1. Field of the Invention
The present invention relates to a filter system. More particularly, the present invention relates to a filter system that can be controlled digitally, i.e., without controlling a resistor or a capacitor.
2. Description of the Related Art
Analog low pass filters (LPFs) and high pass filters (HPFs) may be formed using a capacitor. Thus, a capacitance value and a resistance value may be changed i, so in order to regulate a cut off frequency (fc) of such filters. Accordingly, the analog LPFs and HPFs have disadvantages in terms of varying the fc and integration.
Further, an offset may be generated due to a level integral by the capacitor. Additionally, a large driving capability may be required in order to operate the LPFs or HPFs. Accordingly, the analog LPFs and HPFs may consume a large amount of current. Moreover, it may be difficult to control a passive device, e.g., a resistor or a capacitor forming a filter, to respond to a rapidly changing input signal.
Filters using a variable resistor, a plurality of switched capacitors and/or an externally regulated capacitor have been used to improve the range of fcs available. However, all of the other attendant disadvantages noted above regarding analog filters are still present in these arrangements.
To improve integration of the filter, a capacitor may be regulated through a single pad. To aid in compensating for offset, an output signal may be converted into a digital signal, DC components removed, and then converted back into an analog signal.
However, these filters still may have the following problems.
First, an operational-amplifier offset, generated during a subtraction using a passive device, may occur in an output.
Second, chip integration is too loaded with resistors and capacitors, which are passive devices. Also, the more fcs required, complexity of an array of a switch for selecting a passive device, e.g., a resistor or and a capacitor, increases.
Third, it is difficult to apply various fcs. That is, only limited fcs can be used due to the use of passive devices.
Fourth, an appropriate response to a rapid change in an input signal may not be possible. That is, a capacitor may react even to an abnormal input signal. Accordingly, an error may occur, and an adaptive response to a rapid change in an input signal may not be executed.
Fifth, an external pad, i.e., a pin, has to be used.