JP2003207529A - Sensor capacity sensing apparatus and sensor capacity sensing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stabilize the circuit operation even in the use of the capacitor of the feedback circuit of an arithmetic amplifier. <P>SOLUTION: A capacitor C and a impedance converter Hiz are serially inserted to the feedback circuit of a first arithmetic amplifier OP1, and the electrode P1 of a capacity sensor is connected to the connecting point between the capacitor and the converter through a signal line L. The signal line L is connected to a prescribed reference potential through a resistor R3 having a high resistance value. Although the signal line is laid in a floating state to make the circuit operation unstable when the capacitor is inserted to the feedback circuit, the operation is stabilized since the signal line L is fixed to the prescribed potential through the high resistor R3. The impedance converter may be formed of a voltage follower to connect the resistor R3 to its output. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for detecting a sensor capacitance, and more particularly, to precisely measure a minute capacitance, the potential of a connecting line between a capacitance sensor and a detector for capacitance measurement is measured. The present invention relates to a device and method for detecting a sensor capacitance that can be fixed.

[0002]

2. Description of the Related Art FIG. 5 shows a conventional sensor capacitance measuring device for measuring the sensor capacitance when the capacitance value changes at various frequencies, such as a condenser microphone. As shown in FIG. 5, the sensor capacitance measuring device includes an operational amplifier OP having a feedback resistance R _f and an AC voltage generator OSC that generates an AC voltage V _in , and an input terminal of the operational amplifier OP and an AC voltage. The sensor capacitance C _s is connected to the voltage generator OSC via the signal line L.

The conventional sensor capacitance detection device shown in FIG. 5 uses the AC voltage V _in from the AC voltage generator OSC to
A current flows through the sensor capacitance C _s . Since the input impedance of the operational amplifier OP is ideally infinite, and further, the two input terminals of the operational amplifier OP are in the immagnari short state, V _out = − from the output terminal of the operational amplifier OP. A voltage of (jω _in C _s ) · R _f · V _in is output. By processing the output voltage V _out , a value corresponding to the sensor capacitance Cs can be obtained.

In the conventional sensor capacitance detecting device shown in FIG. 5, a resistor R _f is used as a feedback impedance. V _in = V · sin ω _in t, and the sensor capacitance C _s changes at an angular frequency ω _c centered on a fixed standard capacitance C _d , that is, C _s = C _d , depending on the applied physical quantity. When + ΔC · sinω _c t, the output voltage V _out can be represented as follows. _{V out = -R f [(C} d + ΔC · sinω c t) · ω in · c
_{osω in t + ΔC · ω c} · cosω c t · sinω in t]
V · sin ω _in t As is apparent from this equation, the output voltage V _out includes a term proportional to the angular frequency ω _c of the sensor capacitance, and has a frequency characteristic that depends on the change frequency of the sensor capacitance C _s. Will end up. Therefore, it is necessary to provide a processing circuit for canceling the term proportional to the angular frequency ω _c in the subsequent stage of the sensor capacitance detection device, which increases the scale of the entire device.

Therefore, there has already been proposed a device capable of obtaining an output voltage V _out independent of the angular frequency ω _c of the sensor capacitance C _s by replacing the feedback resistor of the operational amplifier OP with a feedback capacitor. FIG. 6 shows a sensor capacitance detecting device using such a feedback capacitor C _f , and the output voltage V _out in this device can be expressed as follows. _{V out = - (C d +} ΔC · sinω c t) / C f · V · si
n ω _in t As is apparent from this equation, the output voltage V _out does not have the change frequency dependency of the sensor capacitance, and thus an additional circuit for canceling the term proportional to the angular frequency ωc component is not required.

[0006]

[Problems to be Solved by the Invention] Sensor capacitance detection shown in FIG.
In the device, the feedback impedance of the operational amplifier
Feedback capacitor C_fBecause the capacitor is used
C_fAnd sensor capacity C _sIs the signal line L connecting to
There is no current flowing in or out. Therefore, the signal line L is electrically
The potential becomes unstable because it becomes a floating state
And the circuit output will be saturated with the power supply voltage.
As a result, there is a problem that the circuit will not operate normally.
is there. The present invention solves the above-mentioned problems of the conventional example.
The purpose is to detect the sensor capacitance.
Use a capacitor in the feedback circuit of the operational amplifier in the circuit
Even if it is, you can fix the potential of the signal line.
To do so.

[0007]

In order to achieve the above-mentioned object of the present invention, a sensor capacitance for detecting the capacitance of a capacitance sensor according to the first aspect of the present invention, the capacitance of which changes in response to a change in a physical quantity. The detection device includes a voltage generator that supplies at least one of an AC voltage and a DC voltage, an operational amplifier, a capacitor, an impedance converter, a sensor connection portion to which a capacitance sensor can be connected at one end, and the impedance at the other end. The converter includes an input terminal of the converter and a signal line connected to the capacitor, and a first resistor having both ends connected to the signal line and a reference voltage. An output terminal of the voltage generator is an input of the operational amplifier. It is characterized in that the capacitor and the impedance converter are connected to a terminal and are inserted in a feedback path of the operational amplifier.

In the sensor capacitance detecting device according to the first aspect of the present invention, the first resistor is set so that there is substantially no inflow / outflow of current between the signal line and the first resistor. Is preferred. In addition, the first resistor is connected to the signal line, and when the capacitance of the capacitance sensor changes, the first resistor is connected to the signal line rather than the impedance when the feedback path or the capacitance sensor is viewed from the signal line. It is preferable that the impedance is set to be high when the first resistance is viewed.

According to a second aspect of the present invention, a sensor capacitance detecting device for detecting the capacitance of a capacitance sensor whose capacitance changes according to a change in a physical quantity is a voltage generator for supplying at least one of an AC voltage and a DC voltage. An operational amplifier, a capacitor, an impedance converter, a sensor connection portion to which a capacitance sensor can be connected at one end, a signal line to which the input terminal of the impedance converter and the capacitor are connected at the other end, and both ends. Includes a second resistor connected to the signal line and the output terminal of the impedance converter,
An output terminal of the voltage generator is connected to an input terminal of the operational amplifier, and the capacitor and the impedance converter are inserted in a feedback path of the operational amplifier.

In the sensor capacitance detecting device according to the second aspect of the present invention, the second resistor is set so that there is substantially no inflow / outflow of current between the signal line and the second resistor. Is preferred. In addition, the second resistor is connected to the signal line, and when the capacitance of the capacitance sensor changes, the second resistor is connected to the signal line from the impedance of the feedback line or the capacitance sensor when viewed from the signal line. It is preferable that the impedance is high when the resistance of No. 2 is seen.

In the sensor capacitance detecting device according to the first and second aspects of the present invention, when the capacitance changing frequency of the capacitance sensor is in the audio frequency band, the first resistor or the second resistor is used.
The resistance is preferably 10 MΩ or more. Further, it is preferable that the impedance converter is formed by a voltage follower. Furthermore, the sensor capacitance detection device further includes shield means for electrically shielding at least a part of the signal line, and guard voltage application means for applying a voltage having the same potential as the voltage of the signal line to the shield means. It is preferable.

According to a third aspect of the present invention, a sensor capacitance detecting method for detecting the capacitance of a capacitance sensor, the capacitance of which changes in response to a change in a physical quantity, has one end of a capacitance sensor and one end of a resistor fed back to an operational amplifier. Corresponding to the sensor capacitance from the output terminal of the operational amplifier, the step of connecting to the connection point of the capacitor and the impedance converter inserted in series in the path, the step of inputting at least one of AC voltage or DC voltage to the operational amplifier. And a step of outputting an output voltage for changing the capacitance of the capacitance sensor,
The impedance when the resistance is viewed from the signal line is set to be higher than the impedance when the feedback path or the capacitance sensor is viewed from the signal line.

According to a fourth aspect of the present invention, there is provided a sensor capacitance detecting method for detecting the capacitance of a capacitance sensor, the capacitance of which changes in response to a change in physical quantity, in which one end of the capacitance sensor and one end of a resistor are fed back to an operational amplifier. Corresponding to the sensor capacitance from the output terminal of the operational amplifier, the step of connecting to the connection point of the capacitor and the impedance converter inserted in series in the path, the step of inputting at least one of AC voltage or DC voltage to the operational amplifier. And a step of outputting an output voltage for changing the capacitance of the capacitance sensor,
It is characterized in that it is set so that there is substantially no inflow / outflow of current between the signal line and the resistor.

In the above-described sensor capacitance detection device and sensor capacitance detection method of the present invention, "when the capacitance of the sensor capacitance changes" means not only the case where the sensor capacitance changes depending on the frequency change. , Including all cases that change over time. That is, it includes an arbitrary case that changes with time, such as a case where the capacitance value gently rises or falls, or a case where the capacitance value rises or falls instantaneously, that is, digitally.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a circuit diagram showing the structure of a sensor capacitance detecting device according to a first embodiment of the present invention. The sensor capacitance detection device includes a first operational amplifier OP ₁ and an impedance converter H _iz, and includes a first operational amplifier OP 1
The output terminal of ₁ is connected to the input terminal of the impedance converter H _iz via the capacitor C. Note that, here, the impedance converter H _iz may be configured by a voltage follower circuit by short-circuiting the inverting input terminal and the output terminal of the second operational amplifier OP ₂ as shown in FIG. Good. The voltage follower circuit has a non-inverting input terminal as an input terminal of the circuit, has high input impedance and low output impedance, and has an absolute value of input / output gain of 1. A signal line L is further connected to the input terminal of the impedance converter H _iz , and the other end of the signal line L is connected to the electrode (electrode of the sensor connection part) P ₁ forming one end of the capacitance sensor. Has been done. In FIG. 1, the portion of the signal line L is shown by a thick solid line. The sensor connection portion is not shown. The electrode P _{2 at} the other end of the capacitance sensor is connected to a reference potential, that is, a predetermined potential. The reference potential may be ground potential. The electrode P _{2 at} the other end of the capacitance sensor may be in a floating state, but if it is connected to a reference potential, highly accurate measurement is possible. The capacitance sensor receives the physical quantity (acceleration, pressure,
The capacitance between the electrodes P ₁ and P ₂ , that is, the sensor capacitance C _s is changed according to (gas, light, sound waves, etc.), and is a condenser microphone, a minute displacement capacitance sensor, or the like.

First operational amplifier OP₁Non-inverting input terminal
Is connected to a reference potential (predetermined DC potential including ground potential)
The inverting input terminal is connected to the first terminal from the AC voltage generator OSC.
Resistance R₁AC input voltage V via_in(Angular frequency ω_in)But
Is applied. The AC voltage generator OSC also has a first resistor
R₁And the second resistor R₂Through the impedance converter H
_izIs connected to the input terminal of. First operational amplifier OP
₁Is an output terminal OUT of the sensor capacitance detection device.
Output voltage V from the output terminal OUT. _outOut
I will be forced. Second resistor R₂, Capacitor C and impedance
-Dance converter H_izIs the first operational amplifier OP₁Return of
It constitutes the circuit.

The signal line L is also connected to one end of the third resistor R ₃ , and the other end of the resistor R ₃ is connected to a reference potential (a predetermined potential including the ground potential). The third resistor R ₃ is
When the capacitance sensor is connected to the signal line L and the capacitance of the capacitance sensor changes, the third resistance R ₃ from the signal line L to the feedback path or the impedance when the capacitance sensor is viewed from the signal line L It is set so that the impedance when viewed is high.

FIG. 4 shows the results of an actual machine test performed using the sensor capacitance detecting device shown in FIG.
In this actual machine test, the sensor capacitance C _s changes in the audio frequency band (20HZ to 20 KHz),
For example, the frequency is f _c (= ω _c / (2π)) = 1 KHz
And the first capacitor C = 0.5 pF. Then, the value of the third resistor R ₃ was variously changed, the signal and the noise were measured, and the S / N ratio was obtained based on these measured values. As shown in FIG. 4, as a result of this actual machine test, the third
It was found that it is desirable to use a resistance of 10 MΩ or more as the resistance R ₃ of. However, since the S / N ratio also changes depending on the time constant determined by the change frequency f _c of the sensor capacitance C _{s and} the capacitance of the first capacitor C, the third resistor R ₃ is changed according to the time constant. The value of may be determined. It should be noted that it is empirically estimated that other frequencies in the audio frequency band in the above-described actual device test have similar tendencies. By using a high resistance as the third resistance R ₃ , when the signal line L, that is, the input terminal of the impedance converter H _iz and the reference potential are connected via the resistance, a potential difference is generated across the resistance. However, the alternating current flowing through the sensor capacitance C _s hardly flows through the third resistor R _3, and there is no inflow or outflow of current.

Next, the sensor of the first embodiment shown in FIG.
The detection operation of the capacitance detection device will be described. The following smell
The first operational amplifier OP₁Non-inverting input terminal, capacitance
Sensor electrode P₂And one end of the AC voltage generator OSC is connected
And the impedance converter H
_izIs a voltage follower with the configuration shown in Fig. 3.
It will be described as being.Sensor capacity detection First and second operational amplifier OP₁And OP₂And first and second
And the second resistance R₁And R₂By the second operational amplifier O
P₂AC output voltage V_in-R ₂/ R₁Double
Voltage V₂Is obtained. That is, V₂= -R₂/ R₁・ V_in                              (1)

On the other hand, almost all of the alternating current flowing through the sensor capacitance C _s is high because the input impedance of the _second operational amplifier OP ₂ is high and the output impedance of the potential fixing circuit is high. Will flow to. That is, there is substantially no inflow / outflow of current between the resistor R ₃ of the potential fixing circuit and the signal line L. In addition, since the two input terminals of the second operational amplifier OP ₂ are in an imaginary short-circuit state and have the same potential,
If the voltage at the non-inverting input terminal of the operational amplifier OP ₂ is also V ₂ and V _in = V · sin ω _in t, the sensor capacitance C _s
The current flowing through is I _s = dC _s · V ₂ / dt (2). Therefore, from the equation (1), I _s = − (R ₂ / R ₁ ) · dC _s · V _in sinω _in t / dt (3) is obtained. On the other hand, the current I _c flowing through the capacitor C becomes _{I c = dC (V out -V} 2) / dt (4).

Since the current I _c flowing through the capacitor C is equal to the current I _s flowing through the sensor capacitance C _s , the output terminal OUT
The output voltage V _out is expressed by the following formula (3) and (4): V _out = − (R ₂ / R ₁ ) · (1 + C _s / C) · V · sin ω _in t (5) As is clear from the equation (5), the output voltage V
_out has a linear relationship with the sensor capacitance C _s , and the output voltage V
By processing _out , the value of the sensor capacitance C _s can be obtained.

Detection of Change in Sensor Capacitance Next, detection of the change ΔC when the sensor capacitance C _s changes at an angular frequency ω _c around a certain capacitance value C _d as in a condenser microphone, that is, , C _s = C _d +
The detection of ΔC in the case of ΔC sinω _c will be described.
As described above, all the current i flowing through the first capacitor C flows through the sensor capacitance C _s , and therefore the sensor capacitance C _s
The electric charge stored in the first capacitor C is equal to the electric charge stored in the first capacitor C. C (V _out −V ₂ ) = C _s · V ₂ (6) Then, when the equation (6) is transformed, V _in = V · sin ω _in
Since t, the following formula (7) is obtained. _{V out = (- R 2 /} R 1) · (1 + C s / C) · V · sinω in t = (- R 2 / R 1) · V · sinω in t (1 + C d / C + ΔC · sinω c t / C ) (7) since the output voltage V _out does not have a dependency on the change frequency of the sensor capacitance C _s, it is possible to obtain an output which depends linearly on the variation ΔC of the sensor capacitance C _s.

In the embodiment shown in FIG. 1, the signal line
Third resistor R for fixing the electric potential ₃Is the reference potential (zero
Connected to a predetermined electric potential (including electric potential)
When the one shown in FIG. 3 is used as the dance converter,
Instead of the quasi-potential, the second operational amplifier OP₂Output terminal
May be connected to. Fig. 2 shows that high resistance
Resistance of 3₃To the signal line L and the second operational amplifier OP₂Output
The 2nd example when connected to a terminal is shown. this
In the second embodiment, the signal line L has an AC voltage V_inOver
And fixed to the potential determined by the potential of the non-inverting input terminal
become. In addition, the third resistor R₃Is the second operational amplifier O
P₂Connected between the inverting and non-inverting input terminals of
And these two input terminals are
Since it is ideally the same potential in the short state, the third resistance
R₃There is no potential difference between both ends of the current, and the flowing current is zero.
Therefore, the sensor capacitance C_sAll current flowing through
The third resistance R is applied to the signal line L.₃When
Higher accuracy with no current flowing in or out between
It is possible to realize the capacity detection of.

In the first and second embodiments, the third resistor R ₃ may be an electronic element such as a diode or a transistor. When a diode is used, it is preferable to use the high impedance in the reverse bias state, and when a transistor is used, it is preferable to use the high impedance in the off state. Further, although the AC voltage generator OSC is used in the first and second embodiments, a DC voltage generator may be used. When a direct current voltage is V and a physical quantity is applied to the capacitance sensor, the capacitance of the capacitance sensor changes and the output V _out also changes. At that time, the equations (5) and (7) are expressed as the following equations (5) ′ and (7) ′, respectively. V _out =-(R ₂ / R ₁ ) · (1 + C _s / C) · V (5) ′ V _out = (− R ₂ / R ₁ ) · (1 + C _d / C + ΔC · sin ω _c t / C) · V (7) ′ Further, a part or all of the signal line L is covered with a shield wire (not shown) to be electrically shielded, and a guard voltage having the same potential as the signal line is applied to the shield line. As a result, the influence of the stray capacitance formed between the signal line and the reference potential can be reduced, and the S / N ratio of the output voltage can be further improved.

[0025]

Since the present invention is configured as described above, the potential of the signal line connecting the capacitance sensor and the second operational amplifier is set to a predetermined resistance so that the capacitance sensor is connected to the signal line. A resistance that is set so that the impedance when the resistance is seen from the signal line is higher than the impedance when the feedback path or the capacitance sensor is seen from the signal line when connected and the capacitance of the capacitance sensor changes. The floating state of the signal line can be avoided by fixing the potential to a predetermined reference potential via, and thus the circuit operation can be stabilized. Further, by connecting the resistor for fixing the potential of the signal line as described above between the output terminal of the second operational amplifier and the signal line, the resistor and the signal line are both connected. Ideally, the flowing current can be zero, and more accurate capacitance measurement can be performed.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing a sensor capacitance detection device according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing a specific configuration of an impedance converter included in the first embodiment shown in FIG.

FIG. 4 is a graph showing a result of an actual machine test performed using the sensor capacity detecting device shown in FIG.

FIG. 5 is a circuit diagram showing a conventional sensor capacitance detection device.

FIG. 6 is a circuit diagram showing another conventional sensor capacitance detection device.

Claims (11)

[Claims] 1. A sensor capacitance detection device for detecting the capacitance of a capacitance sensor, the capacitance of which changes in response to a change in physical quantity, the voltage generator supplying at least one of an AC voltage and a DC voltage, and an operational amplifier. A capacitor, an impedance converter, a sensor connection part to which a capacitance sensor can be connected at one end, a signal line to which the input terminal of the impedance converter and the capacitor are respectively connected at the other end, and both ends are the signal line and a reference line. A first resistor connected to a voltage, an output terminal of the voltage generator is connected to an input terminal of the operational amplifier, and the capacitor and the impedance converter are inserted in a feedback path of the operational amplifier. A sensor capacitance detection device characterized by the above. 2. The sensor capacitance detection device according to claim 1, wherein the first resistance is set so that an inflow / outflow of current is substantially eliminated between the signal line and the first resistance. A sensor capacitance detection device characterized in that 3. The sensor capacitance detection device according to claim 1, wherein the first resistor has a capacitance sensor connected to a signal line and a capacitance of the capacitance sensor changes.
A sensor characterized in that the impedance when the first resistance is viewed from the signal line is set to be higher than the impedance when the return path or the capacitive sensor is viewed from the signal line. Capacitance detection device. 4. A sensor capacitance detection device for detecting the capacitance of a capacitance sensor, the capacitance of which changes in accordance with a change in a physical quantity, the voltage generator supplying at least one of an AC voltage and a DC voltage, and an operational amplifier. A capacitor, an impedance converter, a sensor connection portion to which a capacitance sensor can be connected at one end, a signal line to which the input terminal of the impedance converter and the capacitor are connected, respectively, at the other end, the signal line and the A second resistor connected to the output terminal of the impedance converter, the output terminal of the voltage generator is connected to the input terminal of the operational amplifier, and the capacitor and the impedance converter in the feedback path of the operational amplifier. A sensor capacitance detecting device characterized in that and are inserted. 5. The sensor capacitance detection device according to claim 4, wherein the second resistor is set so that there is substantially no inflow / outflow of current between the signal line and the second resistor. A sensor capacitance detection device characterized in that 6. The sensor capacitance detecting device according to claim 4 or 5, wherein the second resistor is configured such that when the capacitance sensor is connected to a signal line and the capacitance of the capacitance sensor changes.
The sensor is set so that the impedance when the second resistance is viewed from the signal line is higher than the impedance when the return path or the capacitive sensor is viewed from the signal line. Capacitance detection device. 7. The sensor capacitance detection device according to claim 1, wherein when the capacitance change frequency of the capacitance sensor is in an audio frequency band, the first resistance or the second resistance is 10 MΩ or more. A sensor capacitance detection device characterized by: 8. The sensor capacitance detecting device according to claim 1, wherein the impedance converter is formed by a voltage follower. 9. The sensor capacitance detection device according to claim 1, wherein the device further includes a shield unit that electrically shields at least a part of the signal line, and the shield unit includes the signal line. And a guard voltage applying means for applying a voltage having the same potential as the voltage of the sensor capacitance detecting device. 10. A sensor capacitance detection method for detecting the capacitance of a capacitance sensor, the capacitance of which changes in accordance with a change of a physical quantity, wherein one end of the capacitance sensor and one end of a resistor are inserted in series in a feedback path of an operational amplifier. Connecting to a connection point between the capacitor and the impedance converter, inputting at least one of an AC voltage and a DC voltage to the operational amplifier, and outputting an output voltage corresponding to the sensor capacitance from an output terminal of the operational amplifier. When the capacitance of the capacitance sensor changes, the impedance when the resistance is seen from the signal line is more than the impedance when the feedback path or the capacitance sensor is seen from the signal line. A sensor capacitance detection method characterized by being set to a high value. 11. A sensor capacitance detection method for detecting the capacitance of a capacitance sensor, the capacitance of which changes in accordance with a change in a physical quantity, wherein one end of the capacitance sensor and one end of a resistor are inserted in series in a feedback path of an operational amplifier. Connecting to a connection point between the capacitor and the impedance converter, inputting at least one of an AC voltage and a DC voltage to the operational amplifier, and outputting an output voltage corresponding to the sensor capacitance from an output terminal of the operational amplifier. And a step of setting a capacitance of the capacitance sensor, wherein the capacitance of the capacitance sensor is set so that an inflow / outflow of a current is substantially eliminated between the signal line and the resistor.