switchedcapacitor

Switched-capacitor circuits are electronic networks that use capacitors together with controlled switches to perform sampling and transfer of charge, effectively implementing resistive and other continuous-time functions in discrete time. In a typical switched-capacitor resistor, a capacitor is alternately connected to an input node and to a reference node by non-overlapping clock phases.

Over each clock period, the capacitor charges to the input voltage during one phase and transfers its charge to the next node during the other phase. The average current through the network behaves as if a resistor with value R = 1/(f_clk · C) were present, where f_clk is the switching frequency and C is the capacitor value. This allows precise, temperature-stable resistance values that can be integrated on silicon.

In switched-capacitor active circuits, such as switched-capacitor op-amp integrators and switched-capacitor filters, the switched-capacitor network replaces resistors in feedback paths, yielding programmable or highly accurate time constants independent of process variations. Common implementations use CMOS switches (transmission gates) and a capacitor array driven by two non-overlapping clocks.

Applications include high-precision analog filters in integrated circuits, sample-and-hold stages, programmable gain stages, and ADC/DAC front ends. They are especially favored in integration where large resistors would take too much area or drift with temperature.

Advantages include high matching accuracy, ease of integration, and clock-controlled resistance that is largely independent of absolute resistor values. Limitations involve nonidealities such as clock feedthrough, charge injection, jitter, leakage, finite op-amp bandwidth, and the need for a stable clock.