On 9/15/2012 12:26 PM, Perry Curling-Hope wrote:
> The maximum recommended source impedance for a PIC ADC input is 10K which
> means approx.  1mA drain during an ADC conversion.

FWIW, I've seen that spec vary from PIC to PIC.  The source impedance is=20
really the parallel combination of the high and low voltage dividing=20
resistors (assuming an ideal voltage source of zero ohms), so you can=20
meet the 10k spec with 20k || 20k.  That means 9v/(20k + 20k) =3D=20
~0.225mA.  Still relatively high I know.

Another thing is that the recommended max impedance spec is to meet the=20
acquisition time.  If you can allow more time, and don't need a=20
gazillion samples per second, you can go much higher.  I've used 100k ||=20
100k for an effective 50k source impedance without problems.  I only=20
needed a few samples per second, which I averaged.

--=20
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