My project has 9V and 3.3V on the board. I would like to use a precision voltage reference IC for the analog reference on the MCU (MCU powered by 3.3V).
The precision of the voltage reference IC is 0.5%, which is more precise than the 3.3V regulated output voltage from the regulator powering the MCU. The datasheet for the MCU says the AREF High voltage should never exceed the VDD.
In an ideal world, the voltage of the reference IC and the voltage from the switching regulator would both be exactly 3.3V. However, I am nervous of the real world possibility that the reference IC, with its high precision, will be a higher voltage that the VDD. For example, the reference IC could output 3.29V and the regulated voltage could end up being 3.22V. Is this enough to cause a problem? Or is this small of a difference negligible?
My alternative option would be to use a reference voltage of 3.0V and guarantee the reference voltage will ALWAYS be below the regulated 3.3V, However, it is easier to voltage clamp 3.3 volts as a ceiling for the analog inputs since I already have a 3.3V plane on the board.
Or would it be possible to use a 3V reference and still clamp to the 3.3V plane? According to the datasheet, the analog input voltage cannot exceed the reference voltage :( but IDK why...
Shouldn't the ADC value just max out at anything above the reference voltage? Obviously going above the 3.3V could cause damage...
Any thoughts are appreciated!