Patent ID: 8044710

Claim:
A filter cut-off frequency correction circuit for correcting a cut-off frequency of a filter, the cut-off frequency correction circuit being inputted with a step function increasing from a first voltage to a second voltage, the cut-off frequency correction circuit comprising: a linear passive filter comprising a resistor and a capacitor, the linear passive filter integrating the step function to obtain a third voltage; a first comparator having one terminal connected to an output end of the linear passive filter for receiving the third voltage and another terminal inputted with a first predetermined reference voltage, the first comparator comparing the third voltage and the first predetermined reference voltage, the first comparator outputting a first high-level signal when the third voltage is greater than the first predetermined reference voltage; a second comparator coupled with the linear passive filter and the first comparator, for outputting a second high-level signal in a first period from the time that the second voltage is applied to the linear passive filter to the time that the first comparator outputs the first high-level signal; a counter having a first input end connected to an output end of the second comparator for receiving the second high-level signal and a second input end inputted with a reference clock, the counter counting a number of clock pulses of the reference clock inputted during the first period; a digital block for calculating a variation rate of time constant of the linear passive filter according to the number of clock pulses of the reference clock calculated by the counter to generate a correction code for correcting the variation rate of time constant, in which the correction code is outputted to the filter for correcting its cut-off frequency.