Hello.
I have two questions.
I want to measure 0.1-1.1 mA current using ACS712 current sensor. I have the one which can measure upto 30 A. Is it possible to measure this small current using it, as I have failed to do so?
I'm using a 12 V 1 A adapter to power up my Arduino Uno. I got 5.7 volts as the VCC after powering up. My question is, which equation I need to use in the sketch?
a) I = (Pin voltage - 2.50)/0.066
b) I = (Pin voltage - (5.7/2))/0.066
Thanks in advance.
You should be using the AS712 that measures 5A not 30. The resolution on the 30A module won't give the results you want.
Bill
The 5.7V is too high. The maximum for an ATmega328P microcontroller is 5.5V. Is your multimeter calibrated ?
The ACS712 is very noisy and sensitive for wires, computers, transformers, and so on. You will never be able to measure 0.1mA.
I think that I know vaguely that the ACS712 is linear to the voltage, therefor you can use the normal formula with 5.0V, and if the VCC changes, the formula will still be okay. I'm almost 20% sure of this 
What current do you want to measure ? Must it be electrically isolated from the Arduino ?
Koepel:
What current do you want to measure ? Must it be electrically isolated from the Arduino ?
Thanks a lot. I want to measure the current which is flowing through 25 kilo-ohms. The input voltage will vary from 0 to 30 volts.
Is the current through the 25k resistor electrically connected to the Arduino somehow ?
The ACS712 is very inaccurate, but it is isolated. It is even possible to measure the AC mains current, but many modules are not made according to the rules for the mains voltage.
This high side current sensor connects the Arduino to the current that is measured : SparkFun Current Sensor Breakout - INA169 - SEN-12040 - SparkFun Electronics
I think you want to measure voltages, not currents. Perhaps an external 16-bit digital ADC that measures the voltage before and after the 25k resistor will do. It could be in differential mode, but perhaps that's not needed.