Page 187 - Troubleshooting Analog Circuits

I 74 14. Real Circuits and Real Problems

Figure 14. I. Basic Op-Amp Inverter.

Table 14.1 Troubleshooting Inverting Operational Amplifiers
Indication of Trouble Possible Cause Solution
Output voltage offset is Feedback R too high? Use lower R,
excessive (when input or better op amp
is zero). with lower I,.
Oscillation causes offset? Use scope, check for
oscillation.
Sneak path for leakage Check for dirty, leaky
into input? PC board or connectors.
R+ is wrong value? Make R+ = Ri, II RF
Op amp’s VOS too big? V,,t should be < Vos x
[(Rf/R,) + 11. Use a
VOs trimpot or a better
amplifier for low Vos.
Amplifier out of spec? If all other causes are
negative, remove op amp
and test it.
Output pegged near one Other power supply may Check voltage on each
power supply rail. be missing? pin of the part, not
just the PC board.
Output is shorted? See if amplifier is hot.
Check continuity.
Bad amplifier? Pull out unit, test it.
Output is oscillating? Input oscillating? Check input.
Power supply osc- Check each supply.
illating?
P.S. bypass caps miss- Try more caps, closer
ing or inadequate? to unit, or bigger
or better ones.
Cap. load too heavy? Look for cables; measure
the C load.
No feedback cap? See text; try different.
values of CF
Oscillation in air? Turn off power and watch.
Comp cap too small? Try adding more capac-
(LM301A or similar) itance.
(Continued)