Sensor and sensor module

1. A sensor comprising:

a capacitor having a capacitance varying with a physical quantity;

a capacitance-voltage conversion circuit that converts the capacitance of the capacitor into a voltage; and

a control signal generation circuit that generates a plurality of control signals,

wherein the capacitor has a frequency-capacitance characteristic with a resonant frequency,

wherein, in a measurement of the physical quantity, the capacitance of the capacitor is measured with one of the control signals having a first frequency which is much higher or much lower than the resonant frequency, and

wherein, in a self-diagnosis of the sensor, the capacitance of the capacitor is measured with another one of the control signals having a second frequency which is equal or close to the resonant frequency.

2. The sensor of claim 1 , wherein the capacitor comprises:

a fixed electrode; and

a movable electrode arranged to face the fixed electrode with a gap therebetween, the movable electrode being displaced with a variation in the physical quantity.

3. The sensor of claim 1 , wherein the physical quantity is a pressure.

4. The sensor of claim 1 , wherein the capacitor is formed by a surface micro electro mechanical systems (MEMS) process.

5. The sensor of claim 1 , wherein the capacitor and the capacitance-voltage conversion circuit are formed on the same semiconductor substrate.

6. The sensor of claim 1 , further comprising a boosting circuit,

wherein the capacitance of the capacitor is measured under the condition that a bias voltage boosted by the boosting circuit is applied to the capacitor.

7. The sensor of claim 1 , further comprising a heater installed around the capacitor,

wherein the capacitance of the capacitor is measured under the condition that the capacitor is heated by the heater.

8. A sensor comprising:

a capacitor having a capacitance varying with a physical quantity;

a capacitance-voltage conversion circuit that converts the capacitance of the capacitor into a voltage; and

a control signal generation circuit that generates a plurality of control signals,

wherein the capacitor has a frequency-capacitance characteristic with a resonant frequency,

wherein a first measurement is performed to measure the capacitance of the capacitor with one of the control signals having a first frequency which is much higher or much lower than the resonant frequency, and

wherein, when a result of the first measurement is beyond a standard range or a deviation thereof is beyond a standard range, or when the first measurement has been performed a predetermined number of times, a second measurement is performed to measure the capacitance of the capacitor with another one of the control signals having a second frequency which is equal or close to the resonant frequency.

9. The sensor of claim 8 , further comprising a boosting circuit,

wherein, when a result of the second measurement is beyond a standard range, a third measurement is performed to measure the capacitance of the capacitor under the condition that a bias voltage boosted by the boosting circuit is applied to the capacitor.

10. The sensor of claim 9 , further comprising a heater installed around the capacitor,

wherein, when a result of the third measurement is beyond a standard range, a fourth measurement is performed to measure the capacitance of the capacitor under the condition that the capacitor is heated by the heater.

11. A sensor module comprising:

a capacitor having a capacitance varying with a pressure in a tire;

a capacitance-voltage conversion circuit that converts the capacitance of the capacitor into a voltage;

a control signal generation circuit that generates a plurality of control signals; and

a constant voltage source for driving the capacitance-voltage conversion circuit and control signal generation circuit,

wherein the capacitor has a frequency-capacitance characteristic with a resonant frequency,

wherein a first measurement is performed to measure the capacitance of the capacitor with one of the control signals having a first frequency which is much higher or much lower than the resonant frequency, and

wherein, when a result of the first measurement is beyond a standard range and a deviation thereof is beyond a standard range, or when the first measurement has been performed a predetermined number of times, a second measurement is performed to measure the capacitance of the capacitor with another one of the control signals having a second frequency which is equal or close to the resonant frequency.

12. The sensor module of claim 11 , wherein the sensor module has a package structure, the package structure including:

a semiconductor chip on which at least the capacitor and the capacitance-voltage conversion circuit are formed; and

a battery which is the constant voltage source.

13. The sensor module of claim 11 , wherein the capacitor comprises:

a fixed electrode; and

a movable electrode arranged to face the fixed electrode with a gap therebetween, the movable electrode being displaced with a variation in physical quantity.

14. The sensor module of claim 11 , wherein the capacitor is formed by a surface MEMS process.

15. The sensor module of claim 11 , further comprising a boosting circuit,

wherein, when a result of the second measurement is beyond a standard range, a third measurement is performed to measure the capacitance of the capacitor under the condition that a bias voltage boosted by the boosting circuit is applied to the capacitor.

16. The sensor module of claim 15 , further comprising a heater installed around the capacitor,

wherein, when a result of the third measurement is beyond a standard range, a fourth measurement is performed to measure the capacitance of the capacitor under the condition that the capacitor is heated by the heater.