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looking for a 12v precision voltage reference or something else to do the job

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queglay

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hey guys. im using an adc on a vellman k8055 to measure battery voltages.
my battery voltage range is 12 (flat)-14.4 volts (charged). my adc range is 0-5v and its only 8 bit.

to maximise the resolution i raise the ground voltage with a 12v regulator so the input on the adc will be from 0-4.4v. problem is that the regulator has a variance of about .05v on the output which is stuffing up my measurements considerably.

so what i believe im looking for is a precision 12v reference.
I cant find any 12 references anywhere though. i found this 10v one though - **broken link removed**

close but no cigar.

is there something else that can do the job here? thanks for any tips.
 
the other thing I'm wondering is if i have vout set to 12v, what does the input voltage have to be? would i get away with a 12v battery charged between 12.3 and 13v?
 
As all you're wanting to do is drop the voltage to the ADC range, then just use a simple attenuator, messing about with regulators is pointless - your multimeter does it with attenuators, your scope does it with attenuators, EVERYTHING does it with attenuators.
 
im not familiar with attenuators, so i may be wrong here, forgive me if i am - but after reading wikipedia it appears attenuators will drop the voltage range by a factor. it appears they will scale the whole range, and not just clip the bottom like i am after.

eg 12-14.4 volts will become (divide by 3) will become 4-4.8 volts.

this is undesirable as it is not taking advantage of the full resolution of the adc.

with a precision reference that range is instead brought down to 0-2.4 volts which can be multiplied by a circuit included in the adc to become 0-5 volts, taking advantage of the full 8 bits.

correct me if im wrong though, im a newb to these circuits. thanks for the help. if this circuit actually does the right thing, how would i calculate the parts required to simply subtract 12v from the input?
 
You are correct, an attenuator simply scales it down - but your resolution isn't going to be grossly affected - and it's still likely to be more accurate than trying to subtract 12V from it.

If yoiu do want to, use an opamp as a differential amplifier, and subtract that way (using a precision reference on the other input).
 
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