Are Your Filters Filtering?

摘要

Why is my A/D converter's anti-aliasing filter showing inadequate spurious and noise rejection?rnIt is important to realize that an A/D converter's internal front-end bandwidth, sometimes referred to as full power bandwidth, can be very wide, even if the A/D converter is "slow." The converter needs bandwidth on the internal front-end in order to settle in time for the next sample, preserving the information the designer is trying to capture and represent digitally.rnAn anti-aliasing filter (AAF) typically precedes the A/D converter and can vary from a simple single-pole RC network to complicated multi-pole topology. In either case the idea is the same, to remove unwanted noise and spurs that can fold back, or alias, into the frequency band of interest. Designers should use caution when building or using an AAF. It is important to understand not only the band of interest (pass band), but the out-of-band (stop-band) rejection of the filter design.