JPH095190A - Capacitance detection device - Google Patents

Abstract

PURPOSE: To improve temperature characteristics and linearity of capacitance detection output for pressure by canceling out the temperature fluctuation of a low-pass filter generated due to the difference in the temperature coefficient between a capacitance-change-type sensor and a reference capacitor caused by the temperature fluctuation between two reference power supplies. CONSTITUTION: A low-pass filter 12 inputs the output pulse of a comparison circuit 9 and converts it to a DC voltage and then outputs it as a pressure or a weight value. In this case, to reduce the fluctuation of an output voltage after passing through the filter 12 for temperature change, two reference voltage sources 13 and 14 are provided, the temperature coefficient (temperature characteristic) of both power supplies 13 and 14 is allowed to differ from each other using a resistor with a different temperature coefficient, and the fluctuation of the output voltage of the filter 12 generated due to the temperature coefficient difference between a capacitance-change-type sensor 4 and a capacitor 5 is canceled out, thus reducing the temperature fluctuation of the filter 12 for the same pressure and improving measurement accuracy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitance detecting device for converting pressure or weight into a DC voltage in a capacitance change type sensor whose capacitance changes due to physical changes such as pressure or weight.

[0002]

2. Description of the Related Art As an example of a conventional capacitive sensor, a sensor as disclosed in JP-A-57-70404 has been generally used. First, the principle of the case where the variable capacitance sensor detects pressure will be described with reference to FIG. If the distance between two parallel plate electrodes changes due to pressure, weight, etc.,
The electrostatic capacitance between the two parallel plates changes, and this change is extracted as an electric signal.

In FIG. 9, 1 is a movable electrode, 2 is a fixed electrode which is a pair of the movable electrode 1, and the movable electrode 1 and the fixed electrode 2 constitute a capacitance change type sensor. In the figure, 3A is a reference electrode, 3B is a fixed electrode, and the reference electrode 3A and the fixed electrode 3B form a reference capacitor. In the figure, P is the applied pressure, d0 is the inter-electrode distance when pressure is not applied, Δd is the change in inter-electrode distance when pressure P is applied, and d is the inter-electrode distance when pressure P is applied. Show. C is the capacity of the pressure sensor when pressure is not applied.
Let r be the capacitance when the pressure P is applied and the distance between the electrodes be d, and let S be the electrode area. Since the change Δd in the distance between the electrodes is proportional to the applied pressure P, d = d0−Δd = d0−k1 × P (k1 is a constant) (1) ∴P = (d0−d) / k1 = k2 (1-d / D0) (k2 = d0 / k1: constant) (2) On the other hand, the capacitances Cr and Cp are Cr = εS / d0 and Cp = εS / d (ε is the dielectric constant) (3), so d / d0 = Cr / Cp (4) Substituting the equation (4) into the equation (2), the following equation is obtained.

P = k2 (1-Cr / Cp) (5) Therefore, (1-Cr / Cp) is proportional to the pressure P.

Here, the reference capacitor Cr is generally composed of electrodes 3A and 3B formed in the vicinity of a spacer for adhering two substrates, as shown in FIG.

Next, the circuit operation of the conventional capacitance detecting device will be described with reference to FIGS. 10 and 11. FIG.
In 0, 4 is a capacitance change type sensor (Cp), 5 is a reference capacitor (Cr), 6 is a first charge / discharge circuit, and 7 is a second.
Charging / discharging circuit, 8 is a first comparing circuit, 9 is a second comparing circuit, 10 is a reference voltage source, 11 is charging / discharging control means, and 12 is a low-pass filter. Variable capacitance sensor (Cp)
4, the first charging / discharging circuit 6 and the first comparing circuit 8 are connected at one input terminal at point a, and the reference capacitor (Cr) 5, the second charging / discharging circuit 7 and the second comparing circuit 9 are connected. One input terminal is connected at point b, and the other input terminals of the first comparison circuit 8 and the second comparison circuit 9 are connected to the output of the reference voltage source 10. The output of the second comparison circuit 9 is connected to the low-pass filter 12, and the output of the first comparison circuit 8 is the charge / discharge control means 1.
1 connected. The first charging / discharging circuit 6 is composed of a transistor Q1 that discharges the electric charge of the capacitance change sensor (Cp) 4 and a resistor R1 that determines a time constant when the capacitance change sensor (Cp) 4 is charged. Has been done. On the other hand, the second charging / discharging circuit 7 having a similar function to the reference capacitor (Cr) 5 includes a transistor Q2 and a resistor R2.
2 and. Here, the resistors R1 and R2 have the same value.

Next, with reference to the potential waveforms of Ep and Er and the output of the second comparison circuit shown in FIG. 11, the operation of each part will be described together with FIG. Introduction transistor Q
It is assumed that 1 and Q2 are in the ON state. At that time, the potentials Ep and Er at the points a and b are the power supply voltage Vcc, and the reference voltage source 10
The output of the second comparison circuit is low because it is higher than the potential of the. The transistor Q is controlled by the charge / discharge control means 11.
When 1 and Q2 are switched from on to off, the capacitance change type sensor C
When the charging of p and the reference capacitor Cr is started, the potentials Ep and Er at the points a and b decrease as shown in FIG. When the potential Er at the point b falls below the value Vref of the reference voltage source 10 after the lapse of T1 time, the output of the second comparison circuit 9 is inverted from low to high.

When the potential Ep at the point b falls below the value Vref of the reference voltage source 10 after the lapse of time T2, the first comparison circuit 8 outputs a signal to the charge / discharge control means 11 to control the charge / discharge. By means 11 the transistors Q1 and Q2 are turned on again to rapidly discharge the electric charge of the capacitance change type sensor 4 and the reference capacitor 5 so that the potentials Ep and Er at the points a and b are Vcc.
The output of the second comparison circuit 9 is inverted from high to low and returns to the original level.

By repeating this operation, the second comparison circuit 9
A pulse is output to the output of, and it is converted into a DC voltage by passing through the low-pass filter 12.

The potentials at points a and b change exponentially, and the respective values are given by the following equations. Cp side: Ep = Vcc * exp (-t / (Cp * R1)) (6) Cr side: Er = Vcc * exp (-t / (Cr * R2)) (7) Therefore, the output voltage of the low-pass filter Vout is Vout = Vcc × T2 / (T1 + T2) = Vcc × (1- (Cr × R2 × ln (Vref / Vcc) / (Cp × R1 × ln (Vref / Vcc)) (8) = Vcc × (1 −Cr / Cp) (9) By dividing the right side and the left side of equations (5) and (8), P / Vout = k2 / Vcc = constant, and the output voltage Vout of the low-pass filter is obtained. Is proportional to the pressure P applied to the sensor.

[0011]

However, in the above conventional configuration, when the temperature is changed because of the difference in temperature characteristics between the capacitive sensor 4 (Cp) and the reference capacitor 5 (Cr), As shown in FIG. 12, there is a problem that the output voltage drifts for the same pressure. Further, since the sensor configuration is not an ideal parallel plate structure but a structure in which the diaphragm bends, there is a problem that the linearity deteriorates as shown in FIG. 13 as the applied pressure increases.

The present invention is intended to solve the above-mentioned problems.
The purpose of is to improve the temperature characteristic of the output of the capacitance detection device with respect to the pressure, and the second purpose is to improve the linearity of the output of the capacitance detection device with respect to the pressure.

[0013]

In order to achieve the first object of the present invention, a capacitance change sensor whose capacitance changes according to a physical change and a charge and discharge of the capacitance change sensor are performed. 1 charge and discharge circuit, a first reference voltage source, a first comparison circuit for comparing the potential of the capacitance change type sensor and the potential of the first reference voltage source, the reference capacitor, the charge and discharge of the reference capacitor Driven by a second charge / discharge circuit, a second reference voltage source, a second comparison circuit for comparing the potential of the reference capacitor and the potential of the second reference voltage source, and the output of the first comparison circuit. A charge / discharge control circuit for controlling the first charge / discharge circuit and the second charge / discharge circuit, and a low voltage for converting the output pulse of the second comparison circuit into a direct current to generate a direct current voltage signal corresponding to a physical change. And a first reference voltage source and a second reference voltage source. The temperature fluctuations of the quasi-voltage source or the temperature fluctuations of the first charging / discharging circuit and the second charging / discharging circuit cancel the temperature fluctuations of the low-pass filter caused by the difference in the temperature coefficient between the capacitance change sensor and the reference capacitor. I try to work in the direction.

In the second invention, in order to achieve the second object, a capacitance change type sensor whose capacitance changes according to a physical change and a first charge / discharge circuit for charging / discharging the capacitance change type sensor. And a first reference voltage source, a first comparison circuit for comparing the potential of the capacitance change sensor and the potential of the first reference voltage source, a reference capacitor, and a second charging and discharging device for charging and discharging the reference capacitor. The discharge circuit, the second reference voltage source, the second comparison circuit for comparing the potential of the reference capacitor with the potential of the second reference voltage source, and the first comparison circuit are driven by the output of the first comparison circuit.
A charge / discharge control circuit for controlling the charge / discharge circuit and the second charge / discharge circuit, and a low-pass filter for converting the output pulse of the second comparison circuit into a direct current to generate a direct current voltage signal corresponding to a physical change. , And a reference voltage control circuit for controlling the first reference voltage source or the second reference voltage source according to the output of the low-pass filter.

Further, in order to achieve the second object, the third aspect of the invention is to provide a capacitance change type sensor whose capacitance changes in accordance with a physical change, and a first charge / discharge circuit for charging / discharging the capacitance change type sensor. And a first reference voltage source, a first comparison circuit for comparing the potential of the capacitance change sensor and the potential of the first reference voltage source, a reference capacitor, and a second charging and discharging device for charging and discharging the reference capacitor. A discharge circuit, a second reference voltage source, and a second comparing the potential of the reference capacitor and the potential of the second reference voltage source.
And a charge / discharge control circuit for controlling the first charge / discharge circuit and the second charge / discharge circuit driven by the output of the first comparison circuit, and converting the output pulse of the second comparison circuit into DC. And a feedback circuit that controls the first charge / discharge circuit or the second charge / discharge circuit according to the output of the low-pass filter. There is.

[0016]

With the above structure, in the first invention, the first comparison circuit compares the potential of the capacitance change sensor with the potential of the first reference voltage source, and the second comparison circuit compares with the potential of the reference capacitor. The potentials of the two reference voltage sources are compared. Further, the charge / discharge control circuit is driven by the output of the first comparison circuit, and the output causes the first charge / discharge circuit and the second charge / discharge circuit to charge and discharge the variable capacitance sensor and the reference capacitor. on the other hand,
The output of the second comparison circuit is input to the low pass filter and converted into a DC voltage. Here, the temperature variation between the first reference voltage source and the second reference voltage source or the temperature variation between the first charging / discharging circuit and the second charging / discharging circuit is caused by the temperature coefficient of the capacitance change sensor and the reference capacitor. It works to cancel the temperature fluctuation of the low-pass filter caused by the difference.

In the second invention, the first comparison circuit compares the potential of the capacitance change type sensor with the potential of the first reference voltage source, and the second comparison circuit compares the potential of the reference capacitor with the second reference voltage. Compare the potentials of the voltage sources. Then, the charge / discharge control circuit is driven by the output of the first comparison circuit, and the output causes the first charge / discharge circuit and the second charge / discharge circuit to charge / discharge the variable capacitance sensor and the reference capacitor. On the other hand, the output of the second comparison circuit is input to the low pass filter and converted into a DC voltage. The reference voltage control circuit controls the first reference voltage source or the second reference voltage source according to the output of the low-pass filter to improve the linearity.

In the third invention, the first comparison circuit compares the potential of the capacitance change type sensor with the potential of the first reference voltage source,
The second comparison circuit compares the potential of the reference capacitor with the potential of the second reference voltage source. Then, the charge / discharge control circuit is driven by the output of the first comparison circuit, and the output of the charge / discharge control circuit
And a second charge / discharge circuit charge and discharge the variable capacitance sensor and the reference capacitor. On the other hand, the output of the second comparison circuit is input to the low pass filter and converted into a DC voltage. The feedback circuit controls the first charging / discharging circuit or the second charging / discharging circuit according to the output of the low-pass filter to improve the linearity.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the first invention will be described below with reference to FIGS. In FIG. 1, a capacitance change sensor 4, a reference capacitor 5, a first charge / discharge circuit 6, a second charge / discharge circuit 7, a first comparison circuit 8, a second comparison circuit 9,
The charging / discharging control circuit 11 and the low-pass filter 12 have the same components as those of the conventional example, and therefore their explanations are omitted. In the first invention, except that the reference potential of the first comparison circuit 8 is the first reference voltage source 13 and the reference potential of the second comparison circuit 9 is the second reference voltage source 14, each part The connection of is similar to the conventional example.

In the above structure, the first comparison circuit 8 compares the potential of the capacitance change type sensor 4 with the potential of the first reference voltage source 13, and the second comparison circuit 9 compares the potential of the reference capacitor 5 with the second. The electric potentials of the reference voltage sources 14 are compared. Also, the first
The charge / discharge control circuit 11 is driven by the output of the comparison circuit 8 and the output thereof causes the first charge / discharge circuit 6 and the second charge / discharge circuit 7 to charge / discharge the variable capacitance sensor 4 and the reference capacitor 5. On the other hand, the output of the second comparison circuit 9 is input to the low pass filter 12 and converted into a DC voltage.

On the other hand, the temperature characteristics of the first reference voltage source 13 and the second reference voltage source 14 have a negative temperature gradient, as shown in FIG. Is steeper than the second reference voltage source 14. Therefore, as shown in FIG. 3, as the temperature rises, the potential difference (Vref1−Vref2) between the first reference voltage source 13 and the second reference voltage source 14 increases, and the output pulse width T2 ′ becomes the pulse period T1 ′ + T.
The ratio (duty) for setting 2'is small. That is, the output voltage Vout after passing through the low pass filter decreases as the temperature rises. Therefore, as shown in FIG. 4, the fluctuation of the output voltage becomes small with respect to the temperature change. Thus, the first reference voltage source 13 and the second reference voltage source 1
In order to make the temperature coefficients of 4 different, it is possible to realize by using resistors having different temperature coefficients and creating a reference potential by resistance division.

As described above, according to the configuration of the first embodiment,
Since the first reference voltage source 13 and the second reference voltage source 14 have different temperature coefficients and work to cancel the output fluctuation caused by the difference between the temperature coefficients of the capacitance change type sensor 4 and the reference capacitor 5, they are the same. It is possible to reduce the temperature fluctuation of the low-pass filter 12 with respect to the pressure and improve the accuracy.

Next, a second embodiment for achieving the first object will be described with reference to FIGS. 5 and 6. The capacitance change sensor 4, the reference capacitor 5, the first comparison circuit 8,
The second comparison circuit 9, the charge / discharge control circuit 11, and the low-pass filter 12 have the same components as those of the conventional example. The first reference voltage source 13 and the second reference voltage source 14 are reference voltage sources having the same temperature coefficient. On the other hand, the first charging / discharging circuit 6 and the second charging / discharging circuit 7 have different temperature coefficients, and the difference in the temperature coefficient is caused by the difference in the temperature coefficient between the capacitance change type sensor 4 and the reference capacitor 4. It works to offset temperature fluctuations.

In the above structure, the operation of each part at room temperature is similar to that of the first embodiment. That is, the first comparison circuit 8
Compares the potential of the variable capacitance sensor 4 with the potential of the first reference voltage source 13, and the second comparison circuit 9 compares the potential of the reference capacitor 5 with the potential of the second reference voltage source 14. Also,
The output of the first comparison circuit 8 drives the charging / discharging control circuit 11, and the output thereof drives the first charging / discharging circuit 6 and the second charging / discharging circuit 6.
The charge / discharge circuit 7 charges and discharges the variable capacitance sensor 4 and the reference capacitor 5. On the other hand, the output of the second comparison circuit 9 is
It is input to the low-pass filter 12 and converted into a DC voltage.

Next, the temperature characteristics of the second embodiment will be described. In the second embodiment, the temperature characteristics of the first charge / discharge circuit 6 and the second charge / discharge circuit 7 are different. This can be realized, for example, by providing the discharge resistors R1 and R2 in the circuit of FIG. 10 showing the conventional charge / discharge circuit with different temperature characteristics. A case where the temperature coefficient of the charging / discharging resistor R1 is smaller than the temperature coefficient of the resistor R2 as shown in FIG. 5 will be described. Since the resistance R2 becomes larger than the resistance R1 as the temperature rises, the slope of the voltage drop at the point b becomes gentle as shown in FIG. 6, and the output pulse width T2 ′ and the output pulse width T2 ′ become the pulse period T1. The proportion of "+ T2" is reduced. That is, the output voltage Vout after passing through the low pass filter 12 decreases as the temperature rises. Therefore, the fluctuation of the output voltage becomes small with respect to the temperature change.

According to the configuration of the second embodiment, the temperature coefficients of the first charging / discharging circuit 6 and the second charging / discharging circuit 7 are different, and the temperature coefficients of the variable capacitance sensor 4 and the reference capacitor 5 are different. Since it works so as to cancel out the fluctuation of the output caused by the above, it is possible to reduce the temperature fluctuation of the low-pass filter 12 for the same pressure and improve the accuracy.

Next, a third embodiment for achieving the second object will be described with reference to FIG. Capacity change type sensor 4, reference capacitor 5, first charge / discharge circuit 6, second
The charging / discharging circuit 7, the first comparing circuit 8, the second comparing circuit 9, the charging / discharging control means 11, and the low-pass filter 12 are the same constituent elements as those in the first embodiment. The reference voltage control circuit 17 controls the voltage value of the first reference voltage source 13 or the second reference voltage source 14 according to the output of the low-pass filter 12.

The operation of the above configuration will be described.
The first comparison circuit 8 compares the potential of the capacitance change type sensor 4 with the potential of the first reference voltage source 13, and the second comparison circuit 9 compares the potential of the reference capacitor 5 with the potential of the second reference voltage source 14. To compare. Further, the charge / discharge control circuit 11 is driven by the output of the first comparison circuit 8, and the output thereof causes the first charge / discharge circuit 6 and the second charge / discharge circuit 7 to operate.
And the reference capacitor 5 is charged and discharged. On the other hand, the output of the second comparison circuit 9 is input to the low pass filter 12 and converted into a DC voltage. Then, the output of the low-pass filter 12 is input to the reference voltage control circuit 17, and the reference voltage control circuit 17 controls the potential of the first reference voltage source 15 or the second reference voltage source 16 based on the value thereof. . As shown in FIG.
The reference voltage control circuit 17 functions to change the potential Vref2 of the second reference voltage source 16 according to the signal from the low pass filter 12. When the change amount of the capacitance change type sensor 4 is small, that is, when the duty of the output pulse from the second comparison circuit 9 is small, the potential V of the second reference voltage source 16 is increased.
Make ref2 a little lower. On the contrary, when the change amount of the capacitance change type sensor 4 is large, that is, when the duty of the output pulse from the second comparison circuit 9 is large, the potential Vref2 of the second reference voltage source 16 is greatly lowered. By such an operation of the reference voltage control circuit 17, the output of the low-pass filter 12 has a characteristic of higher linearity as shown by the improved characteristic of FIG.

According to the configuration of the third embodiment, as the capacitance change of the capacitance change type sensor 4 is larger and the output pulse duty from the second comparison circuit 9 is larger, the reference voltage control circuit 17 has the second reference voltage. Since control is performed so as to greatly reduce the reduction range of the reference potential of the voltage source 16, the linearity of the output of the low pass filter 12 with respect to the applied pressure can be improved.

The reference voltage control circuit 17 further lowers the reference potential Vref2 of the second reference voltage source 16 as the change of the capacitance change type sensor 4 becomes larger,
Although the linearity of the output of the low-pass filter 12 is improved, in order to improve the linearity of the output of the low-pass filter 12, the output reference voltage control circuit 17 includes the first reference voltage source 15
May be controlled.

Finally, a fourth embodiment for achieving the second object will be described with reference to FIG. In FIG. 8, a capacitance change type sensor 4, a reference capacitor 5, a first comparison circuit 8, a second comparison circuit 9, a charge / discharge control means 11 and a low pass filter 12 are the same constituent elements as in the first invention, The first reference voltage source 13 and the second reference voltage source 14 are reference potentials having the same temperature coefficient. Further, the feedback circuit 20 receives the signal from the low-pass filter 12 as an input signal and controls the time constant of the first charging / discharging circuit 18 or the second charging / discharging circuit 19.

The operation of the above configuration will be described.
The first comparison circuit 8 compares the potential of the capacitance change type sensor 4 with the potential of the first reference voltage source 13, and the second comparison circuit 9 compares the potential of the reference capacitor 5 with the potential of the second reference voltage source 14. To compare. Further, the charge / discharge control circuit 11 is driven by the output of the first comparison circuit 8, and the output thereof causes the first charge / discharge circuit 6 and the second charge / discharge circuit 7 to operate.
And the reference capacitor 5 is charged and discharged. The output of the second comparison circuit 9 is input to the low pass filter 12 and converted into a DC voltage. Next, the operation of the feedback circuit 20 will be described. Here, as shown in FIG. 6, the feedback circuit 20 functions to change the time constant of the second charge / discharge circuit 19 according to the signal from the low-pass filter 12. When the change amount of the capacitance change type sensor 4 is small, that is, the second comparison circuit 9
When the duty of the output pulse from is small, the time constant of the second charge / discharge circuit is slightly increased. On the contrary, when the change amount of the capacitance change type sensor 4 is large, that is, when the duty of the output pulse from the second comparison circuit 9 is large, the time constant of the second charge / discharge circuit is greatly increased. By such operation of the feedback circuit 20, the low-pass filter 12
The output has a higher linearity characteristic as shown in FIG.

According to the configuration of the fourth embodiment, as the capacitance change of the capacitance change type sensor 4 is larger and the output pulse duty from the second comparison circuit 9 is larger, the feedback circuit 20 becomes the second charge / discharge circuit. Since the change width of the time constant 7 becomes large, the linearity of the output of the low-pass filter 12 with respect to the applied pressure can be improved.

The feedback circuit 20 is the capacitance change type sensor 4
The linearity of the output of the low-pass filter 12 is improved by increasing the time constant of the second charge / discharge circuit 19 as the change of the
The feedback circuit 20 may control the first charging / discharging circuit 18 in order to improve the linearity of the output of.

[0035]

As described above, the temperature coefficients of the first reference voltage source and the second reference voltage source are different, and the difference between the temperature coefficients is that of the temperature coefficient of the variable capacitance sensor and that of the reference capacitor. It acts so as to cancel the fluctuation of the output caused by the difference, or the temperature coefficients of the first charge / discharge circuit and the second charge / discharge circuit are different, and the difference in the temperature coefficient is the temperature coefficient of the variable capacitance sensor and the reference capacitor. Since it works so as to cancel out the fluctuation of the output caused by the difference of, the temperature fluctuation of the low-pass filter can be reduced for the same pressure, and the accuracy can be improved.

Further, as the capacitance change of the capacitance change type sensor is larger and the output pulse duty from the second comparison circuit is larger, the reference voltage control circuit greatly reduces the reduction range of the reference potential of the second reference voltage source. Therefore, the linearity of the output of the low-pass filter with respect to the applied pressure can be improved.

Further, the larger the capacitance change of the capacitance change type sensor and the larger the output pulse duty from the second comparison circuit, the larger the change width in which the feedback circuit increases the time constant of the second charge / discharge circuit. Therefore, the linearity of the output of the low pass filter 12 with respect to the applied pressure can be improved.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of a capacitance detection device according to a first embodiment of the present invention.

FIG. 2 is a temperature characteristic diagram of first and second reference voltage sources of the same circuit.

FIG. 3 is a waveform diagram of points a and b of the circuit and an output waveform diagram of a second comparison circuit.

FIG. 4 is a temperature characteristic diagram of the output of the low-pass filter of the same circuit.

FIG. 5 is a temperature characteristic diagram of discharge resistances of the first and second charge / discharge circuits of the capacitance detection device according to the second embodiment of the present invention.

FIG. 6 is a waveform diagram of points a and b of the circuit and an output waveform diagram of a second comparison circuit.

FIG. 7 is a circuit configuration diagram of a capacitance detection device according to a third embodiment of the present invention.

FIG. 8 is a circuit configuration diagram of a capacitance detection device according to a fourth embodiment of the present invention.

FIG. 9 is a structural diagram of a variable capacitance sensor.

FIG. 10 is a circuit configuration diagram of a conventional capacitance detection device.

FIG. 11 is a waveform diagram of points a and b of the circuit and an output waveform diagram of a second comparison circuit.

FIG. 12 is a temperature characteristic diagram of the output of the same capacitance detection device.

FIG. 13 is a characteristic diagram of pressure vs. output of the same capacity detection device.

[Explanation of symbols]

 4 Capacitance Change Sensor 5 Reference Capacitor 6 First Charging / Discharging Circuit 7 Second Charging / Discharging Circuit 8 First Comparison Circuit 9 Second Comparison Circuit 11 Charge / Discharge Control Circuit 12 Low-pass Filter 13 First Reference Voltage Source 14 second reference voltage source 17 reference voltage control circuit 20 feedback circuit

Claims (3)

[Claims] 1. A capacitance change sensor whose capacitance changes according to a physical change, a first charge / discharge circuit for charging / discharging the capacitance change sensor, a first reference voltage source, and the capacitance change. Comparison circuit for comparing the electric potential of the type sensor and the electric potential of the first reference voltage source, a reference capacitor, a second charging / discharging circuit for charging / discharging the reference capacitor, and a second reference voltage source. A second comparison circuit for comparing the potential of the reference capacitor with the potential of the second reference voltage source; and the first charge / discharge circuit and the second drive circuit driven by the output of the first comparison circuit. A charge / discharge control circuit for controlling the charge / discharge circuit and a low-pass filter for converting the output pulse of the second comparison circuit into a direct current to generate a direct current voltage signal corresponding to the physical change, Temperature of the first reference voltage source and the second reference voltage source Or the temperature fluctuation between the first charge / discharge circuit and the second charge / discharge circuit acts to cancel the temperature fluctuation of the low-pass filter caused by the difference in temperature coefficient between the capacitance change sensor and the reference capacitor. Capacitance detection device configured. 2. A capacitance change sensor whose capacitance changes according to a physical change, a first charge / discharge circuit for charging and discharging the capacitance change sensor, a first reference voltage source, and the capacitance change. Comparison circuit for comparing the electric potential of the type sensor and the electric potential of the first reference voltage source, a reference capacitor, a second charging / discharging circuit for charging / discharging the reference capacitor, and a second reference voltage source. A second comparison circuit for comparing the potential of the reference capacitor with the potential of the second reference voltage source; and the first charge / discharge circuit and the second drive circuit driven by the output of the first comparison circuit. A charging / discharging control circuit for controlling the charging / discharging circuit; a low pass filter for converting the output pulse of the second comparison circuit into a direct current to generate a direct current voltage signal corresponding to the physical change; According to the output, the first reference voltage source or Capacitance detection device composed of a reference voltage control circuit for controlling the serial second reference voltage source. 3. A capacitance change sensor whose capacitance changes in accordance with a physical change, a first charge / discharge circuit for charging / discharging the capacitance change sensor, a first reference voltage source, and the capacitance change. Comparison circuit for comparing the electric potential of the type sensor and the electric potential of the first reference voltage source, a reference capacitor, a second charging / discharging circuit for charging / discharging the reference capacitor, and a second reference voltage source. A second comparison circuit for comparing the potential of the reference capacitor with the potential of the second reference voltage source; and the first charge / discharge circuit and the second drive circuit driven by the output of the first comparison circuit. A charging / discharging control circuit for controlling the charging / discharging circuit; a low pass filter for converting the output pulse of the second comparison circuit into a direct current to generate a direct current voltage signal corresponding to the physical change; According to the output, the first charge / discharge circuit or Capacitance detection device composed of a feedback circuit for controlling the serial second charging and discharging circuit.