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.

[slight]

Please advise. According to my read of the datasheet, the only options for reference are VDD_GPIO or the internal reference. Given the equation for the output code includes this reference, it appears necessary to provide an external reference to VDD_GPIO for best accuracy. Applying an external reference to one of the input pins as you suggest will result in the output code being affected by the reference (internal or external), so it will be necessary to apply the external reference to VDD_GPIO too (and thus I need to know how much current VDD_GPIO will draw.)

[ketiljo]

The input resistance of the pins is very high so there's no current that will flow into the pin. But then the analog pin is sampled, there is an internal capacitor, typical 2.5 pF, that is charged. If you have a voltage source with a very high internal resistance, several kohm, the charging of this cap will load the source. Most voltage sources should be able to handle this just fine. 

Note that the accuracy of the internal reference is +/- 0.2% at room temperature. 

[slight]

I am asking for a specification for the input current of the VDD_GPIO pin. Obviously as a power pin, that does not have a high input resistance.