Hello, I've been trying to implement this circuit:
What it does is: takes an input voltage of 0.55-2.75V, subtracts 0.55 and multiplies it by 1.5 so that on PIN 1 of LM358N I can get 0-3.3V. It works well on the simulation software.
I tried to build the circuit and it works. I have 0-3.3V as an output voltage. The thing is: I want to connect this voltage to an Arduino Due so that I can get a 0-4095 proportional digital range to the original 0.55-2.75 voltage range. The problem is that the digital reading on the Arduino Due is not right. For exemple: if I apply 1.65V to it (via opamp circuit output), I read 2500 or something close to this number, but i was expecting to read something close to 2047 since 1.65V is half of the input voltage on this Due. This problem happens for the whole scale, but it is not a fixed offset, it varies.
One thing that I noticed it that I if I disconnect the ground pin from the Arduino Due it still reads the same range with the same problem. That makes me believe it is some type of ground connection error. I tried connecting a capacitor, a resistor, both between the opamp output and its ground, but there were no better results.