Em 5/9/2011 13:47, PICdude escreveu:
> According to the FAE, the spec did change (because of a new =20
> manufacturing process), so I'm guessing microchip hasn't gotten around =20
> to correcting the datasheets after using an older part datasheet as a =20
> starting point.  Seems like 3k is the correct value.
>
> However, assuming that your voltage source is close to ideal (very low =20
> impedance), then the impedance seen by the A/D should be the parallel =20
> combination of the voltage-divider resistors.  The cap helps, and =20
> increasing the sampling-cap-charge time also helps.  And of course, =20
> the 3K is to achieve a specific (really quick) acquisition time, which =20
> I'd guess most apps won't need.  At least at this point, I'm not =20
> worried about it for my app.
>
> I'm not clear on why the sampling caps are discharged after each =20
> measurement though.  There's an explanation in section 17.2.3 in the =20
> datasheet, but I'm not clear on its meaning.  I'd think starting from =20
> the last value would allow for faster sampling, especially for sources =20
> that change slowly.
>
> Cheers,
> -Neil.



Unless your analog source can only source current, not sink.
--=20
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