The wiper arm of the potentiometer connects to the negative supply voltage. It consists of a 10-kilohm potentiometer connected between the offset null pins (1 and 5) of the op amp. Appendix 4 shows the recommended nulling circuit for an MC1741SC. the input signal from the Arduino PWM output and add offset voltage value. Amplifying the output to get better low current readings doesn't really work because you are just amplifying this noise inherent in the chips. Most op amps, including the 741, have provisions for nulling or canceling the output offset voltage. Now input the frequency you want (which you dont specify) and the amplitude. 3) There is actually a fair bit of noise on the output, so really low currents aren't very accurate.
Turn one of these over while monitoring the output and you can see it change with orientation. The first and most dominant factor is the balance of the resistor ratios across A 3. Two factors influence an instrumentation amplifier’s CMR. 2) They are hall sensors that measure magnetic fields so it weakly picks up the earths magnetic field. With CMR, the inner workings of the op amp are straightforward the change of offset voltage is the only concern. 1) Like was said above, they need some time to thermally settle after power up.
If you are concerned with super low levels of current, you might be better off with a shunt solution, but then you have to figure out your own voltage isolation which can make the circuit much more complex very quickly. If you are using the full range of many 10s of amps, the super low levels shouldn't be as important. These are all small effects, but still something to keep in mind.
These allegro hall sensors are great chips for certain applications, but there are definitely some things to watch out for.
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