Sensor apparatus capable of preventing high frequency noise

A sensor apparatus comprising:

• a first power supply line connected to a power supply terminal;
• a second power supply line separately branched from said first power supply line and connected thereto;
• a constant current producing circuit connected to said first power supply line and producing a constant current from a power supply voltage fed through the first power supply line;
• a sensor circuit activated by the constant current, said sensor circuit detecting a predetermined environmental characteristic and outputting a detection signal based on the detected predetermined environmental characteristic;
• an amplifier circuit connected to said second power supply line and said sensor circuit, said amplifier circuit amplifying the detection signal to generate an amplified detection signal at an output terminal thereof;
• each of said constant current producing circuit and said amplifier circuit including an operational amplifier that receives power supplied from said second power supply line;
• an output line connected to the output terminal of said amplifier circuit;
• a first filter circuit formed as part of one of said amplifier circuit and said constant current producing circuit, said first filter circuit being connected to said first power supply line between a branch point at which said second power supply line branches from said first power supply line and other portions of said one of said constant current producing circuit and said amplifier circuit;
• a second filter circuit connected to said second power supply line;
• and
• a third filter circuit connected to said output line, wherein said first filter circuit, said second filter circuit, and said third filter circuit reduce high frequency noise.

A sensor apparatus comprising:

• a first power supply line connected to a power supply terminal;
• a second power supply line separately branched from said first power supply line and connected thereto;
• a constant current producing circuit for outputting a constant current and including a first operational amplifier that receives power from said second power supply line, a first resistor connected between one of an inverting input terminal and a non-inverting input terminal of said first operational amplifier and for producing a reference voltage used to determine a value of the constant current, a second resistor connected between the other of said non-inverting input terminal and said inverting input terminal of said first operational amplifier and said first power supply line, said second resistor determining said value of the constant current based on a difference between said reference voltage and a power supply voltage supplied from said first power supply line, a first capacitor connected to said first resistor and together constituting a first filter circuit, and a second capacitor connected to said second resistor and together constituting a second filter circuit;
• a bridge circuit including a strain gage having a resistance value that varies in response to a detected pressure, said bridge circuit generating a detection signal corresponding to said resistance value in response to receiving the constant current from said constant current producing circuit;
• an amplifier circuit which amplifies the detection signal generated by said bridge circuit and including a second operational amplifier which receives power from said second power supply line, a third resistor connected between an input terminal of said second operational amplifier and said first power supply line in order to determine an operation point of said second operational amplifier, and a capacitor connected to said third resistor and together constituting a third filter circuit;
• an output line connected between said amplifier circuit and an output terminal;
• a fourth filter circuit comprising a fourth resistor and a fourth capacitor connected to said second power supply line;
• and
• a fifth filter circuit comprising a fifth resistor and a fifth capacitor connected to said output line, wherein said first filter circuit, said second filter circuit, said third filter circuit, said fourth filter circuit, and said fifth filter circuit reduce high frequency noise.