| CPC G01R 27/2605 (2013.01) [H01J 37/32917 (2013.01); H01J 2237/24564 (2013.01)] | 14 Claims |
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1. A measuring instrument comprising: a disc-shaped base board; at least one sensor chip on the base board; and a circuit board on the base board,
wherein the at least one sensor chip includes a sensor unit including a signal electrode having a front surface intersecting the base board in a radial direction, a guard electrode disposed on a rear side of the signal electrode while being spaced apart from the signal electrode and extending along the signal electrode, and a first ground electrode disposed on a rear side of the guard electrode,
the circuit board includes
a radio frequency oscillator configured to apply a radio frequency signal to each of the signal electrode and the guard electrode, and
a C/V conversion circuit configured to generate a voltage signal according to an electrostatic capacitance formed by the signal electrode,
the C/V conversion circuit has an amplifier circuit including an operational amplifier,
the radio frequency oscillator is connected to a non-inversion input terminal of the operational amplifier so that the radio frequency signal applied to the guard electrode is input to the non-inversion input terminal, and is connected to an inversion input terminal of the operational amplifier so that the radio frequency signal applied to the signal electrode is input to the inversion input terminal,
the at least one sensor chip includes a second ground electrode extending along a lower surface of the sensor unit,
the signal electrode, the guard electrode, and the first ground electrode of the sensor unit all extend to a lower end of the sensor unit, and
a space between the second ground electrode and the sensor unit is filled with a material having insulating properties.
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