> If you can allow more time, and don't need a gazillion
> samples per second, you can go much higher.  I've used
> 100k || 100k for an effective 50k source impedance
> without problems

You need to keep an eye on the sampling cap's leakage,
which varies from PIC to PIC

The sampling cap's leakage is what will be determining the
recommended source impedance for a particular device.
Current from the source, eg through a resistor, will be leaking
out through the cap and the cap won't reach its, er, full
potential

Looking at the leakage figure in the Analog Input Model
diagram in some handy datasheets,

+/- 500nA for the 18F1220 but only +/- 100nA for the
18F4550. The diagram for the 18F1827 notes that

"I leakage =3D leakage current at the pin due to various junctions"

and refers you to the Electrical Specs but I don't believe it
actually quotes a figure other than "Recommended impedance
10k. Can go higher if external 0.01uF capacitor is present on
input pin

Joe
--=20
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