Current sensing circuit with LT1495 : need help

[radioactive1014]

Hi,
I am designing a circuit for my University project which involved measuring the current drawn by the load. I am using LT1495 rail to rail opamp. I have attached the circuit here. It's a simple differential amplifier with 4 resistors. Circuit works fine when the current is higher than .34 A. Whenever the current goes below .34 it can't sense anymore ( the output remains .34 ).

current_sensing.png (22.34 KiB) Viewed 5687 times

I would appreciate if anyone can help me solving the problem. Another thing is as I am on deadline I already ordered the PCB (it was very stupid of me ). so I can't change the circuit but all I can do is to change the value of the resistors. Is it possible to make it work?

Thanks for your help in advance.

[pebe]

There is nothing wrong with your circuit.

I assume that it is battery powered, so there is complete galvanic insulation between supply and input.

If so, there appears to be a 34mV offset on the +ve input relative to the -ve input. The maximum offset in the data sheet is 375µV so it's way out of limit. Is there any way in your physical layout that there could be a leak between the +Vsupply and the +input?.

[pebe]

Just a further thought. It could be that you have AC ripple on the input. That would give a sine wave centred on 0V. As the output cannot go negative relative to the -ve supply rail, you would be measuring the mean voltage of the positive peaks.

You could prove the point by disconnecting the input wires to the 0.1ohm sensor resistor.