Page 169 - Troubleshooting Analog Circuits

I56 13. Letters to Bob

10. Probes work into a certain, specified scope-input capacitance. You can’t always take
a probe that came with one scope and use it at high frequencies on another scope.

11. A simple test technique is waving your hand over a circuit to feel for the hot spot.
If something has gone into a latch-up but is not smoking, you can frequently find it
this way.

12. Edmund Scientific (Barrington, NJ) sells thermally sensitive liquid-crystal sheets,
which you can lay over a circuit to find moderate hot spots. This material works well
when you have a known-good PC board to compare with the circuit under test.
13. Drafting departments sometimes erroneously think that they own the schematic and
that its only purpose is to serve as a wiring diagram for the PC-board layout. Long
after PC-board layout, the production-test, sustaining-engineering, and service de-
partments will still need the schematic. Drafting tends to lose notes that I place on the
schematic, such as filter poles and zeros, temperature coefficients, normal AC and
DC voltages, waveforms, and thermal information. I save myself a lot of calls by
putting this information in front of the techs from the beginning.
14. You can make an extremely low-distortion (and slightly microphonic) sine-wave
oscillator from a light bulb and an op amp. I got the circuit from Linear Technology’s
(Milpitas, CA) AN 5 application note. I built a 3-frequency (400-, lOOO-, and 2800-
Hz) oscillator in a small metal Bud box. It had a THD lower than -80 dB.
15. If a circuit’s DC values change when you breathe on it, you may have dirty circuit
boards.

16. When testing high-gain, low-signal-level circuits, repeat the measurements with the
lights off. You may be surprised to learn that many components are photosensitive
and have infrared transparent bodies. One of my colleagues had a photosensitive
metal-can op amp that leaked light in around the leads.
17. Protection diodes can rectify high-frequency noise and oscillations.
18. Micro Technical Industries (Laguna Hills, CA) makes a handy thermal probe with
which you can individually heat components. The probe has tips to fit various compo-
nents, such as small and large resistors, metal-can op amps, and DIPS of various sizes.
19. Some sample-and-hold circuits are sensitive to slew rates on the digital inputs.
20. Even Schmitt triggers can exhibit metastability.
2 1. As paraphrased from an Analog Devices (Norwood, MA) application note, “You
may be able to trust your mother, but you should never trust your ground.”
22. Wrapped-wire circuits work pretty well if you can distribute power and ground prop-
erly. I use large-diameter bus wire in a rectangular grid for high-frequency logic if I
don’t have a wrapped-wire board with internal power and ground distribution.
23. Sometimes powering your test circuit with batteries breaks ground loops and elimi-
nates power-line noise.

24. A handy thing to have is a 60-Hz, passive, twin-T notch filter in a small Pomona box
with dual banana plugs for input and output.
25. Another handy thing to have is a 20-dB high-impedance amplifier in another Pomona
box. The circuit in Figure 13.1 works at audio frequencies.
26. The CMRR of an op amp is not a constant function of the common-mode voltage.
This inconsistency often dominates nonlinearity in noninverting circuits.