I'm reviewing an ugly ADC input circuit which so far has worked very=20
adequately. An op amp's output signal (nom. 0-5v, but could rise to 12v=20
in fault condition) is clamped for safety by a 680R/5V1 zener diode=20
combination. So the voltage into the PIC ADC pin is externally limited=20
to 5v1 and the current from the opamp is externally limited to 10mA if=20
the op amp ever hits 12v.

Possibly due to some 5v1 zener diodes going obsolete and being replaced=20
by a close alternative, I find that some PIC ADC inputs are being=20
dragged down by the presence of the zener diodes, as the incoming=20
voltage approaches 5v. This seems to suggest that the current batch of=20
diodes are turning on earlier than those previously specified.

Removing the zeners seems to make the problem go away. I'm then left=20
with the worry that merely limiting the over-voltage ADC input current=20
to 10mA may be risky, even though the ADC inputs are very unlikely to=20
rise above 5v during the service life of the equipment.

Questions:
a) is just a current limiting resistor of 680R (for 10mA) sufficient due=20
diligence, for protecting the PIC ADC input clamping diodes if the=20
source voltage rises to 12v?
b) is relying on limiting the internal protection diode current, asking=20
for SCR latchup?

Ponderings appreciated,

Matt.
--=20
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