The effect varies on the internal reference that you've chosen for the ADC in following way:
1. When a reference based on Vdd is selected (Vdd/2 +/- Vdd/2, Vdd/2 +/- Vbg), then the internal reference will be directly affected by the ripple on Vdd.
2. The bandgap voltage is actually generated by a bandgap diode of 1.2V and an amplifier to generate 1.3V from this bandgap diode. This op-amp has a high PSRR because of which the ripple will have very negligible effect on the bandgap output. So, if the reference is based on the bandgap voltage (2Vbg +/- Vbg, 1.6Vbg +/- 1.6Vbg etc), the effect of ripple on the reference would be negligible.