AVR-Chat

From the datasheet, the recommendation is for the source impedance to be 10K or less as there is a sample and hold circuit inside that can give incorrect readings if not charged sufficiently quickly between samples. 
The sample time can be adjusted in the registers (by reducing the A/D clock time) If you have a spare pin you could use that to turn on a transistor while you do the measurement then turn it off so that there is no current flow when not measuring. 
For low current appliactions it is not generally a good idea just to hace a potential divider as the current is always there no matter what you are doing (it's a waste) I have used resistances up 1Meg before but had to slow the sample time down and take multiple samples to get a pattern rather than a spot measurement, in a battery application this is preferrable than having the constant current draw.
 
Regards
 


________________________________
From: stu_ki6j <ki6mwn@yahoo.com>
To: AVR-Chat@yahoogroups.com 
Sent: Tuesday, 3 April 2012, 4:32
Subject: [AVR-Chat] A/D current requirements


  

Greetings,

I am designing an tiny13 based voltage monitor. I wish to use the
internal reference to measure input voltages up to 15V, thus I need a
voltage divider at the A/D input. I wish to reduce power consumption to
a minimum. What minimum current through the divider would provide a
reasonably stiff voltage at the A/D pin?

A suggestion where to find such documentation, or an informed guess
welcome.

Stu




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