jcantara@comcast.net wrote:
> I had come up with this solution: Use a resistor and a zener diode in
> series, connected between the supply voltage and ground. The zener
> would have to have a voltage drop of less than the minimum supply.
> Measure the voltage drop across the resistor, do som

Your message got cut off somewhere.  When are people finally going to learn
not to send paragraphs as single long lines!?

Anyway, I've done something similar too.  Zeners at those voltages are not
that accurate, so I used a 2.5V reference because I wanted better accuracy
than what such a zener reference could provide.  In this particular
application the PIC read several voltages including the 2.5V reference.  In
this case I had sufficient cycles so I did a divide for each value to get
the true reading, and compute the Vdd voltage at the same time.

If cycles were more important than complexity, I might use a lookup table
for the 2.5V reading to get a correction factor that can be multiplied by
the other readings.  With reasonable Vdd regulation, you know that the 2.5V
reading can only occur over a small range of the 1024 possible A/D readings,
and the table only needs to cover this worst case range.  64 table entries
gives you +-6% correction range for the combination of the 2.5V reference
voltage and Vdd supply accurracy, with about .2% resolution.


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