Spehro Pefhany wrote:
> Here are some general suggestions, but there are lots of other
> possibilities depending on the exact requirements. Note that if you
> use another voltage and divide it down or buffer it you'll need to
> consider resistor errors and drift or offset voltage etc. in the
> error budget. Buffering for the PIC ADC reference requires a very
> low impedance source; probably a bit more than a simple op-amp output,
> like a couple of resistors and a cap to get a stable low-Z source.

Or you can feed the 2.5V reference into the A/D as another input, run the
A/D reference off the Vdd supply, and do the correction in firmware.  This
is what I did in the ProProg.  The 2.5V reference from a LM385 goes directly
to an A/D input.  The reference voltage is still read with about 1 part in
500 resolution, which is well beyond its accuracy.  Multiplying it up to a
higher voltage would have introduced much more error than it would have
given back in terms of A/D resolution.  Correcting the other readings to the
measured reference is just a little math in the firmware, which costs
nothing in parts and introduces virtually no additional error.


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