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Capacitance detecting apparatus

  • US 20080122454A1
  • Filed: 11/28/2007
  • Published: 05/29/2008
  • Est. Priority Date: 11/29/2006
  • Status: Abandoned Application
First Claim
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1. A capacitance detecting apparatus comprising:

  • a first differential amplifier including an inverting input terminal, a non-inverting input terminal, and an output terminal, the non-inverting input terminal inputting a first fixed voltage;

    a first reference capacitor including a first electrode connected to the output terminal of the first differential amplifier and a second electrode connected to the inverting input terminal of the first differential amplifier;

    a first on/off switch including a first end connected to the output terminal of the first differential amplifier and a second end connected to the inverting input terminal of the first differential amplifier;

    a second on/off switch including a first end connected to the inverting input terminal of the first differential amplifier;

    a third on/off switch including a first end connected to a first power-supply voltage and a second end connected to the second end of the second on/off switch;

    a first sensor electrode connected to the second end of the second on/off switch and facing a ground electrode having a constant electric potential, a first variable capacitance being formed between the first sensor electrode and the ground electrode in response to a distance between the first sensor electrode and the ground electrode;

    switch controlling means for performing a first switch operation in which the first on/off switch is shifted to a closed state and returned to an open state, and then alternately repeating a second switch operation in which the second on/off switch is shifted to a closed state and returned to an open state and a third switch operation in which the third on/off switch is shifted to a closed state and returned to an open state;

    a comparator including a first input terminal connected to the output terminal of the first differential amplifier and a second input terminal inputting a voltage, the comparator comparing an output voltage from the first differential amplifier and the voltage input to the second input terminal;

    counting means for counting the number of times the second switch operation is repeated; and

    determining means for determining changes in the first variable capacitance formed between the first sensor electrode and the ground electrode based on the number of times the second switch operation is repeated that is counted by the counting means before an output level of the comparator is changed.

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