Unlike an Op-Amp with high gain and error feedback sensitive somewhat to supply noise, this is a Track & Hold MUX ADC which also sensitive to supply, Va , the analog & reference voltage.
- good question, unusual result
I would estimate the PSRR as 0 dB for all Vref and Agnd supply signals on SAR ADC's.
They indicate \$V_A\$ ...
This pin should be connected to a quiet 2.7 V to 5.25 V source and bypassed to GND with 1-µF and 0.1-µF monolithic ceramic capacitors located within 1 cm of the power pin.
I suspect you will want to analyze Va with an AC coupled 50 Ohm Spectrum Analyzer from a pair of test points and define your spec for this noise.
I recall my 1st ADC (late 70's) was an X-RAY inspected MIL-STD 883B Burr Brown 12 bit ADC that had missing codes at boundaries of xxxxxx111111. I ended up having to use the industrial part. At that cost, difference, being a newbie, I was surprised, not thinking about buyer/supplier feedback and getting the supplier to fix the problem < I did it my way to save time.
Now that was an metal cased ceramic hybrid with an internal problem. I surmised it was due to internal logic current offset Vref or gnd shift of a couple bits. It was corrected by luck, but that was an internal crosstalk or ground shift or Vref PSRR problem.