What is a Switched Capacitor?

A switched capacitor is an electronic component or, more correctly, an electronic circuit or module typically made up of a capacitor and two switches used to simulate other components in an integrated circuit (IC). The resistor is one of the components most commonly simulated; resistors tend to be far too large and inaccurate to incorporate into micro sized ICs. The switched capacitor module is commonly used in discrete time signal processing and voice frequency filtering applications. These functions are made possible by the unique characteristics of circuits where electric charges are alternately moved into and out of capacitors.

The ability of switched capacitor circuits to simulate a resistance in any give application has been extremely fortuitous for the electronics industry as it has allowed for the production of more complex integrated circuitry in smaller packages. Conventional resistors are particularly problematic in the IC arena due to their physical size and the differences in resistive values encountered in differing production runs. The capacitor and metal oxide semiconductor (MOS) switches used in a switched capacitor module are, on the other hand, extremely compact and very stable regarding their values and tolerances.

These characteristics make for extremely compact and accurate internal circuitry for microprocessors and integrated circuits. Another of the switched capacitor circuit advantages is the fact that the use of these modules, as opposed to conventional resistors, allow for circuit designers to incorporate a degree of frequency tuning in active filter applications. This tuning is achieved by varying the clock frequency or switching tempo of the circuit.

The actual value of the space saving characteristics of the switched capacitor module can be seen when one considers that a 1 MΩ resistance can be simulated with a tiny 10 pF capacitor switched at a 100 kHz clock rate. If a normal resistor were to be used in this application, the complete circuit would be many times the size of that employing the capacitor module. A low-pass switched capacitor filter with a 100 Hz rating, for instance, will require a resistance of 16 MΩ which would clearly be impossible to achieve using a normal resistor.

The advances in programmable analog IC technology seen in the past decade or so would not have been possible without the benefits gained from using switched capacitor modules. The considerable improvements in the field of multi-pole filter and analog-to-digital converter technology would not have been possible either, considering the physical bulk, non-linear, and inconsistent nature of conventional resistors. These points make the switched capacitor one of the most significant electronic component advances since the introduction of the transistor.