Each time I power up the Arduino, the max reference voltage which is represented by 1023 changes. I observed this by measuring the voltage from the Aref pin each time. Is there any internal function or way to find this max voltage? Because, the voltage reading from the analog input pins of a constant voltage source keeps changing every time as a result of varying max reference voltage.
Electrical – Arduino mega 2560 Analog max reference voltage
analogarduinoatmegavoltage-reference
Related Solutions
I don't see any problem with applying an external voltage, through a 5 kohm resistor, to the Arduino reference input. Or better, with using a resistor divider, so that you turn 5 V into your desired AREF voltage, while at the same time exhibiting a source resistance of approximately 5 kohm. This second requirement does not have to be accurate. That is just to limit the current that will flow from AVCC to ground, through the external circuitry.
If you want to end up with 1.8 V at the AREF input of the MCU, just choose R1 and R2 so that \$ V_{AREF} = 5ยท\frac{R2||32000}{R1+(R2||32000)} =\$ 1.8 V and \$ R_{source} = R1||R2 \approx\$ 5 kohm.
When you need to work with the [0, 1.8] V range, disable the references internal to the ATMega, and when you need to work with the [0, 5] V, enable the internal AVCC reference (if that is 5 V). If the MOSFET shown in Fig. 24-1 (that connects the internal references to the AREF line) has an on resistance much lower than 5 kohm (which I suppose it has), the internal circuitry will see AVCC. In this second situation, the current drain from the internal AVCC (assumed 5 V) to your external resistor divider will be \$\leqslant\$ 1 mA, but that is not a problem.
In summary: it would be bad advice if something could get damaged, but 1 mA won't damage anything.
Not sure which Arduino you have but on page 377 the datasheet for the atmega2560 says Vref min for the adc is 1.0V. So unfortunately that probably won't help you.
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Best Answer
The default reference should be 5V, fed from Vcc to Aref by way of an internal reference select multiplexer. If might be less than 5V if the board is powered from USB.
Measure your 5V source - is that varying as well?