Filtering noisy buffer output
BUF634 is the WRONG part to use. Use an op-amp and bias the input to approximately half of the ADC input voltage peak value (2.5 volts). At the moment you have a very powerful buffer chip on a single supply being fed an input that is approximately 0 volts. The BUF634 normally works with a split positive and negative supply so feeding it with an input that is nominally at one of the power rails is illegal and it may well produce a pile of output noise.

If you are trying to make a sizable attenuator it's highly likely that you need to swap the 10 kohm and 309 kohm resistors.