I have exactly that situation on a recent project - the analog input pins could
se negative voltages so have to be protected. There are input dividers on all
the A/D input pins, so they are current limited - I ended up putting a 5.1 volt
Zenar from each to Gnd.  The Zenar voltage clips the positive excursions and
the forward diod clips the negative ones.  So far no strange results and no
chip failures, but they haven't been in use long enough to give me a meaningful
statistics run.  Two diodes - to Gnd and +5 - would work just as well, but
requires more parts. The dual transistor circuit would be even better - its a
tradeoff.

I am somewhat surprised that none of the chip makers have given us GOOD
clamping devices wiht low leakage and very sharp +/- turn-on knees.

Roy J. Gromlich
Renaissance Technologies



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