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I want to measure the voltage of a lead-acid battery bank (48V nominal, 42-64V operational range).

In order to get more accurate readings, I want to substract a reference voltage (40V) and then convert the remaining 2-24V range into 0-3.3V range (ADC).

I guess this could be achieved with the circuit above. Do you have any other suggestions?

LE: Here's the original schematic (post #1) with the actual component values. [USER=152255]@FvM[/USER], could you elaborate on untrusting operating conditions of TL431? R1 current will always be around 1 mA and the R4 (and TL431 K-A) current will be 4 mA, hence the voltage drop across R4 will be 30V.

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