Hello guys,
@jonisonvespaa: it is absolutely not possible to measure resistances around 0.001 Ohm with any standard multimeter. Even the resistance of the multimeter wires is much higher (~200 times!) than the measured value.
In this case
a) just believe the datasheet, the internal resistance should be around 0.0012 ohm
b) make a simple test - solder one precise value 0.001 ohm resistor as close as possible in parrallel to the ACS7xx current sensor and check the reported current value. You can then calculate the internal chip resistance precisely. For example with 10A current the reported current should be 4.55 A after the modification, if the internal chip resistance is really 0.0012 ohm. (for ACS715 0-30A sensor)
@to this topic: I was experimenting with many current sensors lately, and no one met my expectations:
a) ACS7xx based hall sensors : highly sensitive to magnetic fields, very high noise. Not usable for measuring precise values under 1 Amp.
b) AMPLOC hall current sensors (amploc.com) : not suitable for DC currents - the core gets magnetized and zero value shifts by +-0.5 Amps after each day of measurement. Zero internal resistance. Not usable for measuring precise values under 2 Amps. So the error of this sensor may be over 0.5 Amp in any moment, that is not good at all. To be used in cars or such equipment, where it's enough to know the result with +- 1 Amp error.
c) ATTOPILOT current sensor (45A) : shunt resistor type sensor. Not isolated from the measured circuit! Stable zero value. Does not measure currents under 0.22 A (reports zero). All reported values are lower by this 0.22A offset (you have to add 0.21-0.22 Amps to all results). Internal resistance 0.001 Ohm.
d) LTC6102 based shunt current sensor (I haven't seen any sensor using it, I had to make my own). Same as ATTOPILOT, but the offset error is lower - around 0.1 A. Internal resistance 0.0005 Ohm.
So if you do not care about currents under 0.1 resp. 0.2 Amps and you do not care about the electrical isolation, use the ATTOPILOT 45 or LTC6102 based sensor. The LTC6102HV is able to measure on voltages up to 105 Volts or something like that (see the datasheet). Main advantage is, that the results are very precise (above the offset mentioned), low noise and not influenced by any magnetic fields.