Electronic – No amplification with LMV358 compared to LM358 in crude current sensing

I had first imagined that your problem was that the opamp was oscillating and suggested that you try something like this (thanks to Spehro, too):

^{simulate this circuit – Schematic created using CircuitLab}

Per further discussion, where you added that you were supplying the output of the opamp to an MCU input pin on your Arduino, it's also clear that even if you did (or would have had) an oscillation problem (and you would, I'm pretty sure) it is more likely now that your measured limit of around \$6V\$ was due to a heavily loaded protection diode at your MCU's analog input pin. The protection diode limitations for any given MCU can usually be found under the "Absolute Maximum Ratings" section of the datasheet, where that section will include a line described as something similar to: "diode current at any device terminal." Usually, the value listed there will be around \$2mA\$ or less.

If you must measure the opamp output voltage, then you will need to consider a different circuit arrangement to condition the opamp output (without disturbing its MOSFET control behavior) and make it suitable for input to the Arduino analog input pin.

Yes, your problem with the physical circuit is CMRR -- common mode rejection ratio. The CMRR of the opamp itself is pretty good, but you have used very low tolerance (5%) resistors, and that has given your circuit a very poor common mode rejection.

To illustrate what is going on, open up your LTspice simulation and change one of the 10K resistors to 10.5k (i.e, 5% error). You will see a ~500 mV change in the output.

The reason for this is that the gain of your positive and negative inputs are different. What you want to measure is G*(V2-V1). In reality you have (G2*V2-G1*V1). When the gains are not matched (because your resistors are not matched), you have a term that doesn't precisely cancel that is proportional to the voltage at the input terminals, rather than only the voltage difference. This is called the common mode.

It is common to use high precision resistors (0.01% or even better) when constructing differential amplifiers with high common mode rejection. Frequently people use instrumentation amplifier ICs that have laser-trimmed resistors on the chip.

What can you do to fix this with what you have? The simplest is to move the current shunt resistor to the "low" side of the load (i.e., between RL and ground). This will reduce the common mode signal, and thus the impact of the poor CMRR.

You should use the most precise resistors you can find. 1% are common and cheap, but if you don't have them available, you can just grab a handful of 5% resistors and an ohmmeter and find matching pairs. Selecting parts like this isn't as good as actual precision resistors, since low spec parts usually have worse drift and temperature coefficient, but it still helps.