Reference Accuracy

[slight]

I cannot find a specification for the SAADC converter's reference accuracy. The product specification indicates it is 0.6V, but beyond that I cannot find anything. Looking for absolute accuracy at room temp, over operating temperature range and any drift specification available.

Also, is there any guidance for GPIO voltage and input voltage range for best ADC accuracy? Ie., is it better to have a 3.3V VGPIO and ADC input range up to 3.3V, or a 1.8V VGPIO with ADC input range of 1.8V (assuming external circuitry can bring the input signal to the desired range.) Or, does it matter (ie., could have VGPIO=3.5V with ADC input range of 2.5V and use the internal reference.)

Any general guidance on attaining best ADC accuracy? Ie., better to use internal reference or external reference feeding GPIO (or one of the inputs assuming input signal is single ended.)

[ketiljo]

The accuracy of the internal reference is +2/-3% over full operating temperature and voltage. 

The best it to use the full scale of the ADC. How you to it it not important but the highest analog value should give close to max. If you have a max input of 3.3 V on the AIN pin, you can divide it with 6. The input to the ADC must be lower than the reference. 

If you need to use an external reference, you must use the ADC in differential mode and supply the reference voltage to the positive input. 

[slight]

Thank you, I understand that given the equation for the output code includes the reference voltage, in order to get better accuracy than +2/-3%, it will be necessary to use an external reference on VDD_GPIO. In order to specify an external reference, I need the specification for input current on the VDD_GPIO pin and I can't find that on the datasheet. Of course I understand that any current supplied by any of the GPIO pins will also have to come through that pin, but I need to understand the initial current before any IO pin output current is added.