Page 123 - Troubleshooting Analog Circuits

I IO 9. Quashing Spurious Oscillations

in real-time step response. If that is consistent with the frequency-domain response,
fine; if not, I get suspicious.. . . )
Secondly, if an earlier version of your circuit has worked OK, what’s the differ-
ence between the new one that does not work well and the old one that does? Be sure
to keep one or more examples of the old version around so that you can make com-
parisons when the new circuits have troubles. (Note that I said when, not if.) Thirdly,
look for components such as capacitors whose high-frequency characteristics can
change if someone switched types or suppliers.
Optoisolators in switching regulators are another possible cause of oscillation
trouble due to their wide range of DC gain and AC response. A switching-regulator
IC, on the other hand, is not as likely to cause oscillations, because its response
would normally be faster than the loop’s frequency. But, the IC is never absolved
until proven blameless. For this reason, you should have an extra module with
sockets installed just for evaluating these funny little problems with differing sup-
pliers, variant device types, and marginal ICs. You might think that the sockets’ stray
capacitances and inductances would do more harm than good, but in practice, you
can learn more than you lose.

When Is an Oscillation Not an Oscillation?
We still get a phone call, every month or so, from somebody complaining about a
“120-Hz oscillation” on one of our circuits. (It’s a good thing we do, because one of
our applications engineers was mentioning such a case recently, and I realized I had
forgotten to mention this type of oscillation, so this paragraph got plopped into the
text at the last minute. If I hadn’t remembered to include this class of “oscillation,” I
would have been terribly embarrassed.) Now, how can an op amp be “oscillating” at
60 or 120 Hz? Well, it is not impossible for an op amp or regulator to oscillate at this
frequency, but it is extremely unlikely. What is surely happening is that there is some
noise at power-line frequency getting into the circuit. There are four major ways for
this to happen.
1. When there is a diode connected to a delicate input, the ambient light in the room can
shine in and generate photocurrents. With fluorescent lights, this is usually at 120 Hz,
but the higher harmonics can boom in, too, along with some DC current. As soon as
you realize this is happening, it’s fairly easy to troubleshoot this by adding a little
darkness to the circuit-cover it up with a dark cloth, jacket, or book. You can then
localize it and “darken” it permanently. In the case of extremely bright light, it can
even come in through the insulators in the base of a TO-99 can-the little ceramic
feed-throughs are not really opaque-they let a small fraction of the light in.
Fortunately, plastic DIP packages are very opaque, these days.
2. A power supply can have more 60- or 120-Hz ripple (saw-tooth shape or pulse-
shape) than you expect. This can be caused by bad connections, a bad capacitor, an
open rectifier, or a ground loop. Again, as soon as you recognize that this kind of
thing can happen, it’s easy to search and cure the problem.
3. Magnetic flux from a transformer gets coupled into your circuit. The two most
common sources are a soldering iron close by, or, a power transformer that is satu-
rating a little bit, spraying flux around. This usually has a distinctive shape at 60 Hz
with lots of harmonics, and is quite position sensitive. This, too, is fairly easy to
recognize. But if you discover that your power transformer is not only running hot,
but spraying flux badly, it’s not usually easy to relocate it if you are nearly done with
your project. I would love to recommend that you assemble your power supply in a
little box, 3 feet away from your main instrument, but that is not always feasible. You