I have to make an ADC conversion with an ATXmega MCU, in an air quality application.
The input is a voltage divider between a voltage reference. The sensing resistor vary in relation to a gas concentration. I read the voltage between a load resistor and ground. Since the conversion is not high precision (12 bit) we don't want to wast any bit. The sensing resistor can reach the max value of 60 k: in this case the V on the Load Resistor will be at minimum.
The solution we found is to subtract this minimum voltage from the Vin with an op amp.
And now the questions: The voltage divider interact with the resistors of the diff op amp circuit (https://en.wikipedia.org/wiki/Differential_amplifier)?
Will it be useful to place a unity gain buffer on between the Vref_min and the negative input pin of the op amp?
Is this idea is achievable with a normal low noise op amp like this?
Here the schematic. I have to put an unity gain buffer instead of "???" node?
Is the circuit ok?