Analog MOS Integrated Circuits for Signal Processing

Chapter Five

SWITCHED-CAPACITOR FILTERS

The most common, and most difficult, application of switched-capacitor circuits is to perform the frequency-domain filtering of signals. This chapter deals with the design principles, building blocks, and actual circuits of such filters. The relative advantages and disadvantages of the two most useful filter realizations—cascade and ladder-type circuits—are described and illustrated by a typical design example. The important scaling process is also discussed in detail.

In special applications, very narrow-band switched-capacitor bandpass filters are required. In such a case, SC N-path filters may provide the only feasible realization. The design of these filters is hence also discussed. Finally, integrated MOS active-RC filters are described. These circuits are continuous-time filters, and hence do not require antialiasing or smoothing peripheral circuits; in fact, they can themselves be utilized for such tasks. In addition, they do not require clock signals and switches, and hence are free of some of the inherent shortcomings (clock feedthrough noise, noise aliasing, etc.) of switched-capacitor filters.

5.1. INTEGRATED FILTERS¹, ²

Probably the most important reason for the development of analog MOS integrated circuits was the need to fabricate fully integrated high-quality analog filters. Historically, such filters were first realized as ...