Filter circuit

1. A filter circuit that is provided at a previous stage of a differential input type Analog/Digital Converter to remove high-frequency components from an input signal, comprising:

a first filter circuit generating a positive-side output signal from the input signal to be inputted to a positive-side input terminal of the Analog/Digital Converter; and

a second filter circuit inverting the positive-side output signal to generate a negative-side output signal to be inputted to a negative-side input terminal of the Analog/Digital Converter; wherein

the first filter circuit includes,

a first operational amplifier;

a first resistor whose one end is connected to a non-inverting input terminal of the first operational amplifier and whose another end is connected to a positive-side of an input signal source;

a first capacitor whose one end is connected to the non-inverting input terminal of the first operational amplifier and whose another end is connected to a ground potential;

a second resistor whose one end is connected to the non-inverting input terminal of the first operational amplifier and whose another end is connected to a predetermined potential;

a third resistor whose one end is connected to an inverting input terminal of the first operational amplifier and whose another end is connected to a negative-side of the input signal source;

a second capacitor whose one end is connected to the inverting input terminal of the first operational amplifier and whose another end is connected to an output terminal of the first operational amplifier; and

a fourth resistor whose one end is connected to the inverting input terminal of the first operational amplifier and whose another end is connected to the positive-side input terminal of the Analog/Digital Converter,

wherein a resistance value of the first resistor and a resistance value of the third resistor are set to be equal, a capacitance value of the first capacitor and a capacitance value of the second capacitor are set to be equal, and a resistance value of the second resistor and a resistance value of the fourth resistor are set to be equal, by which the first operational amplifier, the first to fourth resistors and the first and second capacitors, which constitute the first filter circuit, function as a ground isolation circuit as well, and

the second filter circuit includes,

a second operational amplifier whose non-inverting input terminal is connected to the predetermined potential;

a fifth resistor whose one end is connected to the output terminal of the first operational amplifier and whose another end is connected to the positive-side input terminal of the Analog/Digital Converter;

a sixth resistor whose one end is connected to the another end of the fifth resistor and whose another end is connected to the inverting input terminal of the second operational amplifier;

a seventh resistor whose one end is connected to the inverting input terminal of the second operational amplifier and whose another end is connected to the negative-side input terminal of the Analog/Digital Converter;

an eighth resistor whose one end is connected to the output terminal of the second operational amplifier and whose another end is connected to the negative-side input terminal of the Analog/Digital Converter;

a third capacitor whose one end is connected to the inverting input terminal of the second operational amplifier and whose another end is connected to the output terminal of the second operational amplifier; and

a fourth capacitor whose one end is connected to the positive-side input terminal of the Analog/Digital Converter and whose another end is connected to the negative-side input terminal of the Analog/Digital Converter.