Page 120 - Troubleshooting Analog Circuits

Different Methods Uncover Different Errors I07

set of errors? A resistor of the wrong value? A short? An open? Then I try to cook up
a test to confirm my theory. Sometimes I have to disconnect things, but I try to mini-
mize that. Sometimes adding a resistor or voltage or current will yield the same re-
sult, and it’s much easier than disconnecting components.
If an amplifier circuit isn’t running at all, sometimes the right thing to do is to
reach into the circuit and “grab’ one amplifier’s input and force it to go above and
below the other input. If the output doesn’t respond at all, you have a dead amplifier.
an amplifier with no connections, or a stuck output. It is not obvious to try this open-
loop test-no book tells you that this is a good idea-but after you try it, you will
agree that its results usually tell you an obvious story. Refer to Figure 14.1 in Chapter
14 for more detailed techniques and notes on troubleshooting basic op-amp circuits.
Many of these op-amp troubleshooting tips are applicable to other components as
well. The next chapter will continue with buffers, comparators, and related devices.

References

I. Data Converter Handbook, Analog Devices, P.O. Box 9106, Norwood MA 02062, 1974.
2. Pease, Robert A., “Improved unity-gain follower delivers fast, stable response,” EDN,
February 20, 1979, p. 93. (Also available as LB-42 in NSC’s Linear Applications Book, 1980,
1986, 1989, etc., “Get Fast Stable Response from Improved Unity-Gain Followers.”)
3. DeMichele, Glenn, “Compensate op amps without capacitors,” EDN, July 21, 1988, p. 33 1
4. Frederiksen, Thomas M., intuitive Operational Amplifiers, McGraw-Hill, New York, NY. 1985.
Available from Heath Company, P.O. Box 8589, Benton Harbor, MI 49022. (800) 253-0570
(Part NO. EBM-I), $19.95.
5. Nelson, Carl T., Super Matched Bipolar Transistor Pair Sets New Standards for Drift arid
Noise, Application Note AN-222, Linear Applications Databook, p. 5 17. National
Semiconductor, Santa Clara, CA, 1986.
6. Pease, Robert A., “Low-noise composite amp beats monolithics,” EDN, May 5, 1980. p. 179.
(Also available as LB-52 in NSC’s Linear Applications Databook, 1982, 1986, 1989. etc. as
“A Low-Noise Precision Op Amp.”)
7. Pease, Robert A., “Bounding, clamping techniques improve on performance,” EDN,
November 10. 1983, p. 277.
8. Pease, Bob, and Ed Maddox, “The Subtleties of Settling Time,” The New Lighrnin,q
Empiricist, Teledyne Philbrick, Dedham, MA, June 1971.
9. Addis, John, “Versatile Broadband Analog IC.” VLSI Systems Design, September 1988. p. 18.
10. Pease, Robert A., “How To Get The Right Information From A Datasheet,” EE Times, April
29, 1985, p. 3 1. (Also available as Appendix F in NSC’s General-Purpose Linear Devices
Databook, 1988, 1989, etc. and as Appendix F in this book.)