Page 211 - Troubleshooting Analog Circuits

I98 E. VF vs. IF on Various Diodes

right away. Then I tried measuring the Vf of a diode with a battery-powered DVM in
series with the diode. I got ridiculous answers such as 30 nA of leakage on a good
transistor at 100 mV. As soon as I plotted the data, I realized I was on the verge of
being fooled. Plots are valuable because you can see the shape of a curve, and if it
doesn’t make sense, you can spot it. It turns out-the DVM was pumping enough AC
noise current into the diode to cause rectification, and false values of current.
(Remember, I warned you about that back in Chapter 2.) When I then put 0.47 pF
across the diode, I got valid data. You should be aware that you, as well as I, can
easily take bad data-it’s easier than taking good data. You just have to be suspicious
and stop and go back and make sure to throw out the bad data, and then get valid
data.