Low-pass filter and voltage-current conversion circuit used in the same
View Patent ↗A low-pass filter with a high signal-to-noise ratio is provided. A low-pass filter includes a first RC filter circuit, a differential operation circuit subtracting an output signal from a low frequency component of an input signal and outputting a differential signal, a second RC filter circuit, a voltage-current conversion circuit, and a capacitor. The voltage-current conversion circuit includes an operational amplifier, a first resistor, and a feedback circuit generating a feedback voltage in accordance with a voltage applied to the first resistor. If resistance values of first to fourth resistors are R 1 , R 2 , R 3 , R 4 , a relational expression of R 1 *R 3 =R 2 *R 4 is satisfied, and output current i becomes constant regardless of a resistance value of a load resistance. Since only a first resistor is connected between the input and the output of the voltage-current conversion circuit, thermal noise decreases, and a signal-to-noise ratio becomes high.
1. A low-pass filter attenuating a high frequency component of an input signal and outputting a low frequency component of the input signal as an output signal, comprising:
a differential operation circuit subtracting the output signal from the input signal and outputting a differential signal;
a voltage-current conversion circuit converting a voltage of the differential signal outputted from the differential operation circuit into a current; and
a capacitance accumulating charges in a current received from the voltage-current conversion circuit to generate the output signal,
wherein the voltage-current conversion circuit including:
a first operational amplifier having a noninverting input terminal connected to an input terminal of the voltage-current conversion circuit and an inverting input terminal receiving a feedback voltage;
a first resistor connected between an output terminal of the voltage-current conversion circuit and an output terminal of the first operational amplifier; and
a feedback unit for generating the feedback voltage in accordance with a voltage applied to the first resistor.
2. The lowpass filter according to claim 1 , wherein the feedback unit includes:
a second operational amplifier having an inverting input terminal connected to the output terminal of the voltage-current conversion circuit and an output terminal connected to the inverting input terminal of the first operational amplifier;
a second resistor connected between the output terminal of the first operational amplifier and a noninverting input terminal of the second operational amplifier;
a third resistor connected between the noninverting input terminal of the second operational amplifier and a ground; and
a fourth resistor connected between the output terminal of the second operational amplifier and the inverting input terminal of the second operational amplifier.
3. The lowpass filter according to claim 2 ,
wherein resistance values R 1 to R 4 of the first to fourth resistors are set so as to satisfy a relational expression of R 1 *R 3 =R 2 *R 4 .
4. The lowpass filter according to claims 1 , further comprising:
a first filter circuit attenuating a high frequency component of the differential signal outputted from the differential operation circuit, and outputting a low frequency component of the differential signal to the voltage-current conversion circuit.
5. The lowpass filter according to claims 1 , further comprising:
a second filter circuit attenuating a high frequency component of the input signal and outputting a low frequency component of the input signal to the differential operation circuit.
6. A voltage-current conversion circuit converting a voltage into a current, comprising:
a first operational amplifier having a noninverting input terminal connected to an input terminal of the voltage-current conversion circuit and an inverting input terminal receiving a feedback voltage;
a first resistor connected between an output terminal of the voltage-current conversion circuit and an output terminal of the first operational amplifier; and
a feedback unit for generating the feedback voltage in accordance with a voltage applied to the first resistor,
wherein the feedback unit includes a second operational amplifier having an inverting input terminal connected to the output terminal of the voltage-current conversion circuit and an output terminal connected to the inverting input terminal of the first operational amplifier;
a second resistor connected between the output terminal of the first operational amplifier and a noninverting input terminal of the second operational amplifier;
a third resistor connected between the noninverting input terminal of the second operational amplifier and a ground; and
a fourth resistor connected between the output terminal of the second operational amplifier and the inverting input terminal of the second operational amplifier.
7. The voltagecurrent conversion circuit according to claim 6 ,
wherein resistance values R 1 to R 4 of the first to fourth resistors are set so as to satisfy a relational expression of R 1 *R 3 =R 2 *R 4 .