Amplifier Settling Time and ADC Resolution
The ADC likely grabs a bunch of charge as it begins the conversion. That charge is chopped up and compared against other internally generated charges, during the binary-approximation process.

Some OpAmps tolerate that initial "grabs a bunch of charge" and some opamps do not. Adding a RC LowPassFilter between OpAmp and ADC provides the sudden charge demands to the ADC.

The ADC probably has 10pF input sampling capacitor. Or maybe 5pF. An external capacitor of 1,000pF provides room for 100:1 settling, immediately. You'll need to wait a few more Tau for the ADC voltage to reach 10 bits.

An RC using 1Kohm and 1,000pF should be an easy load for the OpAmp, with 1uS Tau.

Is your chosen opamp fast enough to recharge the 1,000pF quickly?