I trying to build a simple differential measurement circuit to feed a analog to digital chip to monitor current consumption of a board with an Arduino. So I decided to go with a "traditional" differential amplifier with op-amp in front to get a nice high impedance for this improvise differential probe. Here's a simple schematic of my circuit: (Op-amp have 12v supply)
To my surprise, the reading on the 5 ohm shunt resistor is way off the real reading (measure with a multimeter). Instead of having 2.02V which correspond to a 404mA consumption (which is what is expected), I got 43mV!? I used this probe (pin 3 & 5) to check voltage around others components on my board and the reading are correct. But on the shunt R1, it's completely off. I tough that since it's a really low resistance that my high-impedance will be ok be it seems not. I look around on the net and the solution seems to go with this approach but that's not working for me right now... Any suggestion?