Patent ID: 7239197

Claim:
A Gm-C time constant tuning circuit for tuning a Gm-C time constant of a Gm-C filter, wherein the Gm-C filter comprises a set of conductors and a set of capacitors, the tuning circuit comprising: a reference conductor for receiving a plurality of reference voltages and a control signal and outputting two current signals according to the voltage signal and the control signal; a reference capacitor cross-over coupled in between two output terminals of the reference conductor; a comparator comprising two input terminals coupled to the output terminals of the reference conductor respectively, the output terminal of the comparator outputting a first reference signal; a charged pump coupled to the comparator for outputting a reference current according to the first reference signal or a second reference signal; a feedback capacitor coupled in between the charged pump and the reference conductor for generating the control signal according to the reference current and feeding the control signal back to the reference conductor; wherein the tuning circuit utilizes the control signal to adjust a conductance of the reference conductor; a first switch, a terminal of the first switch receiving a first reference voltage, another terminal of the first switch coupled to a first input terminal of the reference conductor; a second switch, a terminal of the second switch receiving the first reference voltage, another terminal of the second switch coupled to a second input terminal of the reference conductor; a third switch, a terminal of the third switch receiving a second reference voltage, another terminal of the third switch coupled to the first input terminal of the reference conductor; a fourth switch, a terminal of the fourth switch receiving a third reference voltage, another terminal of the fourth switch coupled the second input terminal of the reference conductor; a fifth switch, a terminal of the fifth switch receiving a fourth reference voltage, another terminal of the fifth switch coupled a first input terminal of the comparator; and a sixth switch, a terminal of the sixth switch receiving a fifth reference voltage, another terminal of the sixth switch coupled a second input terminal of the comparator; wherein the first switch, the second switch, the third switch, and the fourth switch are controlled to select whether the input terminal of the reference conductor is coupled to the reference voltage according to a clock signal, wherein the fifth switch and the sixth switch are controlled to select whether the output terminal of the reference conductor is coupled to the fourth reference voltage or the fifth voltage according to the clock signal.