ABSTRACT
The need to have monolithic analog filters motivated circuit designers in the late 1970s to investigate alternatives to conventional active-RC filters. A practical alternative appeared: switched-capacitors (SC) filters [1]-[3]. The original idea was to replace a resistor by a switched capacitor 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 switched-capacitor 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. 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.
