Perhaps the most popular approach for realizing analog signal processing in MOS (or BiCMOS) integrated circuits is through the use of switched-capacitor circuits [Poschenrieder, 1966; Fried, 1972; Caves, 1977; Hosticka, 1977; Young, 1977]. A switched-capacitor circuit operates as a discrete-time signal processor (although without the use of A/D or D/A converters). As a result, these circuits are most easily analyzed with the use of
z-transform techniques and typically require anti-aliasing and smoothing filters when combined with continuous-time circuits.
As a filtering technique, switched-capacitor circuits are popular due to their accurate frequency response as well as good linearity and dynamic range. Accurate discrete-time frequency responses are obtained since filter coefficients are determined by capacitance ratios which can be set quite precisely in an integrated circuit (on the order of 0.1 percent). Such an accuracy is orders of magnitude better than that which occurs for integrated RC time constants (which can vary by as much as 20 percent). Once the coefficients of a switched-capacitor discrete-time filter are accurately determined, its overall frequency response remains a function of the clock (or sampling) frequency. Fortunately, clock frequencies can also be set very precisely through the use of a crystal oscillator.
In addition to creating filtering functions, switched-capacitor ...