Do not use the Zener diode (D1) for analog circuits. It will adversely affect the readings.

It can be useful for protection of Digital Inputs though.

HTH,

Hi Darrel,

A simple thanks is not enough for the contribution you make everyday. I use your calculator often, but skipped your warning on the zener. A little bit elaboration might be useful. I have gathered the following info, but might be wrong. Please correct and update.

Zeners exhibit a soft knee while close to the breakdown voltage. Thus a 5.1 Volt zener would start conducting may be from 4.7 volts (depends on brand, temperature). They are also noisy, capacitive.
Advantages are they are cheap, also protects from negative input and easy to implement.

I do not rely on the 7805 regulator as they are themselves very noisy. Internal reference means the supply line. With 7805, internal reference measure a battery using a 10 BIT ADC PIC and consecutive readings alter by more than 1 LSB, almost always.

In my applications I almost always use a voltage reference MCP1541 from microchip which is 4.096 (binary divisible) volt @ 1%. Thus my inputs need not see voltages greater than 4.096. Escapes with a 5.1V zener from Vishay. Capacitor filters removes noise (to some extent). So overall the accuracy is not that effected. Last but not least use ADC oversample/average. The circuit I mentioned here works good http://www.picbasic.co.uk/forum/showpost.php?p=74678&postcount=11

However I would like to correct my knowledge from the experts like you.