When using the internal voltage reference with the ADC, the accuracy of this reference must be considered. The internal voltage reference is proportional to the bandgap voltage, which is characterized in the devices’ datasheets. The characteristics show that the bandgap voltage is slightly dependent on supply voltage and operating temperature.
The ADC accuracy also depends on the ADC clock. The recommended maximum ADC clock frequency is limited by the internal DAC in the conversion circuitry. For optimum performance, the ADC clock should not exceed 200kHz. However, frequencies up to 1MHz do not reduce the ADC resolution significantly.
Operating the ADC with frequencies greater than 1MHz is not characterized.