The effective noise free resolution is 18-bits. Valid data starts from bit 17 and end at 3-0 depending on the resolution configuration, data found in bit locations higher than 17 can seem to represent valid data but are more susceptible to noise and will lead to erroneous data collection that would need to be corrected.
You should configure the ADC resolution according to table 5 so that you can read the full range of current you expect the photodiode to produce in the first 15-18 bits.
