Hello,

It's a lead-acid battery charger project and I'd like to control the voltage applied to the batteries but not in the full charger's range (0V-32V) for accuracy reasons.

I measure the load's current via R 0,1ohm; no problem here (I have made a 10x gain ampli-op circuit to get a usable reading). Reference for this ADC channel is GND.

For the voltage range that will be controlled, the second ADC channel would return a value between 195 (25V) and 250 (32V) if I use the same voltage reference (using a 2 R divider connected between 0 and 32VDC). BUT, doing so, I use only around 20% of the ADC's resolution range.

I want to monitor a range of approx 7 volts (lets say from 25V to 32V) because this is the range I am really going to control the batteries's charging voltage. I would then (theoretically) read ADC values from (approx) 5 to 250 wich would be almost the full range (hope I'm clear enough...).

My idea is to use a 25V Zener.
<img src="http://home.citycable.ch/flotulopex/downloads/I-U-meas.jpg">

The problem is that both channels won't have the same voltage reference (GND).

How do I resolve this?