Intuitive Approach to Active Digital Filter Design. Part II: Principle of Higher-order Low-pass Filters

In digital signal processing (DSP), digital filtering is a crucial signal processing task used to remove unwanted frequencies and noise, enhance signal quality, and increase the signal-to-noise ratio of processed signals. In Part I, the principle of integrator-based closed-loop filters, namely active digital filters, was described in detail with examples of first-order low-pass, high-pass, band-pass, and band-rejection filters. The approach is based on feedback control theory and uses digital integrators to build stable negative feedback digital signal processing systems. This paper continues to reveal the design of second and higher-order active low-pass filters and demonstrates how easy it is to implement various filtering functions such as Bessel, Butterworth, and Chebyshev in closed-loop architectures. The frequency characteristics of the presented filters fully overlap with their analog Sallen-Key equivalents. Finally, an application of active digital filters for ECG processing is disclosed.