JPH11132787A - Electricity quantity detecting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a large output with a simple circuit configuration in detecting the electric quantities of micro capacitance change and charge change. SOLUTION: The circuit for detecting electric quantities of capacitance change or charge change in a first capacitor 11 is provided with an operational amplifier 10, a time differential operation means which is connected to an non- inverting input terminal of the operational amplifier 10 and has the first capacitor 11 and a first resistance 21, and a time integrating operation means which is connected to an inverting input terminal of the operational amplifier 10 and has a second capacitor 23 and a second resistance 22 constituting a feedback capacity. The differential operation means in the non-inverting input terminal side differentially operates the minimum capacity change in the first capacitor and the integral operation means in the inverting input terminal side integrates this so that the micro capacitance change is converted into a voltage and to be detected. Even if the feedback capacity 23 is large, the enlargement of the ratio of the resistances 21, 22 can sufficiently enlarge the amplification factor of the operational amplifier 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electric quantity detection circuit,
For example, the present invention relates to a circuit used for a capacitance-type physical quantity measuring device and the like, particularly to a circuit capable of detecting a minute amount of electricity.

[0002]

2. Description of the Related Art A detection circuit having a configuration as shown in FIG. 3 is used for detecting a minute change in capacitance of a capacitor which is a capacitance sensor used for physical quantity measurement and the like. The detection circuit has an operational amplifier 30, and a capacitive sensor CS is provided at an inverting input terminal of the operational amplifier 30, and a voltage Vbb is applied to the capacitive sensor CS. In addition, a feedback capacitance CF is arranged in a feedback path between the output terminal and the inverting input terminal of the operational amplifier 30, and the non-inverting input terminal of the operational amplifier 30 is grounded. Then, the operational amplifier 30 converts the capacitance change ΔCS in the capacitance sensor CS connected to the non-inverting input terminal into an electric signal to generate an output voltage Vout.

[0003] Small capacitance change ΔC in capacitance sensor CS
The relationship between S and the output voltage Vout of the operational amplifier 30 is expressed by the following equation (1).

[0004]

Vout = (ΔCS / CF) · Vbb (1) As is apparent from the above equation (1), if the feedback capacitance CF can be reduced, the gain of the amplifier 30 can be increased. The desired output voltage Vout cannot be obtained because of the influence of the parasitic capacitance CC existing in parallel with the feedback capacitance CF.
That is, when the parasitic capacitance CC becomes not negligible with respect to the feedback capacitance CF, the equation (1) becomes as follows.

[0005]

Vout = {ΔCS / (CF + CC)} · Vbb (2) Therefore, the magnitude of the output voltage Vout obtained for the small capacitance change ΔCS is limited by the parasitic capacitance CC.
Further, the actually obtained output voltage depends on the parasitic capacitance CC, which makes it difficult to arbitrarily set the output voltage Vout. Further, when the parasitic capacitance CC greatly depends on the environment such as the temperature, the gain becomes very unstable with respect to the temperature change.

In order to solve such a problem caused by the parasitic capacitance CC, the feedback capacitance CF is set to be larger than the parasitic capacitance CC, that is, the gain of the detection circuit formed by the amplifier 30 is reduced, and the feedback capacitance CF shown in FIG. Thus, it is conceivable to provide another stage amplifier so that the next stage amplifier 32 has a desired gain.

[0007]

However, the addition of the next-stage amplifier 32 as shown in FIG. 4 results in an increase in the number of components and the area occupied by the circuit. For example, when a circuit element as described above for detecting a change in capacitance in this minute gap is formed on a semiconductor substrate on which a minute gap is formed as a capacitance sensor, a reduction in the size of the entire device is required. ing. Therefore, the addition of the next-stage amplifier 32 cannot meet the demand for miniaturization of the device.

When the operational amplifier used has an offset voltage, the output voltage may be saturated by the offset voltage due to the large open gain of the operational amplifier. In such a case, as shown in FIG.
A feedback resistor RF is added in parallel with F so that direct feedback is applied. In such a case, the feedback capacitance CF and the resistance value RF of the feedback resistor need to be set as in the following equation.

[0009]

[Number 3] _{RF >> 1 / (ω L ·} CF) ω L: the lowest frequency ... of the capacitance change detection (3) For example, if you have CF = 1pF, and ω _L = 1kHz, 1
/ (Ω _L · CF) ≒ 16 MΩ, which requires a larger resistance value as the resistance RL, which makes it difficult to handle in actual use. If the resistance RF is reduced to give priority to ease of handling, it is necessary to increase the feedback capacitance CF, and as can be seen from the equation (1), the output voltage Vout decreases.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and it is possible to obtain a small capacitance change .DELTA.CS as a larger voltage amplitude while eliminating the need for a conventional next-stage amplifier. It is an object to reduce the number of components in a quantity detection circuit, reduce the area occupied by a circuit, and realize a stable operation of the circuit.

[0011]

In order to achieve the above object, an electric quantity detection circuit according to the present invention is a circuit for detecting an electric quantity of a change in capacitance or a change in electric charge in a first capacitor. A temporal differential operation means connected to the non-inverting input terminal of the amplifier and having the first capacitor and the first resistor; and a second capacitor and a second resistance connected to the inverting input terminal of the operational amplifier. A second integration capacitor, and the second capacitor is connected to a feedback path between the inverting input terminal and the output terminal of the operational amplifier.

Small capacitance change ΔC in first capacitor
With respect to S, the voltage Vd obtained by the differential operation means can be expressed as the following equation (4).

[0013]

Vd = K1 · d / dt (ΔCS) where K1 is a constant (4) Further, the output voltage Vout from the output terminal is the result of integrating the voltage Vd by the integration operation means. The output voltage Vou is proportional to the small capacitance change ΔCS as shown in the equation (5).
t is obtained.

[0014]

(Equation 5) Further, in the present invention, the small capacitance change ΔC of the sensor capacitance CS
The output voltage Vout when S is detected is as shown in the following equation (6).

[0015]

(Equation 6) In the above equation (6), V′out and K are expressed by the following equations (7) and (8).

[0016]

V′out = (ΔCS / CF) · Vbb (7)

K = RL / R1 (8) Therefore, in the electric quantity detection circuit of the present invention, the differential operation means is provided on the non-inverting input terminal side of the operational amplifier, and the integration calculating means is provided on the inverting input terminal side. The resistance ratio K (= RL / R
If 1) is set to be larger than 1, a sufficient amplification effect can be obtained, and the conventionally required next-stage amplifier can be eliminated. Further, as the gain can be increased by the resistance ratio K, the capacitor CF can also be increased. Therefore, the capacitor C
Even if there is a parasitic capacitance CC parallel to F, the large amount of the capacitor CF makes it less likely to be affected by the parasitic capacitance CC. As a result, stable operation can be achieved.

Further, in the circuit of the present invention, if the resistance ratio K is set to a large value, a second capacitor having a large capacitance CF can be used correspondingly, so that the output voltage Vou
t does not decrease. Further, for example, when a conventional feedback resistor RF as shown in FIG. 5 is provided, it is possible to use a feedback resistor having a small resistance value.

In this case, the gain is limited as a frequency band in which a change in the amount of electricity in the first capacitor can be amplified with a stable (constant) gain. Assuming a low-band cutoff frequency FCL and a wide-band cutoff frequency FCH, these can be expressed as follows.

FCL = 1 / (2π · CS · RL) FCH = 1 / (2π · CF · RF) or FCH = (determined by the frequency characteristic of the operational amplifier) Therefore, the first detectable signal with a stable gain G As shown in FIG. 6A, the frequency f of the change in the amount of electricity of the capacitor is approximately FCL <F <FCH.

In the present invention, in addition to the above configuration, a configuration is also provided in which a non-inverting input terminal of the operational amplifier is provided with a third capacitor serving as a reference capacitor in parallel with a first capacitor including a capacitance sensor. Can be. A third capacitor is provided in parallel with the first capacitor.
The two capacitors are set to have the same amount of charge in the initial state, and the amount of electricity such as a change in the capacity or the amount of charge in the first capacitor is detected by comparison with the third capacitor.

In the present invention, in addition to the above-described configuration, a configuration in which a non-inverting input terminal of the operational amplifier is provided with a third capacitor forming a reference capacitance in parallel with a first capacitor including a capacitance sensor is also applied. Can be. For example, the voltage Vbb1 is applied to the input terminal of the first capacitor which is not connected to the non-inverting input of the operational amplifier, and the voltage Vbb2 is applied to the input terminal of the third capacitor. At this time, Vbb1, Vbb
2 is a sine wave AC voltage having the same magnitude and opposite polarity.

Vbb1 = Vbb · sin (ωs · t) =
Vbb · sin (2π · fs · t) Vbb2 = −Vbb · sin (ωs · t) = − Vbb ·
sin (2π · fs · t) In this way, the two capacitors are set so that the electric charge amounts in the initial state are equal, and the electric amount such as a change in capacitance or a change in electric charge amount in the first capacitor is compared with that of the third capacitor. Is detected by This makes it possible to detect a component that slowly changes from a direct current component in response to a change in the amount of electricity of the first capacitor. As shown in FIG. 6B, the frequency change range of the amount of electricity of the first capacitor that can be detected with a stable gain G is substantially from DC to (fs / 2).

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention (hereinafter, referred to as embodiments) will be described below with reference to the drawings.

[First Embodiment] FIG. 1 shows a configuration example of an electric quantity detection circuit according to an embodiment of the present invention. In FIG. 1, a first capacitor 11 forming a capacitance sensor is connected between a non-inverting input terminal (+) of an operational amplifier 10 and an input terminal 1, and a common terminal 2 and a non-inverting input of the operational amplifier 10 are connected. A first resistor 21 is connected between the terminals.
A second resistor 22 is connected between the inverting input terminal (−) of the operational amplifier 10 and the common terminal 2.
In the feedback path between the inverting input terminal and the output terminal 3, a second capacitor 23 forming a feedback capacitance is connected.

In such a configuration, the operational amplifier 10
The first capacitor 11 and the first resistor 21 connected to the non-inverting input terminal of the first capacitor 11 constitute a time differential operation means of an electric quantity such as a change in capacitance or a change in charge in the first capacitor 11,
The second resistor 2 connected to the inverting input terminal of the operational amplifier 10
The second and second capacitors 23 constitute a temporal integration calculating means. Further, the voltage V between the input terminal 1 and the common terminal 2
bb is applied, and the differential operation means
Is differentiated, and the integration operation means integrates the difference to convert the minute capacitance change of the first capacitor 11 into a voltage Vout between the output terminal 3 and the common terminal 2. This voltage Vout is output as an output signal.

In such a detection circuit, the first capacitor 11 can be configured as a well-known semiconductor capacitance sensor. For example, a diaphragm is formed on a semiconductor substrate with a minute gap therebetween, and electrodes are formed so as to be located on the surface facing the minute gap. When the physical quantity to be measured is pressure, the capacity of the minute gap changes due to the change in the formed minute gap due to the pressure, and it is possible to measure the pressure, which is the physical quantity by detecting this change in capacity. I have. Examples of the physical quantity that can be measured by detecting the quantity of electricity in the minute gap include acceleration, angular velocity, and the like in addition to the above pressure. In the first embodiment, the operational amplifier 10
Can be formed on the same semiconductor substrate as the capacitance sensor.

Next, a method of detecting a small capacitance change ΔCS of the capacitance CS of the first capacitor in the circuit of the present embodiment will be described. The resistance value RL of the first resistor 21 and the second resistor 2
2 has a resistance value R1

It is assumed that RL> R1 (9)

The minimum frequency of the small capacitance change ΔCS to be detected is ω _L, and the resistance value RL of the first resistor 21 is
It is assumed that the capacitance CS of the capacitor 11 has a relationship such as the following equation (10).

[0029]

RL >> 1 / (ω _L × CS) (10) An appropriate constant voltage Vbb is applied between the input terminal 1 and the common terminal 2, and the common terminal 2 is grounded. Therefore, in the steady state, the voltage of the non-inverting input terminal and the inverting input terminal of the operational amplifier 10 have the same potential as the common terminal 2, so that the voltage applied to the first capacitor 11 becomes the voltage Vbb.

In this state, when a small capacitance change ΔCS occurs in the capacitance CS of the first capacitor 11, a charge change ΔQS expressed by the following equation (11) is generated in the first capacitor 11 by the small capacitance change ΔCS. Occurs.

[0031]

ΔQS = ΔCS · Vbb (11) The charge change ΔQS is a charge / discharge current ΔIS to the common terminal 2 via the first resistor 21. The time change of the charge change ΔQS is caused by the charge / discharge current ΔIS of the first capacitor 11.
Becomes This ΔIS can be expressed as the following equation (12).

[0032]

ΔIS = d / dt (ΔQS) (12) When the charging / discharging current ΔIS flows through the first resistor 21, a voltage change ΔVS occurs in the first resistor 21, and the non-inverting input of the operational amplifier 10 is obtained. A voltage change ΔVS as shown in the following equation (13) is applied to the terminal.
Will occur.

[0033]

(Equation 13) On the other hand, since the inverting input terminal of the operational amplifier 10 follows the voltage of the non-inverting input terminal, a voltage change ΔVS also occurs at the voltage of the inverting input terminal, that is, at both ends of the second resistor 22. As a result, the current ΔI as shown in Expression (14) is applied to the second resistor 22.
1 flows, and this current ΔI1 becomes a charge / discharge current of the second capacitor 23, and a charge change ΔQF as shown in Expression (15) occurs in the capacitor 23. As a result, the operational amplifier 10
The output voltage Vou as shown in equation (16)
t occurs.

[0034]

[Equation 14]

(Equation 15)

(Equation 16) However, in the above equation (16), K is given by the following equation (17)

K = RL / R1 (17)

The first resistor 21 (RL) shown in equation (17)
It is easy to set the resistance ratio K of the second resistor 22 (R1) to 1 or more. Therefore, according to the circuit configuration of the first embodiment, it is possible to easily obtain an amplification effect of K times the resistance ratio, and it is not necessary to provide a further amplifier in the circuit of FIG.

When there is a parasitic capacitance CC in parallel with the second capacitor 23 shown in FIG.
6) is expressed by the following equation (18).

[0037]

(Equation 18) In the circuit configuration according to the first embodiment, the capacitance of the second capacitor 23 is set to be larger than the parasitic capacitance CC, and the resistance ratio K is set to be larger than 1 by that.
Operation can be performed stably without being affected by the above. At the same time, a large output voltage Vout can be obtained.

The case where the variation ΔQS of the charge QS stored in the capacitance CS of the first capacitor is detected can be considered in the same manner as described above. For example, when the capacitance CS of the first capacitor 11 is constant and the voltage change ΔVbb of the voltage Vbb between the input terminal 1 and the common terminal 2 causes the charge change ΔQS of the first capacitor 11, the charge change ΔQS is represented by the following equation (19).

[0039]

ΔQS = CS · ΔVbb (19) Then, if the above equation (11) is replaced with this equation (19), the output voltage Vout according to the voltage change ΔVbb as in the following equation (20) Is obtained.

[0040]

Vout = ΔQF / CF = K · (CS / CF) · ΔVbb (20) In the equation (20), the first resistor 21 and the second resistor 21 are used as described above.
It is easy to set the resistance ratio K of the resistor 22 to 1 or more, stable operation can be performed without being affected by the parasitic capacitance CC, and a large output voltage Vout can be obtained.

In the circuit according to the first embodiment, FIG.
As shown in (a), generally, the lower cutoff frequency FC
It is possible to detect the change with a stable gain G in an intermediate frequency band between L and the wide cutoff frequency FCH.

[Second Embodiment] FIG. 2 is a diagram showing a circuit configuration example of an electric quantity detection circuit according to a second embodiment. In this example, the first capacitor 1 which is the capacitance sensor of the first embodiment is used.
A third capacitor 12, which forms a reference capacitance in parallel with 1, is added. Note that the third capacitor 12 is disposed between the second input terminal 4 and the non-inverting input terminal of the operational amplifier 10. In the configuration of the second embodiment, when the voltage Vbb2 is applied between the input terminal 4 and the common terminal 2 and the voltage Vbb1 is applied between the input terminal 1 and the common terminal 2, the output terminal 3 A voltage Vout between the common terminal 2 is output as an output signal.

Here, in order to simplify the operation, the voltage V
bb2 is a sine-wave AC voltage having the same magnitude as the voltage Vbb1 and the opposite polarity as shown in the equations (21) and (22) (note that Vbb1 and Vbb2 have opposite polarities). The operation in the case of the DC voltage is similar to that of the first embodiment in terms of the detection signal frequency.)

[0044]

Vbb1 = Vbb · sin (ωs · t) (21)

Vbb2 = −Vbb · sin (ωs · t) (22) Further, it is assumed that the initial capacitance CS of the first capacitor 11 is equal to the capacitance CR of the third capacitor 12, and the voltage is It is assumed that the frequency ωs of Vbb1 and Vbb2, the resistance value RL of the first resistor 21, and the capacitance CS of the first capacitor 11 have the following relationship (23).

[0045]

RL >> 1/1 / (ωs · CS) (23) The charge amount QS of the first capacitor 11 and the third capacitor 12
When the charge amount QR is calculated, the equations (24) and (25) are obtained, respectively.

[0046]

QS = CS · Vbb1 = CS · Vbb · sin (ωs · t) (24)

## EQU25 ## QR = CS.Vbb2 = -CS.Vbb.sin (.omega.s.t) (25) Then, the charge amount change .DELTA.Q at the non-inverting input terminal of the operational amplifier 10 is expressed by the following equation (26). become,

ΔQ = QS + QR = 0 (26) The charge amount change ΔQ is canceled out to zero.

Under such conditions, the capacitance sensor 11
When a small capacitance change ΔCS occurs at the non-inverting input terminal of the operational amplifier 10,

ΔQ = ΔCS · Vbb (27)

The change in charge amount ΔQ becomes a current ΔI flowing through the resistor 21, and causes a voltage change ΔV as shown in the following equation (28) at the non-inverting input terminal of the operational amplifier 10.

[0049]

ΔV = RL · d / dt (ΔQ) = RL · Vbb · d / dt (ΔCS) (28) At the inverting input terminal of the operational amplifier 10, the voltage change ΔV at the non-inverting input terminal is A tracked voltage change ΔV occurs.
Therefore, the current change ΔI is applied to the second resistor 22 on the inverting input terminal side.
Occurs, and the capacitor 23 is charged and discharged by the current change ΔI. As a result, the output voltage Vout at the output terminal 3 of the operational amplifier 10 is as shown in the following equation (29).

[0050]

(Equation 29) However, in the above equation (29), K is K = RL / R1.

As is apparent from the equation (29), if the resistance ratio K between the first resistor 21 and the second resistor 22 is set to 1 or more,
The small capacitance change ΔCS in the first capacitor 11 is amplified by an arbitrary K times by the circuit of the second embodiment, and is output from the output terminal 3 as the output voltage Vout.

The parasitic capacitance C is connected in parallel with the capacitor 23.
If C exists, equation (29) becomes:

[0053]

[Equation 30] Also in the circuit configuration according to the second embodiment, similarly to the first embodiment, the capacitance of the second capacitor 23 is changed to the parasitic capacitance CC.
If the resistance ratio K is set to a value larger than 1, a stable operation can be performed without being affected by the parasitic capacitance CC, and a large output voltage Vout can be obtained at the same time.

In the circuit according to the second embodiment, FIG.
As shown in (b), it is possible to detect the change with a stable gain G in a frequency range from DC to fs / 2. In the second embodiment, the provision of the third capacitor serving as the reference capacitance provides an effect of withstanding noise generated by individual components in the circuit.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a circuit configuration example of an electric quantity detection circuit according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a circuit configuration example of an electric quantity detection circuit according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a circuit configuration for detecting a minute capacitance of a conventional capacitance sensor.

FIG. 4 is a diagram showing a conventional circuit configuration when an output voltage amplitude is amplified by adding a next-stage amplifier to the configuration of FIG. 3;

FIG. 5 is a diagram showing a conventional circuit configuration when a feedback resistor RF is added in parallel with a feedback capacitor CF.

FIG. 6 is a diagram illustrating a frequency band that can be detected by the electric quantity detection circuit according to the present invention.

[Explanation of symbols]

1 input terminal, 2 common terminal, 3 output terminal, 4 second
Input terminal, 10 operational amplifier, 11 first capacitor (capacitance sensor), 12 third capacitor (reference capacitance),
21 first resistor, 22 second resistor, 23 second capacitor (feedback capacitance).

Claims (1)

[Claims] 1. A circuit for detecting an electric quantity of a capacitance change or a charge change in a first capacitor, comprising: an operational amplifier connected to a non-inverting input terminal of the operational amplifier;
A temporal differential calculating means having a capacitor and a first resistor; a temporal integrating calculating means connected to the inverting input terminal of the operational amplifier and having a second capacitor and a second resistor;
An electric quantity detection circuit comprising: a second capacitor connected to a feedback path between the inverting input terminal and the output terminal of the operational amplifier.