Page 124 - Troubleshooting Analog Circuits

Comparators Can Misbehave Ill

should at least start out by installing only a power transformer of known quality, with
good, known freedom from excessive external flux and saturation. Sometimes you
can retrofit your circuit with a toroidal power transformer, but most people don’t
keep these lying around. They are more expensive but often worth it, as they are
more efficient and have less self-heating.
In concept. you might try adding some shielding. Go ahead, put in 1/16 inch of
aluminum. It will have no effect at all-for magnetic shielding, you need iron. Go
ahead. put in 1/16 inch of iron. That’s not much help, because at 60 Hz, it takes about
114 inch thickness of steel to do much good. You can try, but it’s not an easy way.
Sometimes you can arrange your critical circuits to have smaller loops, so they will
pick up less flux. Make neat, compact paths: use twisted pairs; and use layout tricks
like that-those can sometimes help. If you haven’t tried these before, ask an old-timer.
4. A mechanical vibration can be coupled in through a floppy wire or a high-K ceramic
capacitor. If nobody tells you about this one, this is a very tricky tease, not at all easy
to guess. Sometimes if you replace the high-K ceramic with an NPO, or a film capac-
itor, it will solve the problem. Recently we ran a picoammeter, and when the power
supply lead ran near the summing point, there was a certain amount of charge, Q =
C X V. When the wire was vibrated at line frequency, a 60-Hz current I = V X dC/dT
flowed into the input. The current stopped when we guarded the 5-V bus away from
the input, and we also added shock-mounting for the whole assembly, to keep out all
vibrations.
There are probably a few other ways to get 60-Hz noises into a circuit, so you must
be prepared to exercise ingenuity to search for nasty coupling modes. But if the “os-
cillation” is at exactly line frequency, and it synchronizes with the “line synch” mode
of your scope, then it is certainly not a real oscillation. Now, I have seen 59-Hz oscil-
lations, that would fool you into thinking they were at 60 Hz. but that is quite rare. It
just goes to show that there are many noises to keep you on your toes. Some are os-
cillations, and some are “oscillations.”
You can best analyze the design of a slow servo mechanism, such as that in Figure
9.2, with a strip-chart recorder because the response of the loop is so slow. (A storage
scope might be OK, but a smp-chart recorder works better for me.) You might wish
to analyze such a servo loop with a computer simulation, such as SPICE, but the
thermal response from the heater to the temperature sensor is strictly a function of the
mechanical and thermal mounting of those components. This relationship would
hardly be amenable to computer modelling or analysis.

Comparators Can Misbehave

Saying that a comparator is just an op amp with all the damping capacitors left out-
that is an oversimplification. Comparators have a lot of voltage gain and quite a bit of
phase shift at high frequencies; hence, oscillation is always a possibility. In fact, most
comparator problems involve oscillation.
Slow comparators, such as the familiar LM339, are fairly well behaved. And if you
design a PC-board layout so that the comparator’s outputs and all other large, fast.
noisy signals are kept away from the comparator’s inputs. you can often get a good
clean output without oscillation. However, even at slow speeds, an LM339 can oscil-
late if you impress a slowly shifting voltage ramp on its differential inputs. Things
can get even messier if the input signals’ sources have a high impedance (>> 10 kQ )
or if the PC-board layout doesn’t provide guarding.
In general, then, for every comparator application, you should provide a little
hysteresis, or positive feedback, from the output back to the positive input. How