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Digital Signal Processing
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Digital Signal Processing |
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Digital Filters
In preparation for acquiring data in digital form, the analog signal being measured is typically passed through an analog filter to ensure that all components of the signal, and noise with frequencies corresponding to a half, or more, of the digital sampling rate of the analog-to-digital converter (ADC), are removed. As described earlier , this helps ensure that false lower-frequency signals -- called aliases -- are not introduced into the digital data by the sampling process itself. The sampling rate of the ADC is typically much higher than that required to extract the necessary information from the signal within the frequency range of interest. In addition to allowing unwanted higher frequency components (noise) to remain in the data, these higher sampling rates will also increase data storage requirements and analysis time. In order to acquire data at a lower rate while avoiding aliasing errors, it would be necessary to make physical changes to the analog anti-aliasing filter and slow down the sampling rate of the ADC. The disadvantage of this approach is that different analog filter components are required for each sampling rate. A more practical solution is to leave the analog filter and ADC sampling rate unchanged and to mathematically eliminate any unwanted components from the measured signal by passing the digital data through a digital filter. Specifically, low-pass digital filters enable the digital data coming from the ADC to be "decimated." Instead of the software processing and storing data from every analog-to-digital conversion, the digital filter allows data to be sampled at intervals corresponding to every n th conversion, effectively reducing the sampling rate without introducing aliases. And, at the same time, any unwanted higher frequency components of the measured signal are eliminated from the digital data as well. (continued...) 
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