AVR-Chat

On Tue, 3 Apr 2012 11:32:01 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.

Data Sheet. Table 18.7 puts the input impedance at 100 MOhm

Keep your external divider impedance a decade lower.

If the working range of battery voltages is much smaller than the
peak voltage (e.g. 8V to 15V), then you can expand the useful
resolution of measurement by having the tiny13 "float" above the
battery negative slightly below that higher, minimum voltage.

One way of minimising the parasitic loss due to the voltage monitor
is to put a FET in the voltage divider and switch that on using one
of the tiny13's digital outputs when you want to take a measurement.
Check the specs of the FET for its impedance when "off".

-- 
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