Page 125 - Troubleshooting Analog Circuits

I12 9. Quashing Spurious Oscillations

XlOTTtiERMAL
CUTOUT 5oow HEATER BLOWER

MOC 3030

15Ok
Figure 9.2. Stabilizing this heater’s slow servo loop and choosing the proper values for RI, C,, and C2
involved applying a I -V p-p, 0.004-Hz square wave, Vsn, to Rz and observing the LM I IC’s
output with a strip-chart recorder.

much? Well, I like to provide about twice or three times as much hysteresis as the
minimum amount it takes to prevent oscillation near the comparator’s zero-crossing
threshold. This excess amount of feedback defines a safety margin. (For more infor-
mation on safety margins, see the box, “Pease’s Principle,” in Chapter 8.) I have
never seen this hysteresis safety-factor technique outlined in print for comparators,
so you can say you read it here first.
My suggestion for excess hysteresis is only a rule of thumb. Depending on your
application, you might want to use even more hysteresis. For example, a comparator
in an RC oscillator may operate with 1,2, or 5 V of hysteresis, which means you can
always use more than my “minimum amount” of excess hysteresis. Also, if you have
a signal with a few millivolts of noise riding on top of it, the comparator that senses
the signal will often want to have a hysteresis range that is two or three times greater
than the worst-case noise.

Just the Right Touch

Comparators are literally very “touchy” components; that is, you can drastically alter
their performance just by touching the circuit with your finger. And because com-
parators are so touchy, you should be prepared for the probability that your safety
margin will change, for better or worse, when you go from a breadboard to a printed-