Precise measure of current

I have arduino due and current sensor ACS712 (+/-20A). When I put power to jack 12V, I have a big fluctuation about +/-0,2A or more, but when I have power from USB, then I heave fluctuation about +/-0,03. I know that is something with power supply fluctuation. I want to use precise voltage regulator and something like this http://hacking.majenko.co.uk/making-accurate-adc-readings-on-arduino, is it possible to make this in arduino due.

If you want to make accurate voltage measurements using the Arduino, you will need an accurate, external voltage reference. Note that "accuracy" and "precision" are two different concepts.

The Due doesn't seem to have an internal reference ADC internal voltage reference for DUE ? - Arduino Due - Arduino Forum

On an ATmega the internal voltage reference is not very accurate. According to the ATmega328 data sheet (Table 29-16, ADC characteristics) the nominal value is 1.1 V but the actual value can be anywhere from 1.0 to 1.2 V. On the other hand, the long term stability of the voltage reference may be pretty good, so if the internal voltage reference value can be accurately measured (for a particular application) it may be quite reliable.