Page 65 - Troubleshooting Analog Circuits

52 5. Preventing Material and Assembly Problems

Figure 5. I. You can use this circuit to test your board for leakage current. The transistors are connected
across the op amp's feedback path so that they act like a current-to-voltage detector, with a
wide-range logarithmic characteristic.

Figure 5.2. You can calibrate the logarithmic-current-meter circuit of Figure 5. I to sense currents be-
tween - l mA and - l PA and between + l PA and + l mk

Leakage Can Be a Problem

When a PC board comes from its manufacturer, it is usually very clean and exhibits
high impedance. Sometimes a board starts out leaky, but normally a board doesn't
begin to leak until you solder it or wash it with a contaminated solvent-the fifth
problem.
Looking for leakages
When you have a leaky board or a slightly less-than-infinite-impedance connector
or insulator, how do you test for leakage? You can't just slap a DVM on it, because
even on the highest range, (for example, 20 Ma) the display will just read Over-
range. That's no help if you want to read 2000 Ma or 20,000 or 200,000 Mkr
even higher. Some DVMs or digital multimeters have a scale for microSiemens
(conductance) which will let you resolve as high as 100 Ma. But this scale does not
normally have more resolution than that.