Thu 10 May 2007
Final exams are over, so I am back to designing things. The first thing that I worked on was this tester for my pre-amplifier set. The pre-amplifiers are differential inputs into INA116 instrumentation amplifiers. I am trying to model very fine electrodes as the source, so I use a high (>100kΩ ) source with a 250μVpp (with a 300mV offset) test signal. What I didn’t take into account was sub-threshold conduction in the LEDs on the board. The minute current added to very high resistance networks created relatively large offsets in the test circuit and made it unusable. Most LED datasheets don’t display the current profiles below 0.1mA and many diode datasheets don’t display them past the μA range. Moral of the story is save yourself a few hours of debugging and remember that diodes conduct even below threshold.
( ina116.pdf ) ( dan235e.pdf ) ( 1n4004.pdf )
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