Yes, of cource you need!
The AAF is used to remove outside band interference. Which makes the input to the modulator a band limited signal. As you know sampling means frequency spectrum perodic occur at the integer times of sampling frequency. Alias occur if there isn't a AAF.

hanm

Hi min,

You should remember that the modulator will react to signals both inband and out-of-band. Since you are interested in inband signals, it makes
sense to prefilter the input to minimize energy outside your band of interest.

The NTF poles lie outside the signal band of interest and if the input to the modulator contains significant power at frequencies close to the NTF poles,
this can excite it and induce oscillations, which are worse if high-Q poles are used in the NTF. Hence, the need for prefiltering.

For more details, please see Chapter 5 by Bob Adams in the book by Norsworthy, Schreier and Temes.

Regards
Vivek