Analog data input Arduino

Data input ;

values = analogRead(A0)

Configures the reference voltage used for analog input (i.e. the value used as the top of the input range). The options are:

• DEFAULT: the default analog reference of 5 volts (on 5V Arduino boards) or 3.3 volts (on 3.3V Arduino boards)
• INTERNAL: an built-in reference, equal to 1.1 volts on the ATmega168 or ATmega328 and 2.56 volts on the ATmega8 
• 
  
• EXTERNAL: the voltage applied to the AREF pin (0 to 5V only) is used as the reference.

      

       Scaling values ;

0 to 5V give as 0 to 1023 digital values 

minimum voltage that can read is 5/1023 = 4.88mv

when you give power supply to the board through Diode, voltage will reduced up to 4.4V 

then the correction must be 4.4/1023= 4.3mv 

further, it is required sampling of those analog values in for loop at least few hundred 

for ( int i; i<100; i++) { values = analogRead();

 voltage += 4.441*values/1023;}

then average the voltage by dividing the number of samples 

output = voltage/100;

furthermore,  gain must be adjust as per the requirement ; 

( you have to get few values and read the values with external meters and add the gain)

real Volatge = Arduino read voltage values *gain ;

calculate the gain as per the above equation and put the gain to above output ;

output = voltage*gain/100;