Q:

Op Amp High Side Current Sensing

I'm trying to build a high-side current sensing circuit with a differential op-amp which can be seen below:

The LT1006 is actually an LM358 IRL(I know it's a peanut op-amp, but it's what I have available). Anyways, according to this setup, I should have unity gain. 5V/101.1 OHM = 49.4mA. So the shunt has 49.4mA * 1.1 OHM = 54mV across it, and therefore the LM358 should output around the same voltage, but it doesn't. I've made sure the resistors are very matched and also tried using 100k resistors. I've also experimented with different power rail voltages and even tried setups that would give a gain of 5, 10, and 20, and I still can't get the op-amp to output the expected voltage. What am I doing wrong? I've read the datasheet and I'm pretty sure I am within spec of the common mode input range.
Thanks.

A:

The LM358 can can't really get down to the negative rail while sinking current, and your circuit has about 250uA flowing into the output.
Increasing the resistors to 100K probably helped quite a bit, but it still is limited.
Referring to the datasheet for the typical characteristics:

To get down to 50mV it can typically sink perhaps 15uA with a 5V supply @ 25°C, so maybe you can count on 5-10uA in realistic conditions. And that's just to barely get down to 50mV out.

To get something useful out of this, you can increase the gain significantly so the minimum output is (preferably) in the +1V range. But then you'll run into common mode range issues if you just increase R11/R12.

