I am using part of this quad amplifier as a differential amplifier in a low side current sense application. The circuit in question is within the red box labeled "current sense". The voltage sense circuit functions properly, so the power and ground is fine.
To test the circuit I took out the MOSFET and placed a pot between the sense resistor and ground. At 0 ohms, the output of U4C is about 500mV and the current across the pot is about 102mA. I then increase the resistance, and both the output of U4C and the current continue to decrease as then should. The unexpected behavior is around 30mA and the voltage on U4C is about 200mV. As I continue to increase the resistance, the current will continue to decrease, however the amplifier output begins to increase to about 330mV. At that point the voltage will "runaway" and increase on it's own to the positive supply rail, without changing the resistance of the pot.
I can't seem to figure out why this would be happening. Is there a design flaw?
Thanks