback path of an op amp, you’ll need a moderate feedback capacitor
across the meter.

Digital MetercNot So Bad, and Sometimes Better than That

As I mentioned before, digital meters are always more accurate than analog
meters.. . except for when they aren’t. Recently, a manufacturer of power supplies
decided to “modernize” its bench-type supplies by replacing the old analog meters
with digital meters. Unfortunately, these meters came with an accuracy of fi%.
Having a 2-1/2-digit digital panel meter (DPM) with a resolution of 1 part in 200 but
an accuracy of 1 part in 20 certainly is silly. Needless to say, I stopped buying power
supplies from that manufacturer.
The steadiness and irrefutability of those glowing, unwavering digits is psycholog-
ically hard to rebut. I classify the readings of the DVM or DPM with any other com-
puter’s output: You have to learn to trust a computer or instrument when it’s telling
the truth, and to blow the whistle on it when it starts to tell something other than the
truth.
For example, most slow DVMs have some kind of dual-slope or integrating con-
version, so they’re inherently quite linear, perhaps within 1 or 2 least-significant
digits. Other DVMs claim to have the advantage of higher conversion speed; this
higher speed may be of no use to the bench engineer, but it is usable when the DVM
is part of an automated data-acquisition system. These faster instruments usually use
a successive-approximation or recirculating-remainder conversion scheme, both of
which are not inherently linear but depend on well-trimmed components for linearity.
I have seen several DVMs that cost more than $lo00 and were prejudiced against
certain readings. One didn’t like to convert 15 mV; it preferred to indicate 14 or 16.
One time I got a call from an engineer at one of the major instrument companies.
He wondered why the Voltage-to-Frequency converter he made with an NSC LM33 1
was showing him poor linearity-worse than the guaranteed spec of 0.01%. I told
him that was strange, because if it was true, it was the first LM33 1 to have poor lin-