The Charge Pump Oscillator Selection (CPOS) bit selects the charge pump oscillator source. The CPOS bit allows the user to select between the charge pump’s internal oscillator or the oscillator driving the ADC.
When CPOS is set (CPOS = 1), the charge pump utilizes its internal
oscillator. The charge pump’s internal oscillator provides a very steady output voltage,
but at the expense of higher operating current.
When CPOS is clear (CPOS = 0), and the ADGO bit is clear (GO =
0), the charge pump is clock by the ADCRC. When ADGO is set (GO =
1), the charge pump is clocked by a derivative of the
FOSC (as determined by the ADCLK register). This allows the charge pump
to operate at a lower current when the ADC is not converting, while offering higher
performance when the ADC is converting.