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In practice, using the internal fixed voltage reference sets the upper range of the ADC to a fixed value. That value is 4.3V with input scaling enabled and ~1.4785V with input scaling disabled. In this mode, the output is a function of the input voltage multiplied by the resolution in alternatives (not bits) divided by the upper voltage range of the ADC. Output = Input (V) * 2^12 / (ADC range (V))
Using VDDS as a reference scales the upper range of the ADC with the battery voltage. As the battery depletes and its voltage drops, so does the range of the ADC. This is helpful when measuring signals that are generated relative to the battery voltage. In this mode, the output is a function of the input voltage multiplied by the resolution in alternatives (not bits) divided by VDDS multiplied by a scaling factor derived from the input scaling. Output = Input (V) * 2^12 / (VDDS (V) * Scaling factor), where the scaling factor is ~1.4785/4.3 for input scaling disabled and 1 for input scaling enabled.
