INPUT CIRCUIT FOR INSULATING-VOLTAGE MEASURING APPARATUS

PROBLEM TO BE SOLVED: To obtain an input circuit by which an insulating circuit can be miniaturized and by which a power-supply circuit can be simplified by a method wherein a zero- point offset voltage is given to a first amplifier at an analog coupler and a bipolar voltage signal is transmitted to a second amplifier as a unipolar signal. SOLUTION: A reference voltage Vref is input, via a zero-point-offset setting resistance Roff, to the input end of an operational amplifier OPA1 . On the other hand, an operational amplifier OPA2 can be driven by a + side power supply. Then, an input voltage signal is normalized to a prescribed voltage by an attenuator ATT which is set according to a range and by a gain amplifier PGA, and it is insulated in the state of an analog signal by an analog coupler AC. The normalized bipolar voltage signal is changed into a unipolar signal in such a way that a prescribed zero- point offset voltage is generated by the reference voltage Vref and the setting resistance Roff, and the signal is transmitted to the OPA2 so as to be input to a CPU. At this time, an alternating current is used, an insulating circuit is miniaturized, and the OPA2 is operated by a single power supply. As a result, only a + side power supply is used as the power supply of a noninsulating circuit part, and a power-supply circuit can be simplified.