Page 127 - Troubleshooting Analog Circuits

I14 9. Quashing Spurious Oscillations

Figure 9.4. When oscillations get nasty, you need a scope with pinpoint triggering to help you study the
problem.

For my tests of comparator VOs, I usually set up a classic op-amp oscillator into
which I build a specific amount of hysteresis and a definite amount of capacitance, so
that the unit will oscillate at a moderate, controlled frequency. If you’re curious, refer
to Appendix D, which is not trivial.
Another way to avoid VOS trouble with comparators is to use a monolithic dual
transistor as a differential-amplifier preamplifier stage ahead of the comparator. This
preamp can add gain and precision while decreasing the stray feedback from the out-
put to the input signal. Refer to the example of a (fairly slow) precision comparator in
LB-32 (Ref. 1).

Common-Mode Excursions Unpredictable
After curing oscillation, most complaints about comparators are related to their
common-mode range. We at National Semiconductor’s applications engineering
department get many queries from engineers who want to know if they can violate
comparators’ common-mode specs. But they’re not always happy with our answers.
I guess the complaints are partly the fault of the manufacturers for not being clear
enough in their data sheets.
By way of contrast, most engineers know well that an op amp’s common-mode-
voltage range, V,,, is defined provided that both inputs are at the same level. This
spec makes sense for an op amp because most operate with their inputs at the same
level. But in most cases, a comparator’s inputs are not at the same level. As long as
you keep both inputs within the comparator’s specified common-mode range, the
comparator’s output will be correct.
But if one input is within the common-mode range and the other is outside that
range, one of three things could happen, depending on the voltages and the particular
comparator involved. For some input ranges you can overdrive the inputs and still get