AVR-Chat

On Apr 2, 2012, at 8:32 PM, stu_ki6j wrote:

> 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.


I don't know about the 13 but for the ATtiny85, in the section titled Analog Input Circuitry is this line:

    "The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or less."

So the voltage divider needs an impedance of 10K ohms or less.  If it's your basic two resistor voltage divider this means the resistors if in parallel need to be less than 10K. 


 ie. 10K >= 1 / (1 / R1 + 1 / R2)

That is assuming the source impedance of the monitored voltage is very low, otherwise you need to add it to the resistance of the high side of the divider.




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