Filter Basics: Anti-Aliasing - AN928

In a sampled data system, frequency components greater than half the sampling rate "alias" (shift) into the frequency band of interest. Most of the time, aliasing in an undesirable side effect, so the "undersampled" higher frequencies are simply filtered out before the A/D stage. But sometimes, the undersampling is deliberate and the aliasing causes the A/D system to function as a mixer. This application note discusses the different filtering requirements for a sampled data system. It describes aliasing and the types of filters that can be used for anti-aliasing. Filtering is such a common process that we often take it for granted. When we make a cell phone call, the receiver filters out all other channels so we only receive our unique channel. When we adjust the equalizer on a stereo system, we are selectively increasing or decreasing the audio signal in a particular frequency band, using a bandpass filter. Filters play a key role in virtually all sampled data systems. Most A/D converters (ADCs) are preceded by a filter which removes frequency components that are beyond the ADC's range. Some ADCs have filtering inherent in their topology. Let's take a look at a sampled data system, the filtering requirements, and the relationship to aliasing.