JPH10319036A - Interface circuit of capacitive sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the offset output of a circuit and its temperature dependency and improve the stability by providing a switching mechanism for moderating the offset voltage of the output by the input offset voltage of an arithmetic amplifier. SOLUTION: One-side ends of first and second capacitors 7, 8 consisting of variable capacitors are mutually connected and grounded, and a switch SW12 is connected between the other ends thereof. The other end of the first capacitor 7 is connected to a power source Vs through a switch SW11, and the other end of the second capacitor is grounded through a switch SW 13 and connected to the reversed input of the first arithmetic amplifier A1 of an impedance converting circuit 21 through a switch SW14. These switches SW11, SW12, SW13, SW14, SW5, SW6, and SW7 are ON/OFF at a specified timing, whereby generation of the offset output by the input voltage offset of the arithmetic amplifiers A1, A2 can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitance detecting circuit of a capacitive sensor used for measuring pressure, acceleration, angular velocity, etc., and more particularly, to a first and a second capacitor in which at least one of the capacitances fluctuates. And an interface circuit connected to a capacitive sensor in which the common terminal of the first and second capacitors is fixed to a ground potential or a constant potential.

[0002]

2. Description of the Related Art In recent years, a sensor for detecting a pressure of a fluid, an acceleration or an angular velocity applied to a moving object, and the like, particularly by applying a micromachining technology of a semiconductor, detect a change in capacitance of a capacitor. Those that detect the signal are attracting attention. These have advantages such as miniaturization, mass production, high precision and high reliability of the device.

FIG. 6 is a cross-sectional view of a typical capacitive acceleration sensor manufactured by using a semiconductor micromachining process as an example. The silicon mass 1 is supported by beams 3 through anchors 2. Fixed electrodes 4 and 5 are formed on glass or silicon 6 above and below the mass 1, and the capacitors 7 and 8 are formed by the mass 1 and the fixed electrodes 4 and 5 as shown in FIG. doing. These capacitors 7 and 8 each have a capacitance C
1 and C2, and constitutes the sensor element 9.

When an inertial force due to acceleration acts on the mass body 1 in the x direction, the mass body 1 is displaced by a certain distance u in the x direction. Due to this displacement u, the capacitance value between the mass body 1 and the fixed electrodes 4, 5 increases on the one hand (C + ΔC) and decreases on the other hand (C-ΔC).
I do.

As a method of converting a capacitance change according to the displacement of the mass body 1 into a voltage output, for example, an example in which a switched capacitor circuit is applied is described in “Rudlf etc, A
nAsic for High-resolution
Capacitive Microaccelerom
eters, Sensor & Actuator,
A21-A23, 1990, pp278-281 "
It is described in.

FIG. 8 shows an example of this conventional circuit. FIG. 9 shows the clock timing of each switch shown in FIG.

As shown in FIG. 8, the sensor element 9 has the same configuration as that of FIG. 7, and the common terminal of the first and second capacitors 7 and 8 is connected to the first stage operational amplifier A1 of the impedance conversion circuit 10. The other end of the first capacitor 7 is connected to the power supply Vs via SW1 and to the non-inverting input of the first stage operational amplifier A1 of the impedance conversion circuit 10 via the switch SW2. The other end of the second capacitor 8 is connected to a switch SW4
, And connected to a wiring connecting the switch SW2 and the non-inverting input of the first-stage operational amplifier A1 via a switch SW3. The impedance conversion circuit 10 has a first-stage operational amplifier A1 and a second-stage operational amplifier A2, and an inverting input of the operational amplifier A1 is connected to a common terminal of the first and second capacitors 7 and 8, and The output is fed back to the inverting input of the operational amplifier A1 via a feedback capacitor 11, and a switch SW5 is connected in parallel to both ends of the capacitor 11. The non-inverting input of the operational amplifier A1 is connected to the reference power supply Vr via the capacitor 14. The inverting input of the second-stage operational amplifier A2 is connected to the output of the first-stage operational amplifier A1 via the switch SW6 and the capacitor 12, and the non-inverting input of the operational amplifier A2 is connected to the reference power supply Vr and the switch SW7. Through the capacitor 1
The output of the operational amplifier A2 is connected to the capacitor 13
, And is fed back to the non-inverting input of the first operational amplifier A1 via the switch SW8.

The switches SW1, SW4 and SW6 are opened and closed at the first timing φ1 shown in FIG. 9, and the switches SW2, SW3, SW5, SW7 and SW8 are opened and closed in FIG.
At the second timing φ2.

At the timing of φ1, switches SW1, SW
4 is opened and closed, the power supply voltage Vs is coupled to one terminal of the first capacitor 7 constituting the sensor element 9 via the closed switch SW1, and the other terminal of the second capacitor 8 is closed. It is grounded via switch SW4. At this time, if the intermediate electrodes (common terminals) of the first and second capacitors 7 and 8 are not connected to the inverting input of the first-stage operational amplifier A1, the first and second capacitors are not connected.
Equal charges are accumulated in the capacitors 7 and 8, and as a result, the common terminal voltage Vm becomes a value represented by the following equation.

Vm = C1 / (C1 + C2) · Vs = Vs / 2 · [1 + S] (1) S is the sensor sensitivity and is represented by S = (C1−C2) / (C1 + C2). For example, for convenience C1 = Co / (1-X) C2 = Co / (1 + X) X: Assuming the relative displacement of the electrode spacing (between the fixed electrodes 4, 5 and the mass 1) with respect to the initial spacing, S = X And the sensitivity S corresponds to the relative displacement X of the electrode interval.

In the case of an interface circuit for a capacitive sensor, it is preferable to use a method in which a voltage value represented by the above equation (1) is taken out with a low impedance and an output proportional to the relative displacement X of the electrode interval is obtained. However, the sensor element 9 is usually formed of a capacitor having a capacitance value of several pF to several tens of pF, and has an extremely large output impedance, so that impedance conversion is performed using the subsequent impedance conversion circuit 10. Basically, the impedance conversion circuit 10 includes the first and second capacitors 7,
8 is a circuit in which the non-inverting input voltage of the operational amplifier A1 is determined so that the same amount of charge is stored in the circuit 8.

[0012]

However, in the prior art, there is a drawback that the input offset voltage Vos1 of the operational amplifier A1 is amplified and appears at the output as represented by the equation (2). The feedback capacitance C3 of the feedback third capacitor 11 cannot be made too small due to the stability of the operational amplifier A1, and the total capacitance (C1 + C2) of the capacitances C1 and C2 of the first and second capacitors 7, 8 is equal to As the capacitance C3 of the three capacitors 11 becomes smaller, the input offset voltage Vos1 is amplified by the ratio and appears at the output.

Vout = {C1 / (C1 + C2)} · Vs + [C3 / (C1 + C2) −1] · Vos1 (2) As a result, the temperature dependence of the input offset voltage Vos1 is amplified and output, and the output of the sensor is output. There is a problem that the temperature characteristics are deteriorated.

As shown in FIG. 7, when the common terminal 3 of the differential capacitive sensor element 9 is grounded to a ground potential or a constant potential, the circuit shown in FIG. 8 cannot be applied as it is. There was a problem.

Further, when an amplification process is performed by an operational amplifier in the subsequent stage in order to improve the sensitivity output of the sensor, there has been a problem that the temperature offset drift caused by the sensor element 9 is also increased. .

Since the common terminal 3 of the sensor element 9 has a floating potential and a value Vm represented by the equation (1) in accordance with the displacement of the mass body 1, the actuating electrode newly facing the terminal 3 The electrostatic force between the electrodes by the voltage Va applied to the actuating electrodes is
When the actuator is used as the actuating function, there is a problem that the substantial inter-electrode voltage becomes Va-Vm, and the self-diagnosis output is reduced because Va itself cannot be used effectively.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned various problems, and an object of the present invention is to provide a stable capacitive sensor interface circuit having a small offset output and its temperature dependency. It is.

[0018]

In order to achieve the above object, the present invention relates to a differential capacitance type sensor having a common electrode grounded to obtain an output voltage proportional to the relative displacement between the electrodes. And a switching mechanism for reducing the output offset voltage due to the input offset voltage of the operational amplifier. Furthermore, in order to double the sensitivity while not doubling the offset temperature drift amount of the sensor element, a power supply switch is newly provided, and a method of differentially amplifying the voltage sampled in each power supply phase is adopted.

An interface circuit for a capacitive sensor according to the first aspect of the present invention has first and second capacitors whose at least one of the capacitances fluctuates.
In an interface circuit connected to a capacitive sensor in which a common terminal of a capacitor is fixed to a ground potential or a constant potential, a first operational amplifier and a feedback connected between an output terminal and an inverting input terminal of the first operational amplifier. A third capacitor having one terminal connected to an output terminal of the first operational amplifier, and a hold capacitor connected between a non-inverting input terminal of the first operational amplifier and a reference power supply. A sixth capacitor; a second operational amplifier having a non-inverting input terminal connected to the reference power supply;
A fifth capacitor for feedback connected to an output terminal of the operational amplifier and an inverting input terminal, wherein the first capacitor is charged up with a reference voltage in a first switching cycle, and the second and third capacitors are charged. Is reset, the other terminal of the fourth capacitor is connected to a reference power supply, and the output of the second operational amplifier is connected to the non-inverting input of the first operational amplifier.
At the inversion timing of the first switching cycle,
The first and second capacitors are short-circuited and connected to the inverting input of the first operational amplifier, and the other end of the fourth capacitor is connected to the inverting input of the second operational amplifier.

An interface circuit for a capacitive sensor according to a second aspect of the present invention has first and second capacitors whose at least one of the capacitances varies, and a common terminal of the first and second capacitors is grounded. A first operational amplifier, a third capacitor having one terminal connected to an output terminal of the operational amplifier, and a first capacitor connected to the capacitive sensor fixed to a potential or a constant potential. A fourth capacitor having a terminal connected to the output terminal of the first operational amplifier, a sixth capacitor for holding connected between a non-inverting input terminal of the first operational amplifier and a reference power supply, and a non-inverting input terminal. A second operational amplifier connected to the reference power supply, and a fifth capacitor for feedback connected to an output terminal and an inverting input terminal of the second operational amplifier Wherein the first capacitor in the first switching cycle is charged up at the reference voltage, the second charge of the third capacitor is reset, the other terminal of said third capacitor the first
Connected to a non-inverting input terminal of an operational amplifier, the other terminal of the fourth capacitor is connected to the reference power supply, and an output of the second operational amplifier is connected to a non-inverting input of the first operational amplifier; At the inversion timing of the first switching cycle, the first and second capacitors are short-circuited and connected to the inversion input of the first operational amplifier, and the other end of the third capacitor is connected to the inversion input of the first operational amplifier. And the other end of the fourth capacitor is connected to the inverting input of the second operational amplifier.

According to a third aspect of the present invention, there is provided an interface circuit for a capacitive sensor, wherein in a second switching cycle having a period longer than the first switching cycle,
A first and a second that alternately reverse the connection of the power supply and sample and hold the voltage output at each power supply polarity.
It further comprises a sample hold circuit and a differential amplifier for differentially amplifying two hold voltages by the first and second sample hold circuits.

According to a fourth aspect of the present invention, there is provided an interface circuit for a capacitive sensor, wherein an actuating electrode is provided as a new fixed electrode on at least one of the fixed electrodes forming the first and second capacitors. is there.

[0023]

Embodiments of the present invention will be described below with reference to the accompanying drawings. In the following description, portions that are the same as or correspond to those in the above-described conventional example are denoted by the same reference numerals.

Embodiment 1 FIG. 1 shows an interface circuit of a capacitive sensor according to Embodiment 1 of the present invention.

In the first embodiment, when the common terminal (terminal 3) of the capacitive sensor element 9 shown in FIG. 7 is grounded, the impedance conversion for obtaining the output voltage represented by the equation (2) is performed. It relates to the circuit 21.

In FIG. 1, one ends of first and second capacitors 7 and 8 composed of variable capacitors are connected to each other and grounded, and a switch SW12 is connected between the other ends.
Is connected. The other end of the first capacitor 7 is connected to the power supply Vs via the switch SW11, the other end of the second capacitor is grounded via the switch SW13, and the first end of the impedance conversion circuit 21 via the switch SW14. It is connected to the inverting input of the operational amplifier A1. The other configuration of the impedance conversion circuit 21 is substantially the same as the configuration of the impedance conversion circuit 10 of FIG. 8, and thus a detailed description thereof will be omitted.

The switches SW11, SW13, SW
5, SW7 and SW8 are turned on and off at the timing of φ1 in FIG. 9, and the switches SW12, SW14 and SW8
6 is turned on and off at the timing of φ2 in FIG.

As shown in FIG. 1, at the timing of φ1,
The first capacitor 7 of the capacitive sensor element 9 has a power supply V
s and the other second capacitor 8
Is connected to ground, and the charge stored there is released.

When the switch SW12 is turned on at the timing of φ2 to move the electric charge accumulated in the first capacitor 7 to the second capacitor 8, both ends of the first and second capacitors 7 and 8 are redistributed by the electric charge. The common potential is Vm represented by Expression (2). Therefore, an output voltage proportional to the relative displacement X between the electrodes can be obtained. The subsequent impedance conversion circuit 21 controls the sixth capacitor 14 so that the first-stage operational amplifier A1 has the voltage Vm.
It has a function of adjusting the potential of (capacitance C6), and is almost the same as the impedance conversion circuit 10 in FIG.
In the above, the order in which the first capacitor 7 is charged up and the charge of the second capacitor 8 is released in the phase φ1 is adopted, but the first capacitor 7 and the second capacitor 8 may be exchanged.

The system of the first embodiment has a large number of differential capacitive sensor elements described in the first embodiment when the common terminal 3 of the sensor element 9 of FIG. 7 is grounded. The same can be applied to a case or a case where a sensor element according to a second embodiment described later uses a full capacitance bridge.

Embodiment 2 FIG. 2 shows an interface circuit of a capacitive sensor according to Embodiment 2 of the present invention.
In the first embodiment, the configuration in which the common terminal 3 of the sensor element 9 can be grounded has been described. However, in the circuit of the first embodiment, similarly to the conventional method, as shown in Expression (2), The input offset voltage Vos1 of the first-stage operational amplifier A1 is amplified and output, thereby deteriorating the temperature characteristics of the sensor.

In the second embodiment, as shown in FIG.
Two switches SW9 and SW10 are newly added to the impedance conversion circuit 31. That is, the capacitor 1
1 is connected to the inverting input of the operational amplifier A1 of the first stage via the switch SW9, and the switch 10
To the non-inverting input of the first stage operational amplifier A1. As a result, the output voltage difference (Vos2−Vos1) between the input offset voltages of the first-stage and second-stage operational amplifiers A1 and A2 is amplified and output to Vout as represented by Expression (3). Will be.

Vout = {C1 / (C1 + C2)}. Vs + {C3 / (C1 + C2)}. (Vos2-Vos1) -Vos1 (3) On the other hand, in an operational amplifier manufactured through a semiconductor IC process, Vos1 and Vos2 are almost equal to each other. Since they can be set equal, the second term of Expression (3) is cancelled, and the occurrence of offset output can be suppressed.

FIG. 2 shows an example in which an actuation electrode 33 is newly provided. This actuation electrode 3
By applying the voltage Va to the mass 3, the mass body 1 grounded to the ground can be actuated by electrostatic attraction. In this case, unlike the conventional method, the voltage Va itself can be effectively used for generating an electrostatic force between the two electrodes, so that the actuating displacement can be increased as compared with the conventional method.

Embodiment 3 FIG. 3, 4, and 5 show an interface circuit 41 of a capacitive sensor according to a third embodiment of the present invention.

The interface circuit 41 shown in FIG.
3 has basically the same circuit configuration as the impedance conversion circuit 31 and the capacitive sensor element 9 and includes the impedance conversion circuit and the sensor element 9 and has three terminals A, B, and C specified in FIG. have.
These terminals A, B and C are connected to the power switch 4 shown in FIG.
5 and the sample hold circuit 46, and the sample hold circuit 46 is connected to the differential amplifier 47. Switches SW13-SW18 of each element shown in FIG.
Are ON / OFF controlled at the drive clock timings φ3-φ6 shown in FIG.

Switches SW13-SW operated by clocks φ3, φ4 longer in period than clocks φ1, φ2
The direction of the power supplied to the sensor element 9 is controlled by 16. The ON state of the switches SW13 and SW16 is the same as the configuration shown in FIG. 6, and the output immediately before the sample and hold circuit 46 is the switch SW13 and SW16.
Vma = ｛C1 / (C1 +
C2) Converge to ΔVs. This voltage is sampled and held by opening and closing the switch SW17 at the timing of φ5. On the other hand, when the switches SW14 and SW15 are on, the output immediately before the sample hold circuit 46 converges to Vmb = {C2 / (C1 + C2)} Vs,
Similarly, the sample is held by opening and closing the switch SW18 at the timing of φ6. The difference between Vma and Vmb is calculated by the subsequent differential amplifier 47, and as a result, the final output Vm is expressed by the following equation (4).

Vm = {(C1−C2) / (C1 + C2)} Vs = SVs (4) Comparing Expressions (3) and (4), it is found that the sensitivity is doubled by adopting this method. . On the other hand, the sensitivity of the sensor is usually performed, for example, by adjusting the gain of the subsequent operational amplifier. However, if the circuit including the subsequent operational amplifier includes noise components such as DC offset output dependency and noise, These noises appear in the output after being multiplied by the amplification gain. However, in this method, by doubling the sensitivity of the sensor element 9, it is possible to halve the subsequent circuit gain, and as a result, it is possible to double the SN ratio of the sensor.

In the above description of the embodiment, variable capacitors are used as the first and second capacitors 7 and 8 of the capacitive sensor element 9. However, at least one of these two capacitors is used. A variable capacitor may be used, and the other capacitor may be used as a constant capacitor.

[0040]

As described above, according to the present invention, the following excellent effects can be obtained. The interface circuit of the capacitance detection type sensor can be used even when the common terminal of the sensor element is grounded. Further, a circuit for compensating for the influence of temperature fluctuations of the input offset voltage of the operational amplifier included in the impedance converter can provide a detection circuit with less temperature dependence of the DC offset output voltage. Further, since the sensitivity can be doubled without amplifying the DC offset temperature dependency caused by the amplifier circuit, a circuit capable of doubling the SN ratio of the sensor can be provided. Furthermore, a large self-diagnosis output can be obtained using the actuation electrode.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of an interface circuit of a capacitive sensor according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of an interface circuit of a capacitive sensor according to a second embodiment of the present invention.

FIG. 3 is a diagram illustrating an example of an interface circuit of a capacitive sensor according to a third embodiment of the present invention.

FIG. 4 is a diagram illustrating an example of an interface circuit of a capacitive sensor according to a third embodiment of the present invention and peripheral circuits connected to the interface circuit.

FIG. 5 is a diagram showing a clock for driving a switch of an interface circuit of the capacitive sensor according to the third embodiment of the present invention.

FIG. 6 is an electric circuit diagram showing an example of a conventional capacitive acceleration sensor.

FIG. 7 is a diagram showing an equivalent circuit of a conventional capacitive acceleration sensor.

FIG. 8 is a diagram illustrating an example of an interface circuit of a conventional capacitive sensor.

FIG. 9 is a diagram showing a clock for driving a switch of an interface circuit of a conventional capacitive sensor.

[Explanation of symbols]

1 mass body, 2 anchor parts, 3 beams, 4 fixed electrodes,
5 fixed electrode, 6 silicon, 7 first capacitor, 8
Second capacitor, 9 capacitive sensor element, 10
Impedance conversion circuit, 21 impedance conversion circuit, 31 impedance conversion circuit, 41 interface circuit (impedance conversion circuit + sensor element), 33 actuation electrode, 45 power switch, 46 sample hold circuit, 47 differential amplifier.

Claims (4)

[Claims] 1. A capacitor having at least one of a first capacitor and a second capacitor having a variable capacitance, and a common terminal of the first and second capacitors connected to a capacitance sensor fixed to a ground potential or a constant potential. A first operational amplifier; a third capacitor for feedback connected between an output terminal and an inverting input terminal of the first operational amplifier; and one terminal connected to an output terminal of the first operational amplifier. , A sixth capacitor for holding connected between a non-inverting input terminal of the first operational amplifier and a reference power supply, and a second operation circuit having a non-inverting input terminal connected to the reference power supply. An amplifier, and a fifth capacitor for feedback connected to an output terminal and an inverting input terminal of the second operational amplifier. The first capacitor is charged up with a reference voltage, the charges of the second and third capacitors are reset, the other terminal of the fourth capacitor is connected to a reference power supply, and the output of the second operational amplifier is Are connected to the non-inverting input of the first operational amplifier, and at the inversion timing of the first switching cycle, the first and second capacitors are short-circuited and connected to the inverting input of the first operational amplifier, 4. An interface circuit for a capacitive sensor in which the other end of the capacitor is connected to the inverting input of the second operational amplifier. 2. A capacitive sensor having at least one of first and second capacitors whose capacitance varies, and a common terminal of the first and second capacitors being fixed to a ground potential or a constant potential. In the interface circuit to be connected, a first operational amplifier, one terminal is connected to a third capacitor for feedback connected to an output terminal of the operational amplifier, and one terminal is connected to an output terminal of the first operational amplifier. A fourth capacitor, a sixth capacitor for holding connected between the non-inverting input terminal of the first operational amplifier and a reference power supply, a second operational amplifier having a non-inverting input terminal connected to the reference power supply, A fifth capacitor for feedback connected to an output terminal of the second operational amplifier and an inverting input terminal; and The capacitor is charged up with the reference voltage, the charge of the second and third capacitors is reset, the other terminal of the third capacitor is connected to the non-inverting input terminal of the first operational amplifier, and The other terminal is connected to the reference power supply, and the output of the second operational amplifier is connected to the non-inverting input of the first operational amplifier. The first and second operational amplifiers are connected at the inversion timing of the first switching cycle. The second capacitor is short-circuited and connected to the inverting input of the first operational amplifier, the other end of the third capacitor is connected to the inverting input terminal of the first operational amplifier, and the other end of the fourth capacitor is connected to the second Interface circuit for capacitive sensors connected to the inverting input of the operational amplifier. 3. A first and a second sampler for alternately reversing the connection of power supplies in a second switching cycle having a period longer than the first switching cycle, and sampling and holding voltages output at respective power supply polarities. The interface circuit for a capacitive sensor according to claim 1, further comprising: a hold circuit; and a differential amplifier that differentially amplifies two hold voltages of the first and second sample hold circuits. 4. The capacitive sensor according to claim 1, wherein an actuating electrode is provided as a new fixed electrode on at least one of the fixed electrodes forming the first and second capacitors. Interface circuit.