2009 by Taylor & Francis Group, LLC. The need to have monolithic high-performance analog filters motivated circuit designers in the late 1970s to investigate alternatives to conventional active-RC filters. A practical alternative appeared in the form of switched-capacitor (SC) filters [13]. The original idea was to replace a resistor by an SC simulating the resistor. Thus, this equivalent resistor could be implemented with a capacitor and two switches operating with two-clock phases. SC filters consist of switches, capacitors, and op-amps. They are characterized by difference equations in contrast to differential equations for continuous-time filters. Simultaneously, the mathematical operator to handle sample-data systems such as SC circuits is the z-transform, while the Laplace transform is used for continuous-time circuits. Several key properties of SC circuits have made them very popular in industrial environments: 1. Time constants (RC products) from active-RC filters become capacitor ratios multiplied by the clock period T. That is [Formula] where fc is the clock frequency used to drive the SC equivalent resistor. 2. Reduced silicon area, since the equivalent of large resistors can be simulated using small-size capacitors. Furthermore, positive and=or negative equivalent resistors can be easily implemented with SC techniques. 3. Above expression can be realized in real applications with a good accuracy of nearly 0.1%. 4. Typically, the load of an SC circuit is mainly capacitive; therefore the required op-amps do not require a low-impedance output stage. This allows the use of unbuffered cascode operational transconductance amplifiers, which is especially useful in high-speed applications. 5. SC filters can be implemented in digital circuit process technologies; metalmetal capacitors are quite often used in deep submicron technologies. Thus, useful mixed-mode signal circuits can be economically realized. 6. SC design technique has matured. In the audio range, SC design techniques have become the dominant design approach. Many circuits in communication applications and data converters use SC implementations. In what follows, we will discuss basic building blocks involved in low- and high-order filters. Limitations and practical design considerations will be presented. Furthermore, due to the industrial push for lower power supply voltages, a brief discussion on low-voltage circuit design is included.
