The statement in datasheet "which eliminates all digital switching noise from the conversion" cannot be true, not least because of the rc oscillator itself.

The idea of this sleep mode is to avoid togglings at the port lines. If there is considerable load and stray capacitance present, charging currents can be enormously high, considerably eroding the precision of ADC caused by ground bounce on die and other interference. The internal timers and other stuff, which are also turned-off during sleep mode usually, also produce lots of insane noise.

What promises your ADC keeps, you will see, when it's finally working in your application...

Kai