Can nRF51822 ADC external reference (AREF0/1) voltage be as low as 0.5V or less?

[Pablo Gindel]

According to PS, external reference min. voltage is 0.83V. However, I've experimented with voltages of about 0.2V and it appears to work. Specifically, I connected a voltage divider with 47K to VCC=3.2V and 3K3 to GND, then connect the midpoint to pin p0.06 (AREF1), with the following code:

NRF_ADC->ENABLE = ADC_ENABLE_ENABLE_Enabled; NRF_ADC->CONFIG = (ADC_CONFIG_RES_10bit << ADC_CONFIG_RES_Pos) | (ADC_CONFIG_INPSEL_AnalogInputNoPrescaling << ADC_CONFIG_INPSEL_Pos) | (ADC_CONFIG_REFSEL_External << ADC_CONFIG_REFSEL_Pos) | (channel << ADC_CONFIG_PSEL_Pos) | (ADC_CONFIG_EXTREFSEL_AnalogReference1 << ADC_CONFIG_EXTREFSEL_Pos);

NRF_ADC->TASKS_START = 1; while (((NRF_ADC->BUSY & ADC_BUSY_BUSY_Msk) >> ADC_BUSY_BUSY_Pos) == ADC_BUSY_BUSY_Busy) ; value = NRF_ADC->RESULT;

It seems to work, but maybe it's an illusion... So the question is: actually what is supposed to happen exactly if I use an external reference voltage lower than 0.83V?

Regards, P.G.

[Pablo Gindel]

...and a 12bit ADC too! That's pretty cool, but for now I'm "married" to the nrf51822. Going back to the question at the beginning, what I would like to know is in what internal characteristic of the ADC that limitation of 0.83V is based... why is it 0.83V and not 1V or 0.5V, for example?

[MartinBL]

The word from the ADC architects is that:

"0.83V is the limit used in corner simulations of the ADC, outside that value there are no guarantees that the ADC will be operational. Outside this range the linearity will suffer."