PICLIST <piclist <at> platinum.ca> writes:


> My sampling circuit will be a relatively high impedance voltage divider o=
ff
> of the 12V battery, but I'm assuming that if I give the ADC lots of time =
to
> settle (i.e. a large fraction of a second), I should be OK with a 200K+
> input impedance.

You really need to consider the input pin leakage currents before jumping t=
o
any conclusions.  No matter how slow you perform a conversion, or how much
capacitance you use for decoupling, input leakage will give you the same er=
ror.

Datasheet says +-0.1uA typical, +-1.0uA maximum.  With your 200k input
impedance that would give a typical offset error of 0.02v and a maximum of
0.2v .  And since this is a leakage current it will very likely vary with
temperature.

Also check the current that the A/D requires during conversion from the Vre=
f
source, you may well be using more power by running the A/D clock so slowly=
..

Your requirements of simply taking a sample about 1 per second could be ver=
y
easily implemented by using the watchdog timer to wake the part from sleep.
 You could then simply use the internal RC oscillator running at whatever
speed is appropriate (i.e. 125kHz up to 8MHz) to perform any processing
before sleeping again.  The watchdog takes around ~1uA, and the rest of the
device will be in the low picoamps range if correctly configured before
sleeping.

Mike

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