US 2010/0071476 A1Published application

Capacitive electromagnetic flowmeter

AssigneeYamatake Corporation

InventorsYutaka Harada, Ichiro Mitsutake, Kouji Okuda, Taka Inoue, Tetsuya Kajita

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Abstract

In a constant current circuit, a constant current is caused to flow through a resistor, thereby causing a constant voltage to occur across the resistor. This constant voltage is then superimposed on an output signal of an operational amplifier that is to be fed back to the drain of a field effect transistor, thereby maintaining the same potential in an AC manner between the output terminal of the operational amplifier and the drain of the field effect transistor. In this way, the gate and drain of the field effect transistor is caused to exhibit the same potential in an AC manner, so that no current will occur through the stray capacitance between the gate and drain of the field effect transistor. As a result, similarly to a case of using a feedback capacitor, the input impedance of the field effect transistor can be raised.

Claims4

Dependent claims

A capacitive electromagnetic flowmeter comprising:

an excitation coil producing a magnetic field in a direction that is perpendicular to the direction of flow of a fluid that flows within a measuring tube;
a signal electrode, provided within the measuring tube, picking up an electromotive force that is produced in the fluid that flows within the measuring tube due to the magnetic field that is produced by the excitation coil, through electrostatic capacitive coupling with the fluid that flows within the measuring tube;
a first field effect transistor having a gate input of the electromotive force that is picked up by the signal electrode;
an operational amplifier wherein the output of the source of the first field effect transistor is applied to the non-inverting input terminal thereof;
a first and a second feedback path to feedback, to the inverting input terminal of the operational amplifier and to the drain of the first field effect transistor, the output signal from the output terminal of the operational amplifier;
a voltage maintaining circuit maintaining the voltages of the gate and the drain of the first field effect transistor at the same voltage, in terms of alternating current through superimposing a predetermined constant voltage, generated by a flow of an electric current, onto the output signal of the operational amplifier that is fed back to the drain of the first field effect transistor;
a series connection circuit of a first and a second resistor, connected between the gate of the first field effect transistor and a common voltage point;
and
a capacitor connected between the contact point of the first and second resistors and the inverting input terminal of the operational amplifier.

6 more elements

Dependencies are inferred from the claim text.