Page 405 - Op Amps Design, Applications, and Troubleshooting

Troubleshooting Tips for Arithmetic Circuits 381

If your job requires you to do frequent maintenance on arithmetic circuits, it
is probably worth your while to construct a test jig to aid in diagnosing faulty cir-
cuits. The test jig could consist of a number of switch-selectable voltages applied
to several output jacks, and the voltages should be accurate enough to effectively
test the particular class of circuits being evaluated. The jig, coupled with a table
showing me performance of a known good circuit, can be used to very quickly
isolate troubles in arithmetic circuits. Of course, this entire test fixture could easily
be interfaced to a computer for automatic testing and comparison.
If the circuit seems to work properly in the laboratory, but consistently goes
out of tolerance when placed in service, you might suspect a thermal problem.
Nearly all of the components in all of the circuits are affected by temperature
changes. Short of providing a constant temperature environment, your only options
for improving performance under changing temperature conditions are these:

1. Locate a defective component.
2. Substitute compatible components with tighter tolerances.
3. Redesign the circuit using a different technique.

You can artificially simulate temperature changes to a single component by spray-
ing a freezing mist on it. Sprays of this type are available at any electronics supply
store. Although every component you spray may cause a shift in operation, an
abrupt or dramatic or erratic response from a particular one may indicate a failing
part,
There are numerous choices for all of the components in the circuits pre-
sented in this chapter. Improved immunity to temperature variations can often be
obtained simply by substituting components with more stringent tolerances. Resis-
tors with a 5-percent rating can be replaced, for example, with 1-percent resistors.
Similarly, a general-purpose op amp can be replaced with a pin-compatible op amp
having lower bias currents, noise, or temperature coefficients.
Finally, if items 1 and 2 in the list above do not resolve a particular thermal
problem, then redesign may be in order. Frequently, there is a trade-off between
circuit simplicity and circuit stability. Achieving rock-solid stability often requires
a step increase in circuit complexity.