Hi,
One should always place an analog filter in front of an ADC, acting as anti aliasing filter.
It should attenuate at least all frequencies above f_sample/2. This is above the "frequency of interest",
and makes the digital values clean and reliable.
On the digital side hen you are free to design any filter you want.
Digital filters are
* free (no part cost)
* stable, don't suffer from thermal drift (frequency, DC offset)
* accurate, don't suffer from part tolerances
* predictable
* flexible in type and order
* simple to modify (configuration, software update), no soldering
Klaus