Signal processing circuit

A signal processing circuit, comprising:

• an isolator including an input side and an output side that are electrically insulated;
• a variable gain amplifier circuit that amplifies an output signal of the isolator;
• a DC offset adjustment circuit that adjusts a DC offset of the variable gain amplifier circuit;
• a gain adjustment circuit that adjusts a gain of the variable gain amplifier circuit;
• a first voltage setting circuit provided on the input side of the isolator, the first voltage setting circuit setting, based on a first control signal, a reference voltage on the input side of the isolator to zero during DC offset adjustment and to a constant voltage different from zero during gain adjustment, and outputting the reference voltage on the input side to the input side of the isolator;
• a second voltage setting circuit provided on the output side of the isolator, the second voltage setting circuit setting, based on a second control signal, a reference voltage on the output side of the isolator to zero during DC offset adjustment and to a constant voltage different from zero during gain adjustment, and outputting the reference voltage on the output side to the output side of the isolator;
• a comparison circuit that compares an output voltage of the variable gain amplifier circuit with an output voltage of the second voltage setting circuit;
• and
• a control circuit that controls the DC offset adjustment circuit and the gain adjustment circuit in response to an output signal of the comparison circuit, wherein
• the first voltage setting circuit includes:
• a first differential amplifier circuit that includes an inverting input terminal and a non-inverting input terminal to which constant currents are supplied;
• and
• a first switch provided between the inverting input terminal and the non-inverting input terminal of the first differential amplifier circuit;
• the second voltage setting circuit includes:
• a second differential amplifier circuit that includes an inverting input terminal and a non-inverting input terminal to which constant currents are supplied;
• and
• a second switch provided between the inverting input terminal and the non-inverting input terminal of the second differential amplifier circuit;
• and
• the control circuit outputs the first and second control signals, and controls on/off of the first and second switches to exclusively control the DC offset adjustment circuit and the gain adjustment circuit.