Patent ID: 7619465

Claim:
A filter circuit comprising: a low-pass filter; and a calibration circuit calibrating a frequency characteristic of the low-pass filter, wherein the calibration circuit includes: a feedback circuit which forms an oscillation circuit with the low-pass filter when the filter circuit is in a calibration mode, the oscillation circuit being formed by a negative feedback loop having a gain larger than one; and a control circuit which controls the frequency characteristic of the low-pass filter so as to make an oscillation frequency of the oscillation circuit falls within a predetermined range, when the filter circuit is in the calibration mode, in which the feedback circuit comprises: a comparator generating a digital output signal from an output signal of the low-pass filter; a buffer generating a feedback signal from the digital output signal of the comparator; and a switch circuit which selects an input signal to output the input signal as an input signal of the low-pass filter when the filter circuit is in a normal mode, and selects the feedback signal to output the feedback signal as the input signal of the low-pass filter when the filter circuit is in the calibration mode, and the control circuit comprises: a counter which performs a count operation during a predetermined period which is set based on a period of the digital output signal of the comparator, when the filter circuit is in the calibration mode; and an adjustment circuit which compares a count value of the counter and a reference value to adjust the frequency characteristic of the low-pass filter according to a result of the comparison, when the filter circuit is in the calibration mode.