The low-pass filter and the voltage electric current converting circuit which is used for that

摘要

A low-pass filter with a high signal-to-noise ratio is provided. A low-pass filter includes a first RC filter circuit (12), a differential operation circuit (14) subtracting an output signal (OUT) from a low frequency component of an input signal and outputting a differential signal (DF), a second RC filter circuit (16), a voltage-current conversion circuit (18), and a capacitor (20). The voltage-current conversion circuit (18) includes an operational amplifier (181), a first resistor (R 1 ), and a feedback circuit (183) generating a feedback voltage (FB) in accordance with a voltage applied to the first resistor (R 1 ). If resistance values of first to fourth resistors are R1, R2, R3, R4, a relational expression of R1*R3=R2*R4 is satisfied, and output current i becomes constant regardless of a resistance value of a load resistance (Z). The feedback circuit comprises a second operational amplifier (184) whose inverting and non-inverting inputs are connected at the extremities of the first resistor (R 1 ). Resistor (R 2 ) is connected between (R 1 ) and the non-inverting input of the second operational amplifier (184). Resistor (R 3 ) is connected between the non-inverting input of the second operational amplifier (184) and the ground. Resistor (R 4 ) is connected between the inverting input of the second operational amplifier (184) and its output. Since only a first resistor (R 1 ) is connected between the input and the output (OUT) of the voltage-current conversion circuit (18), thermal noise decreases, and a signal-to-noise ratio becomes high.