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A switched capacitor is an electronic circuit element used for discrete time signal processing. It works by moving charges into and out of capacitors when switches are opened and closed. Usually, non-overlapping signals are used to control the switches, so that not all switches are closed simultaneously. Filters implemented with these elements are termed "switched-capacitor filters," and depend only on the ratios between capacitances. This makes them much more suitable for use within integrated circuits, where accurately specified resistors and capacitors are not economical to construct.〔(''Switched Capacitor Circuits'' ), Swarthmore College course notes, accessed 2009-05-02〕 == The switched capacitor resistor == The simplest switched capacitor (SC) circuit is the switched capacitor resistor, made of one capacitor C and two switches S and S which connect the capacitor with a given frequency alternately to the input and output of the SC. Each switching cycle transfers a charge from the input to the output at the switching frequency . Recall that the charge ''q'' on a capacitor ''C'' with a voltage ''V'' between the plates is given by: : where ''V'' is the voltage across the capacitor. Therefore, when S is closed while S is open, the charge stored in the capacitor C is: : When S is closed, some of that charge is transferred out of the capacitor, after which the charge that remains in capacitor C is: : Thus, the charge moved out of the capacitor to the output is: : Because this charge ''q'' is transferred at a rate ''f'', the rate of transfer of charge per unit time is: : Note that we use ''I'', the symbol for electric current, for this quantity. This is to demonstrate that a continuous transfer of charge from one node to another is equivalent to a current. Substituting for ''q'' in the above, we have: : Let ''V'' be the voltage across the SC from input to output. So: : So the equivalent resistance ''R'' (i.e., the voltage–current relationship) is: : Thus, the SC behaves like a resistor whose value depends on capacitance ''C'' and switching frequency ''f''. The SC resistor is used as a replacement for simple resistors in integrated circuits because it is easier to fabricate reliably with a wide range of values. It also has the benefit that its value can be adjusted by changing the switching frequency (i.e., it is a programmable resistance). See also: operational amplifier applications. This same circuit can be used in discrete time systems (such as analog to digital converters) as a track and hold circuit. During the appropriate clock phase, the capacitor samples the analog voltage through switch one and in the second phase presents this held sampled value to an electronic circuit for processing. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「switched capacitor」の詳細全文を読む スポンサード リンク
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