Can we have a Digital Anti Aliasing filter?
I agree with pichenettes's answer but I would like to add that it is pretty common practice to use a simple inexpensive low-order analog anti-aliasing filter, and do the rest of the anti-aliasing filtering in the digital domain. This implies of course that you do not process at maximum sampling rate but that you downsample after the digital anti-aliasing filter. Summarizing:


Of course you need an analog anti-aliasing filter.
The analog filter can be kept very simple if you can downsample your signal. In this case you can do more aliasing removal in the digital domain (before downsampling).