Op Amp Saturation output

[cstiang]

Hello I am doing board repairing with faulty current sensing circuit,

The op amp have -12V saturate rail output with -1.73V measured on the inverting input of the op amp.

I have tried to disconnect the non-inverting input of the op amp and it behave the same, tried replaced the op amp IC as well but no luck!

[JFJ]

The op amp have -12V saturate rail output ...

A typical op-amp has an extremely high open-loop gain. Consequently, even a tiny difference between the voltages at an op-amp's inverting and non-inverting inputs will cause its output to saturate. The inputs are usually maintained at the same voltage by negative feedback.

Presumably, the op-amp is monitoring the voltage drop across a low value current sense resistor, to determine the current flowing. If that sense resistor has gone high or open circuit, then the voltage drop could be too high for the negative feedback loop to handle. Hence, the op-amp's output saturates.