I would like to measure current making use of a shunt. Since the solution needs to be scalable I wanted to have a circuit that would be easily adjustable to various current ranges, and therefor opted to make use of a differential amplifier. It's also necesarry to measure on the high side, since there are a lot of components connected to common groud, which I dont want to disturb.
For my first test I wanted to measure currents up to 30Amps, neatly scaled to my 8bit 0-3v ADC(Not the best resolution, but plenty for my application) I did some online searching, and came across this simple circuit a lot. I calculated the resistor values to get a gain of 40, so the 75mV voltage drop at 30 amps through the shunt would get me 3V at the output of the op-amp.
However when I want to measure current on the high side of my circuit, I dont get the values I expect. When I put it at the low side it does work however. I'm in the dark on what's wrong with this circuit.