Patent ID: 8604955

Claim:
A filter comprising: a sigma-delta modulator configured to sigma-delta modulate an input signal corresponding to a desired cut-off frequency to output a modulated signal; and a fully-differential active filter including, a differential amplifier having a positive-polarity input terminal, a negative-polarity input terminal, a positive-polarity output terminal and a negative-polarity output terminal, and configured to amplify an input differential signal being input to the positive-polarity input terminal and the negative-polarity input terminal to output an amplified output differential signal from the positive-polarity output terminal and the negative-polarity output terminal, a first variable resistor or a first variable capacitor provided between the positive-polarity output terminal and the positive-polarity input terminal, a second variable resistor or a second variable capacitor provided between the positive-polarity output terminal and the negative-polarity input terminal, a third variable resistor or a third variable capacitor provided between the negative-polarity output terminal and the negative-polarity input terminal, and a fourth variable resistor or a fourth variable capacitor provided between the negative-polarity output terminal and the positive-polarity input terminal, values of the first to fourth variable resistors or the first to fourth variable capacitors being set in accordance with the modulated signal output from the sigma-delta modulator so that the fully-differential active filter has the desired cut-off frequency.