We need to measure current from 25mA to 2A. Accuracy of 3 to 5mA. Wondering what is the minimum current INA216 can measure? What would be the better gain (25 or 50)?
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There should not be a significant difference on the accuracy versus gain, but do note than external noise may come into play on the higher gain variants. Things like PCB traces add undesirable voltage drops, which at 25mA may increase the error significantly, nevertheless increasing the sense resistor will decrease the error at the cost of additional voltage drop to the load, which may not be ideal in every application.
At lower currents the error will increase substantially. Here's a simulation of 1mA to 50mA
As you can see, from 10 to 40mA the error will be +/- 2mA, so I would not recommend measuring a current lower than this.
As with most engineering challenges, it is a compromise that will depend on your particular application requirements.
For your current ranges I would recommend a 50mOhm sense resistor and the 25V/V gain part variant, since the 50V/V part would saturate below 5V (here's why)
At 25mA, Vsense = 1.25mV and at 2A, Vensense = 100mV, which gives an output of 31.25 and 2500mV, respectively, which works well on 3V3 and 5V systems.
The gain versus error plot is shown below:
Similarly, given the range, a part with a gain of 20 and a 100mR sense resistor could also be used, assuming the additional voltage drop (100mV) is acceptable. The output would be 4V at 2A current, so it may need to be adjusted for a 3V3 circuit. The example below is the simulation for the INA18x