You might try putting a capacitor from the analog input to ground, since
the A/D input is not connected all the time. This COULD allow the
capacitor to charge to the appropriate voltage when the A/D is not
sampling. However, I expect much of the leakage current is due to the
clamp diodes, which are always connected.
        "Breaking the rules" by using a higer source resistance won't cause the
chip to blow up. You just won't get the specified accuracy of the A/D.
Can you live with less accuracy?  If not, it's probably time for an op
amp  voltage follower in front of the PIC.

Harold


On Tue, 13 Nov 2001 18:08:23 -0500 Eoin Ross <eross@CHEMSTATION.COM>
writes:
> Found that after I sent that e-mail ... oops.
>
> One question about the data sheet for a 12C671 - it says I need to
> (on pg 48) keep impedance at or below 10k on the analog inputs - I
> want to use a 2M/1M resistor divider to drop the input to 3V or less
> (Will be using a 3V low current reg for the PIC)  It looks like I
> would be breaking that rule with using a 1M impedance ??
>
> Anyone have a better idea for sensing the battery level? (low parts
> count/current consumption = good)
>
> >>> electrifried@GEOLOGIST.COM 11/13/01 02:07PM >>>
> Eoin,
>
> Microchip has a good appnote, AN606, that covers low power design,
> including how to measure power consumption.  A 16f877 will run on a
> 32kHz crystal, and a crystal is nice for calibrating the watchdog
> timeout period to adjust the wakeup time if you need pretty good
> accuracy.  Try to get Microchip's CD with their entire site on it,
> it will have the appnotes and is not annoying the way their site is.
>
> Alice
>
>
> >I am trying to develop a unit which:-
> >
> >  * Runs on a 9V battery
> >  *Power is turned on by a float switch ... at which point it needs
> to:-
> >         Wait 1 second -
> >                 if still turned on ...
> >                    check the battery level
> >                          turn on a piezo buzzer for 10 seconds
> >                                  (pulsing if battery is low)
> >                         go into sleep/low power mode indefinitely
> >                                  (until float switch opens turning
> off the device)
> >
> > I only have 16F8XX programming tools (Microchips ICD) availiable
> >    I know these are overkill but so is forking out heaps of $$ for
> only needing to program 15 chips.
> >
> > What is the best oscillator for low low power consumption - I am
> thinking RC. Obviously timing isn't critical - except for maybe the
> AD convertor.
> >
> > --
> > http://www.piclist.com hint: To leave the PICList
> > mailto:piclist-unsubscribe-request@mitvma.mit.edu
>
> --
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>
>

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