Analog subsystem: Analog subsystem is used to configure the flexible voltage references used by the analog modules i.e. ADC and DAC in the device. VREFHIx pin voltage can be driven in externally or can be generated by an internal bandgap voltage reference. The internal voltage reference range can be selected to be 0V to 3.3V or 0V to 2.5V
Ganged pins: Multiple VREFHI pins are ganged together in lower pin-count packages i.e. due to pin count restrictions of the device there is sharing of analog VREF pins among ADCs. In order to avoid potential pin contention between Internal vs External reference modes between ADCs, the reference voltage settings for the ganged pins should always be configured to the same setting. VREFHIB and VREFHIC are always ganged together for F28004x.
By default, an internally generated band gap voltage reference supplies the ADC logic. However, depending on application requirements, you can enable the external reference so that the ADC logic uses an external voltage reference instead.
The ADC logic uses internal bandgap voltage reference range to be 0V to 3.3V or 0V to 2.5V.