Analog read different with external and usb power

Projects Programming
4

Hi

The analogRead is from an external source (variable psu)with 1.2M ohm and 100K ohm as voltage divider.

The external psu negative is connected with the GND pin on the Arduino.

The 0 to 1023 10 bit value is then converted to give a voltage reading.

Value1 = ((1.1/1023)*analogread)*13 ;

where 13 is the divider factor of the resistors.

I did add this line-- analogReference(INTERNAL) and use 1.1 for my calculation.

What is the default setting of the reference voltage in Arduino?

As in the analogInput example in the Arduino Analog example.

Thanks.