It is possible to turn on the VBIAS and at the same time use this pint as a reference in negative AIN to perform measurement referenced to VDDreg/2?
Just like that
pADI_ADC0->CON = (pADI_ADC0->CON & ~ADC0CON_ADCCP_MSK & ~ADC0CON_ADCCN_MSK) | ADC0CON_ADCCP_AIN10 | ADC0CON_ADCCN_AIN11;
/* ON Vbias */ pADI_ADC0->ADCCFG = ADCCFG_PINSEL_AIN11;
Not sure if it can work well due to the limited drive capability. You can have a try.
However I don't suggest to use this way. Because we always hope that the reference has good drift and precision…