Page 162 - Troubleshooting Analog Circuits

Digital Meters-Not So Bad, and Sometimes Better Than That I 49

earity from the first couple million we had produced. I advised him to check the
capacitors and the op-amp waveform, and to call me back, because if he had a part
that didn’t meet specs, I wanted to get my hands on it.
The next day he called me back, feeling very sheepish and embarrassed. He ad-
mitted he had been using a prototype DVM designed by his company, and because it
was a prototype, it was not exactly under calibration control. It was his DVM that had
gone out of linearity, not the LM33 1.
Normally I hate to use a DVM’s autoranging mode. I have seen at least two (other-
wise high-performance) DVMs that could not lock out the autorange feature. The
worst aspect of these meters was that I couldn’t tell where they would autorange from
one range to another, so I couldn’t tell where to look for their nonlinearity; yet I kneKs
there was some nonlinearity in there somewhere. After an hour of searching, I found
a couple of missing codes at some such preposterous place as 10.18577 V. And this
on a $4000 DVM that the manufacturer claimed could not possibly have such an
error--could not have more than 1 ppm of nonlinearity.
Another fancy DVM had the ability to display its own guaranteed maximum error,
saying that its own error could not be more than f 0.0040% when measuring a 1 MR
resistor. But then it started indicating that one of my better 1.000000 MR resistors
was really 0.99980 MR. How could I prove if it was lying to me? Easy-I used jiu-
jitsu-I employed its own force against itself. I got ten resistors each measuring
exactly 100.000 kR-the fancy machine and all the other DVMs in the lab agreed
quite well on these resistors’ values. When I put all 10 resistors in series, all the other
meters in the lab agreed that they added up to 1.00000 Ma; the fancy but erroneous
machine said 0.99980 MR. Back to the manufacturer it went.
So, if you get in an argument with a digital meter, don’t think that you must neces-
sarily be wrong. You can usually get an opinion from another instrument to help
prove where the truth lies. Don’t automatically believe that a piece of “data” must be
correct just because it’s “digital.”
And be sure to hold onto the user’s manual that comes with the instrument. It can
tell you where the guaranteed error band of the DVM gets relatively bad, such as for
very low resistances, for very high resistances, for low AC voltages, and for low or
high frequencies.. . .
Most digital voltmeters have a very high input impedance ( 10,000 MR typ) for
small signals. However, if you let the DVM autorange, at some level the meter will
automatically change to a higher range where the input impedance becomes 10 MR.
Some DVMs change at +2 V or 3 V, others at 10 or 12 or 15 V, and yet others at f220 V.
As I mentioned in the chapter on equipment, I like to work with the DVMs that stay
high-impedance up to at least 15 V. But, the important thing is to know the voltage at
which the impedance changes. A friend reminded me that his technician had recently
taken a week’s worth of data that had to be retaken because he neglected to allow for
the change of impedance. I think I’ll go around our lab and put labels on each DVM.
Still, DVMs are very powerful and useful instruments, often with excellent accu-
racy and tremendous linearity and resolution4ften as good as 1 ppm. I’ve counted
some of these ultralinear meters as my friends for many years. I really do like ma-
chines -such as the Hp3455, HP3456, and HP3457-that are inherently. repeatably
linear, as some of these DVMs are absolutely first-class.
One picky little detail: Even the best DVM is still subject to the adage, “Heat is the
enemy of precision.” For example, some DVMs have a few extra microvolts of
warm-up drift, but only when you stand the box on its end or side. Some of them have
a few microvolts of thermal wobble and wander when COMeCted to a zero-volt signal
(shorted leads), but only when you use banana plugs or heavy-gauge (16, 18, or 20