Switched capacitor
Switched capacitor filter or SC filter are called (English for switched capacitor filters ) electronic filters which have instead of the ohmic resistors are connected in their circuit configuration capacitors. It is discrete-time filter.
Conventional active analog filters as the low-pass or high-pass filter composed of operational amplifiers as active devices and most capacitors and electrical resistances as external frequency determining circuitry. To change the filter parameters such as the cutoff frequency of these external passive components need to be changed in their values , which can be very expensive depending on the circuit. They should use the switched-capacitor filter, which also consist of operational amplifiers as active circuit element, but have the external components are replaced by switched capacitors. The switching frequency determines the cut-off frequency of the filter. This can be only with analog filter capacitors build, which very easily can be accommodated on integrated circuits as in FPAAs and their parameters such as the cutoff frequency are easily changeable. Depending on the method of connection of the switched capacitors can be used in this technique, both high-pass filters ( differentiators ) and low-pass filters ( integrators ) to build.
By varying the switching frequencies at which the capacitors are switched, the filter parameters of the SC filter are very easy to change. The replacement of the ohmic resistors R in a given circuit, such as a low pass through capacitors CS, which are operated with the switching frequency fs can be calculated according to the following equation:
Because it is sensing systems of this type of filter, the switching frequency fs needs to by a factor 50 to 100 greater than the cut-off frequency of the filter to be in order to avoid aliasing. Therefore, SC- filters are usually only for low-frequency applications in question and require, when required, subsequent low-pass filters which filter out the high frequency signal components of the switching frequency. This is one of the disadvantages of this circuit technology.
A significant advantage of this filter is that the filter time constant only slightly dependent on the component tolerances of capacitors used. It need only the ratios of the component values remain constant to each other, since the filtering accuracy is primarily determined by the switching frequency. Accurate switching frequencies can be technically easy to ensure by quartz oscillator circuits.
Literature sources
- Ulrich Tietze, Christoph Schenk: semiconductor circuit technology, Berlin: Springer 2002, ISBN 3-540-42849-6.