Point of ADC saturation really depends on your reference voltage and your chosen prescaling. If you use the 1.2V VBG internal reference voltage the ADC range will be 0-1.2V with a saturation of 1.2V. This means that your AIN signal with 1/1 prescaling should be in the range 0-1.2V in order to obtain proper conversion. Input above 1.2V will just be converted to the maximum ADC value. However if you use for example 1/3 prescaling for your AIN input the input is scaled down by 1/3. The effect is that your AIN voltage range is 0-3.6V, because the 3.6V input voltage is scaled down to 3.6/3=1.2V.
Examples of saturation points for ADC AIN inputs are:
1.2VBG reference, AIN 1/1 prescaling: AIN 1.2V
1.2VBG reference, AIN 2/3 prescaling: AIN 1.8V
1.2VBG reference, AIN 1/3 prescaling: AIN 3.6V
1.0V AREF, AIN 1/1 prescaling: AIN 1.0V
1.0V AREF, AIN 2/3 prescaling: AIN 1.5V
1.0V AREF, AIN 1/3 prescaling: AIN 3.0V
VDD is 3.0V, VDD 1/2 reference, AIN 1/1 prescaling: AIN 1.5V
According to the nRF51882 Product Specification, the input voltage on any IO Pin must not exceed VDD + 0.3V.
This means that ADC inputs must not exceed 2.1V if the nRF51822 is supplied by 1.8V, is this correct? I guess this is also true even if the ADC input is prescaled with 1/3 or 2/3 right?
According to the nRF51882 Product Specification, the input voltage on any IO Pin must not exceed VDD + 0.3V.
This means that ADC inputs must not exceed 2.1V if the nRF51822 is supplied by 1.8V, is this correct? I guess this is also true even if the ADC input is prescaled with 1/3 or 2/3 right?
There are basically two rules to follow when deciding max voltage on AIN pins:
The ADC should not be exposed to higher voltage than 2.4V on an input pin AIN after prescaling
GPIO pin should not be exposed to higher voltage than VDD+0.3V, see Aboslute maximum ratings in the nRF51822 PS
For example, when having 2/3 prescaling you can expose 2.4V/(2/3)=3.6V to an AIN pin. For not to violate rule 2, VDD should be 3.3V or higher in this case.
These are the maximum voltages that can be exposed to an ADC AIN pins, depending on supply voltage and your prescale settings:
VDD 3.6, prescaling 1/1: AIN max 2.4V (rule 1 limitation)
VDD 3.6, prescaling 2/3: AIN max 3.6V (rule 1 limitation)
VDD 3.6, prescaling 1/3: AIN max 3.9V (rule 2 limitation)
VDD 3.3, prescaling 1/1: AIN max 2.4V (rule 1 limitation)
VDD 3.3, prescaling 2/3: AIN max 3.6V (rule 1 and rule 2 limitation)
VDD 3.3, prescaling 1/3: AIN max 3.6V (rule 2 limitation)
VDD 1.8, prescaling 1/1: AIN max 2.1V (rule 2 limitation)
VDD 1.8, prescaling 2/3: AIN max 2.1V (rule 2 limitation)
VDD 1.8, prescaling 1/3: AIN max 2.1V (rule 2 limitation)
