Analog Integrated Circuit Design, 2nd Edition

Integrated analog filters can be realized in either discrete time or continuous time. Discrete-time filters and signal processing are addressed in Chapter 13 and their implementation, switched-capacitor circuits, are discussed in Chapter 14. In switched-capacitor circuits, although the signals remain continuous in voltage (i.e., they are never quantized) they require sampling in the time domain. Because of this time-domain sampling, the clock rate must always be at least twice that of the highest frequency being processed to eliminate aliasing. In fact, typically the clock rate is much greater than twice the signal bandwidth, to reduce the requirements of an anti-aliasing filter. As a result, switched-capacitor filters are limited in their ability to process high-frequency signals.

This chapter focuses on continuous-time filtering. As their name suggests, continuous-time filters have signals that remain continuous in time and that have analog signal levels. Since no sampling is required, continuous-time filters have a significant speed advantage over their switched-capacitor counterparts. However, continuous-time filters do have some disadvantages. One is their need for tuning circuitry. Tuning is required because their filter coefficients are determined by the product of two dissimilar elements, such as capacitance and resistance (or transconductance) values. Thus, whereas switched-capacitor ...