How to calibrate ACS712 current sensor for a 3v3 MCU (Adafruit Feather M0 with LoRa module)?
The ACS712 is obsolete.
The eg ACS723 is a more recent replacement.

ACS712 datasheet here  

Reading the datasheet rather than watching You Tube videos will usually tell you much.
The device is ONLY 5V supply specified.

Fig 4A page 6 (count down 1 2 3 4 ...) shows that Vout is centred on VCC/2.
Vout = Vcc/2 +/- k mV/A with k varying with current rating of the part used (which we do not know).
Vout_max is shown as about 3.5V.
IF you operated the device on 5V you could resistively divide the output slightly to limit Vout_max to 3V3 or less.

The ACS723 datasheet here is also a 5V only part with similar output characteristics - but perhaps ~= 4.5V max Vout so a slightly greater Vout divider needed.

Options:


Use one of these device with 5V supply and an output voltage divider.


or


Use a 3V3 rated part.






Notes:

The ACS712 is obsolete. It can be used but is not recommended. For one off tasks it is fine.  

You can use different voltage MCU and sensor if desired, as long as the interface signals are within spec. 


If you are happy to provide 5V and 3V3 supplies then that's fine. 
IF you can use the same supply for both, so much the better. 


The ADC range of the MCU depends on the device 


it may be 0 to &lt; Vdd, or 
0-Vdd or  
0 - > Vdd (this is much less common, but in some cases is allowed). 


The ACS712 with Vdd=5V has Vout within 0-3V3.   

The ACS723 with Vdd=5V has max Vout > 3V3.   


IF the MCU cannot handle analog Vin > 3V3 then you will need to reduce the analog voltage.