The SAR intenral ADC can take voltage reference as internal Vref, External Vref,VDDA and VDDA/2. The reference option should be from any of these. Regarding SARADC monitoring the supply voltage, there is an example code already available to monitor the battery level( or supply voltage) without external components. The project uses only one external connection to the ADC input. In PSoC Creator(File-> Example Code) find BLE_Battery_Level example.
Thanks for the suggestion.