| 摘要 |
PURPOSE:To enable fault diagnosis including the front-stage side circuit of a preamplifier by providing a solar battery which generates a voltage at the time of the fault diagnosis between input signal lines for an analog signal to be measured. CONSTITUTION:A control circuit 6 normally turns off a switch S, to the photodiode 5a in a photoisolator 5 is supplied with no electric power and does not illuminate. The voltage of the analog signal X to be measured is therefore inputted to an A/D converter 4 as it is. When a fault diagnosis is taken, on the other hand, the control circuit 6 turns on the switch S to supply light from the photodiode 5a to the solar battery 5, whose generated voltage or the composite voltage of the generated voltage and analog voltage X to be measured is inputted to a potential insulating means (insulating circuit 1, etc.) For the purpose, the generated voltage or composite voltage is measured to diagnose a fault as to the whole of a measured analog voltage input circuit including the front-stage circuit of the preamplifier 3.
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