I had a similar issue some time back ... 15V source/environment, =20 voltage divider to drop that to <5V at A/D input, 5.1v zener for =20 protection. I too found that the 5.1V zener was altering the signal =20 as it went higher, and it was inconsistent from one diode to another. Up to then I was using a 4.096V reference to the A/D, so I changed =20 that to 2.5V, and altered the voltage divider to divide by 8 (7k and =20 1k), and removed the zener altogether. With this, I get good =20 resolution still, and the input can safely go to 40V. Cheers, -Neil. Quoting Matt Rhys-Roberts : > I'm reviewing an ugly ADC input circuit which so far has worked very > adequately. An op amp's output signal (nom. 0-5v, but could rise to 12v > in fault condition) is clamped for safety by a 680R/5V1 zener diode > combination. So the voltage into the PIC ADC pin is externally limited > to 5v1 and the current from the opamp is externally limited to 10mA if > the op amp ever hits 12v. > > Possibly due to some 5v1 zener diodes going obsolete and being replaced > by a close alternative, I find that some PIC ADC inputs are being > dragged down by the presence of the zener diodes, as the incoming > voltage approaches 5v. This seems to suggest that the current batch of > diodes are turning on earlier than those previously specified. > > Removing the zeners seems to make the problem go away. I'm then left > with the worry that merely limiting the over-voltage ADC input current > to 10mA may be risky, even though the ADC inputs are very unlikely to > rise above 5v during the service life of the equipment. > > Questions: > a) is just a current limiting resistor of 680R (for 10mA) sufficient due > diligence, for protecting the PIC ADC input clamping diodes if the > source voltage rises to 12v? > b) is relying on limiting the internal protection diode current, asking > for SCR latchup? > > Ponderings appreciated, > > Matt. > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .