| 摘要 |
A current holding circuit of a two-wire instrument which includes a transmitter and a receiving resistor, remotely located from the transmitter, for detecting a signal from the transmitter as a change in loop current, and to which a commuication device is connected, includes a current detecting unit, a sample-hold unit, and a current control unit. The current detecting unit includes a reference resistor for converting the loop current into a voltage and a first operational amplifier for receiving the voltage obtained by the reference resistor at the noninverting input terminal. The sample-hold unit holds an output from the current detecting unit. The current control unit supplies an output from the sample-hold unit to the inverting input terminal of the operational amplifier and controls the loop current on the basis of the output from the operational amplifier, thereby holding the loop current at a constant value.
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