I/O-pins (AINx), as well as the VREFA/VREFB reference voltage pins are analog inputs to the ADC. Any internal reference source, such as a bandgap voltage reference, or DAC must be configured and enabled prior to its use with the ADC.
The analog signals of AC, ADC, DAC and OPAMP can be interconnected. The AC and ADC peripheral can request the OPAMP using an analog ONDEMAND functionality.
See Analog Connections of Peripherals for details.