Page 188 - Op Amps Design, Applications, and Troubleshooting

Nonideal Considerations 171

Incorrect Switching Levels. Although there are many things that can cause
minor shifts in switching levels (e.g., component value drifts), some are more
probable than others. If the circuit has adjustable components, such as a variable
reference voltage, suspect this first. If the variable components are properly
adjusted but the problem remains, suspect any solid-state components other than
the op amp (e.g., zener diodes).
The zeners can be checked for proper operation by measuring the voltage
across them. A forward-biased zener will drop about 0.7 volts; a reverse-biased
zener should have a voltage drop that is approximately equal to its rated voltage.
Keep in mind that zeners are not precision devices. For example, a 5.6-volt zener
that drops 6 volts is probably not defective.
As a last resort, verify the resistance values. Resistor tolerances in a low-
power circuit of this type do not present problems very often.

3.8 NONIDEAL CONSIDERATIONS

For many comparator applications, slew rate is the primary nonideal parameter
that must be considered. This limitation was discussed in earlier sections, along
with methods for determining the effects of a finite slew rate. Additionally, the
zener diodes become less ideal as the input frequency is increased.
Throughout the earlier sections of this chapter, it was assumed that the op amp
changed states whenever the differential input voltage passed through 0. The input
bias current for the op amp, however, can cause the actual switch point to be slightly
above or below 0. This problem is minimized.by keeping the resistance between the
(-) input to ground equal to the resistance between the (+) input and ground.
Input offset voltage is another nonideal op amp parameter that can affect the
switching points of the comparator. The effect of a non-0 input offset voltage can
be canceled by utilizing the offset null terminals (discussed in Chapter 10).
Appendix 4 illustrates the proper way to utilize the null terminals on an
MC1741SC op amp. Note, however, that different op amps use different tech-
niques for nulling the effects of input offset voltage. Therefore, you must refer to
the manufacturer's data sheet for each particular op amp.
The errors caused by the input bias currents and the input offset voltage can
be totally eliminated by utilizing the nulling terminals. Unfortunately, however,
the required level of compensation varies with temperature. Thus, although you
may completely cancel the nonideal effects at one temperature, the effects will
likely return at a different temperature. For many, if not most, comparator appli-
cations, this latter drift does not present severe problems. If the application
demands greater stability, an op amp that offers optimum performance in these
areas should be initially selected.