Switched capacitors are electronic circuits consisting of a capacitor and two switches used to simulate components in an integrated circuit. They are commonly used in signal processing and filtering applications due to their compact size and accuracy. They allow for more complex integrated circuits to be manufactured in smaller packages and can incorporate frequency tuning. The use of switched capacitors has led to significant advances in programmable analog integrated circuit technology.
A switched capacitor is an electronic component or, more correctly, an electronic circuit or module typically consisting of a capacitor and two switches used to simulate other components in an integrated circuit (IC). The resistor is one of the most commonly simulated components; resistors tend to be 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 the circuits in which electric charges are alternately moved into and out of the capacitors.
The ability of switched capacitor circuits to simulate a resistance in any given application was extremely serendipitous to the electronics industry as it allowed for more complex integrated circuits to be manufactured in smaller packages. Conventional resistors are particularly problematic in the integrated circuit arena due to their physical size and the differences in resistance values experienced across different manufacturing runs. On the other hand, the metal oxide semiconductor (MOS) capacitor switches used in a switched capacitor module are extremely compact and very stable with regard to their values and tolerances.
These features make the internal circuits extremely compact and accurate for microprocessors and integrated circuits. Another benefit of the switched capacitor circuit is that the use of these modules, as opposed to conventional resistors, allows circuit designers to incorporate a degree of frequency tuning into active filter applications. This tuning is accomplished by varying the clock frequency or switching time of the circuit.
The actual value of the small footprint characteristics of the switched capacitor module can be seen when one considers that a 1 MΩ resistor can be simulated with a tiny 10 pF capacitor switched at a 100 kHz clock frequency. If a regular resistor were to be used in this application, the complete circuit would be many times larger than the one using the capacitor module. A low pass switched capacitor filter with a power rating of 100Hz, for example, will require a resistance of 16MΩ which would clearly be impossible to achieve using a normal resistor.
The advances in programmable analog integrated circuit technology seen over the past decade or so would not have been possible without the benefits gained from using switched capacitor modules. The dramatic improvements in multipole filter and analog-to-digital converter technology would not have been possible, considering the physical size, non-linear and incoherent nature of conventional resistors. These points make the switched capacitor one of the most significant advances in electronic components since the introduction of the transistor.
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