ADC Max Impendance and Power saving

[Michael Wakileh]

Hi! I am experimenting with power saving designs on a PIC12F. The PIC is regulated at 2.5V, and powered through a cap with a charge of up to 10V. The pic should intermittently determine the voltage before its voltage regulator.

I plan on using one pin for adc and another pin that is already used as an indicator pin. The other pin powers a 10mA indicator led when high (AT LEAST 200mS....), and will now ALSO drive the base ofa 2n2222equivalent transistor switch.

The Voltage diver which feeds the ADC is the LOAD of this 2n2222 transistor.

(there is also a high ohm weak pull-down resistor on the transistor base to keep it turned off completely).
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I'm unsure about good practice using a highohm (powersaving) voltage divider to feed the ADC. The PIC'S ADC has a Recommended max Impedance of Analog Voltage Source of 10K but I don't quite understand what is meant by that. Could anyone explain?. I had planned on using a 100K+25K divider...and i am wondering if this destined to fail...
If yes, could I charge a 100pF or 1nF cap (parallel to the 25K) to store the voltage before aquisition? (I have enough time to charge it up before the measurement).

I cherish all input...

mike

[Michael Wakileh]

(...I meant 100K+33K divider....)

[Michael Wakileh]

... and I forgot to mention that the other pins are in use as well...

[Michael Wakileh]

Alright...I've found a post (from Darell Taylor) from which I can tell that my input impendance is too high. Still I wonder if the method with charging the capacitor might work...I've been looking at the charge times using the "MISC EL" calculator, and have plenty of time to charge even a 1uF cap (I used 100K as the value in the charge curve).