Four ranges, one step

Four ranges

The chip's gain setting, called the PGA, picks how many millivolts across the shunt count as full scale. Across 100 mΩ each one becomes a current:

/8 is what the chip starts in, and what the Adafruit library's default uses.

The step does not change

It is natural to expect a narrow range to measure in smaller steps. It does not. The shunt register counts in 10 µV at every setting, which on this board is 0.1 mA whether the range is 0.4 A or 3.2 A.

What does change is the zero error: how far from zero the chip may read with no current at all. TI's maximum is 100 µV at /1 and twice that at /8, so the narrow range halves the error that matters most at small currents.

Above the range

Above full scale the reading stops. A load drawing 600 mA reads 400 mA on the narrowest setting, and the reading does not move however much more it draws. A reading sitting exactly at full scale is the only sign, which is what makes it easy to miss.

So pick the widest range the load could reach, including the moment it starts, and only narrow it for a load you know stays small.

The three calls in the library

Only the last one uses the narrow range, and it also drops the bus range to 16 V. The middle one keeps /8, so its zero error is the widest setting's.

For the narrow range on a rail above 16 V, write the config register after the library's calibration: 32 V bus range, /1, 12-bit, both measured continuously.

ina.setCalibration_32V_2A();          // 0.1 mA a count
Wire.beginTransmission(0x40);
Wire.write(0x00);                     // config register
Wire.write(0x21);                     // 32 V, gain /1
Wire.write(0x9F);                     // 12-bit, continuous
Wire.endTransmission();               // now +-0.4 A