How to improve this circuit design to meet Arduino ADC input impedance requirement?
How about making R3 10x smaller, and C2 10X larger?
The opa333 has poor settling time; the ADC sampling_charge demand will initiate another settling behavior, and the datasheet shows about 50 microseconds is needed for 100pF connected directly on the Vout pin.
The passive R+C low pass filter is a different matter. The resistor lowers the instantaneous peak output current, causing less of a transient inside the OPA333.
experiment with that R+C filter.
If not satisfactory, and a larger VDD bypass does not help, then as FASTER opamp may be the solution.