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Voltage Divider Problem

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sonicj:
i can't seem to wrap my head around how to do this....

i am trying to measure total voltage of a string of six 48v batteries. the 48v pack should read about 60v tops fresh off the charger. the ADC input range is 0-5v. i have no problem getting the voltage down to 5v with say a 65v input (+5v for ADC safety) and it tracks fine all the way down to zero. the thing is, im not really interested in measuring anything below say 40v, because anything below around 46v has already damaged the batteries. so basically, im wasting most of my dynamic range of the ADC on voltage lower than i'll ever need to measure.

is there anyway to utilize the entire dynamic range of the 10bit ADC for a "window" of voltage? say 40v-65v?

looking forward to some expert insight! cheers!
-sj

oPossum:
Use an external low reference voltage of 3 volts and high reference voltage of 5 volts.

qno:
The easiest way is to put a zener diode in series with your voltage divider.
A zener of 39 Volt will start conducting arround 39 Volts.
Be aware of some non linear behavior when the zener starts to conduct.
Check the zener data for an appropriate current.

You can also do this with an differential opamp. Set Vref on the - input to 3V.
Set the gain to 0.2 and your 40-65 volt will be converted to 0.05 to 4.95 V adding the rail to rail limits of a good opamp.

Simon:
you could use opamps to create an offset, or setup the ADC in the pic to use external lower and upper Vref's so you could set the lower ref at 3V and the upper at 5V, you will need to provide the reference voltages yourself

sonicj:
thanx for the answers! the vref seems like the easiest solution as i already have 3.3v on the board. 41.2v to 62.5 = 3.3v to 5v on the meter without any changes to my divider; nearly perfect! the thing is, i don't think the arduino will let me set 3.3v as the low and 5v as the high. the AVR datasheet leads me to believe its a either or thing.  :-\

it sounds appealing, but im not sure i completely follow how to use the zener....

i might give the opamp a go in a bit. i have a few i salvaged from a old cd player! cheers!
-sj

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