JP2003185688A - Circuit and apparatus for detection of capacitance and microphone device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a capacitance detection circuit by which a very small capacitance can be detected precisely and which is suitable for miniaturization. <P>SOLUTION: The capacitance detection circuit 10 is provided with an AC voltage generator 11, an operational amplifier 14 whose noninverting input terminal is connected to a prescribed potential (a ground in this example), an impedance converter 16, a resistance (R<SB>1</SB>) 12 which is connected across the generator 11 and the noninverting input terminal of the operational amplifier 14, a resistance (R<SB>2</SB>) 13 which is connected across the noninverting input terminal of the operational amplifier 14 and an output terminal of the impedance converter 16, and an impedance element (a capacitor) 15 which is connected across an output terminal of the operational amplifier 14 and an input terminal of the impedance converter 16. A capacitor 17 to be detected is connected across the input terminal of the impedance converter 16 and the prescribed potential, and the circuit 10 and the capacitor 17 to be detected are installed in adjacent positions. <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 circuit and a device for detecting electrostatic capacitance, and more particularly to a circuit, a device and a microphone device for detecting a very small capacitance with high accuracy.

[0002]

2. Description of the Related Art As a conventional example of a capacitance detecting circuit, there is a capacitance type displacement meter (for example, refer to Patent Document 1). FIG. 9 is a circuit diagram showing this capacitance detection circuit. In this detection circuit, the capacitance sensor 92 formed by the electrodes 90 and 91 includes an operational amplifier 9 via a signal line 93.
5 is connected to the inverting input terminal. A capacitor 96 is connected between the output terminal of the operational amplifier 95 and the inverting input terminal, and the AC voltage Vac is applied to the non-inverting input terminal. Further, the signal line 93 is covered with a shield line 94, and is electrically shielded against disturbance noise. The shield line 94 is connected to the non-inverting input terminal of the operational amplifier 95. The output voltage Vd is taken out from the output terminal of the operational amplifier 95 via the transformer 97.

In this detection circuit, the inverting input terminal and the non-inverting input terminal of the operational amplifier 95 are in the state of an emergency short, and the signal line 93 connected to the inverting input terminal and the shield line connected to the non-inverting input terminal. 94 and the same potential as each other. As a result, the signal line 93 is guarded by the shield line 94, that is,
The stray capacitance between 3 and 94 is canceled, and the output voltage Vd that is not easily influenced by the stray capacitance is obtained.

[0004]

[Patent Document 1] JP-A-9-280806 (second
(Figure)

[0005]

However, according to such a conventional technique, when the capacitance of the capacitance sensor 92 is certainly large to some extent, the stray capacitance between the signal line 93 and the shield line 94 is not affected. Although it is possible to obtain a high output voltage Vd, there is a problem that an error becomes large in the detection of a minute capacitance of several pF or fF (femto farad) order or less.

Further, depending on the frequency of the AC voltage Vac to be applied, due to a tracking error in the operational amplifier 95 or the like, the voltage between the inverting input terminal and the non-inverting input terminal in the state of the immagnari short circuit may be delicate as a result. Na phase
There is also a problem that the deviation of the amplitude occurs and the detection error becomes large.

On the other hand, in a lightweight and small-sized voice communication device typified by a mobile phone, a compact amplifier circuit for converting a voice detected by a capacitive sensor such as a condenser microphone into an electric signal with high sensitivity and fidelity is required. Has been. If it is possible to accurately detect a minute capacitance of several pF or fF or less or its change, a high-performance microphone capable of detecting voice with extremely high sensitivity and faithfully is realized. The performance in picking up voice in a voice communication device such as a telephone is dramatically improved.

Therefore, the present invention has been made in view of such a situation, and is capable of accurately detecting a minute capacitance, and is a condenser microphone used in a lightweight and small-sized voice communication device. It is an object of the present invention to provide a capacitance detection circuit and the like suitable for capacitance detection of the capacitance sensor.

[0009]

In order to achieve the above object, the electrostatic capacity detection circuit according to the present invention is an electrostatic capacity detection circuit that outputs a detection signal corresponding to the electrostatic capacity of a capacitor to be detected. , An impedance converter with high input impedance and low output impedance, and a capacitive first
An impedance element, an operational amplifier, an AC voltage generator that applies an AC voltage to the operational amplifier, and a signal output terminal connected to the output of the operational amplifier, and the input terminal of the impedance converter includes the target voltage. One end of the detection capacitor and one end of the first impedance element are connected, the first impedance element and the impedance converter are included in the negative feedback path of the operational amplifier, the detected capacitor and the capacitance detection circuit. And are provided adjacent to each other.

Further, the capacitance detection circuit according to the present invention is
An electrostatic capacitance detection circuit that outputs a detection signal corresponding to the electrostatic capacitance of a capacitor to be detected, the impedance converter having a high input impedance and a low output impedance, a capacitive first impedance element, an operational amplifier, An AC voltage generator for applying an AC voltage to the operational amplifier and a signal output terminal connected to the output of the operational amplifier are provided, and one end of the detected capacitor and the first terminal are connected to an input terminal of the impedance converter. One end of the impedance element is connected, the first impedance element and the impedance converter are included in the negative feedback path of the operational amplifier, and the detected capacitor, the first impedance element and the impedance converter are close to each other. It is characterized by being provided.

As a concrete example, an AC voltage generator, an operational amplifier whose non-inverting input terminal is connected to a predetermined potential, an impedance converter, and an inverting input terminal of the operational amplifier and an output terminal of the impedance converter. The connected resistance,
A capacitance detection circuit is provided that includes a capacitor (first impedance element) connected between the output terminal of the operational amplifier and the input terminal of the impedance converter, and the detected capacitor is the input terminal of the impedance converter and a predetermined potential. Connected between them, the capacitance detection circuit and the capacitor to be detected are adjacent to each other, or short and close to each other so that the stray capacitance of the signal line does not exceed 10 times the maximum value of the capacitance of the connected element. Keep it. Here, the predetermined potential refers to any one of a reference potential, a predetermined DC potential, a ground potential, and a floating state, and an optimum one is selected according to the embodiment. A resistor as a second impedance connected between the AC voltage generator and the inverting input terminal of the operational amplifier may be further provided.

With such a configuration, a constant voltage is applied to the capacitor to be detected, and almost all of the current flowing through the capacitor to be detected is the capacitor (first
The signal corresponding to the electrostatic capacity of the detected capacitor is output from the signal output terminal.

It should be noted that in order to reduce the mixing of noise in the signal line connecting the capacitance detection circuit and the detection capacitor and the generation of stray capacitance of the signal line, the detection capacitor and the capacitance detection circuit And should be placed as close to each other as possible. Alternatively, the detected capacitor, the first impedance element, and the impedance converter are provided in positions as close to each other as possible. Here, in the present specification,
"Proximity" means that the stray capacitance of the signal line does not exceed 10 times the capacitance value of the detected capacitor or the larger capacitance value of the capacitive first impedance element. Say. This is because when the stray capacitance of the signal line is set to a capacitance value that does not exceed the numerical value of the capacitance value of the connected element by one digit, the electrostatic capacitance detection circuit of the present invention significantly deteriorates the detection sensitivity. Was found to be preventable, and this is empirically derived. The stray capacitance of the signal line can be measured by measuring the capacitance without connecting the detected capacitor, the first impedance element, and the impedance converter to the signal line. In the specification of the present application, the state of being adjacent and in contact with each other under the condition of being close to each other is referred to as “adjacent”.

Here, in addition to the capacitance detection circuit,
You may add the inverting amplifier circuit which inverts the signal in a signal output terminal, and the addition circuit which adds the output signal of an impedance converter and the output signal of an inverting amplifier circuit. A resistor may be connected in parallel with the capacitor (first impedance element).

As an application of the present invention, the capacitor to be detected is a capacitance type sensor which detects a physical quantity according to a change in capacitance, and the capacitance detection circuit is formed on a printed board or a silicon substrate, It is preferable that the mold sensor and the substrate are fixed or integrally molded.
As a more specific example, more preferably, a condenser microphone is used as the detected capacitor, the capacitance detection circuit is realized by an IC, and the condenser microphone and the IC are integrated to be used in a mobile phone or the like. Alternatively, it may be housed in one housing (shield box). At this time, the condenser microphone and IC
Are fixed at adjacent positions and are connected by a conductive plate, a wiring pattern, wire bonding, or the like.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. (First Embodiment) FIG. 1 is a circuit diagram of an electrostatic capacitance detection circuit 10 according to a first embodiment of the present invention. In this figure, the capacitance detection circuit 10 includes a detected capacitor 17 to be detected (here, a capacitance sensor such as a condenser microphone that detects various physical quantities by utilizing changes in the capacitance Cs). Are connected.

The capacitance detecting circuit 10 includes an AC voltage generator 11 for generating an AC voltage, a resistor (R1) 12, a resistor (R2) 13, an operational amplifier 14, an impedance element (here, a capacitor having a capacitance Cf). 15 and an impedance converter 16, and a detected capacitor 17
The detection signal (voltage Vout) corresponding to the electrostatic capacitance is output from the signal output terminal 20.

One end of the AC voltage generator 11 is connected to a predetermined potential (ground in this example), and a constant AC voltage (voltage Vin, angular frequency ω) is generated from the other end (output terminal). . A resistor (R1) 12 is connected between the output terminal of the AC voltage generator 11 and the inverting input terminal of the operational amplifier 14.

The operational amplifier 14 is a voltage amplifier having an extremely high input impedance and open loop gain. Here, the non-inverting input terminal has a predetermined potential (ground in this example).
, And the non-inverting input terminal and the inverting input terminal are in the state of an emergency short. This operational amplifier 1
4, the capacitor 15, the impedance converter 16, and the resistor (R2) 13 are connected in series in this order between the negative feedback path of 4, that is, the output terminal of the operational amplifier 14 and the inverting input terminal.

The impedance converter 16 is a voltage amplifier having an extremely high input impedance, an extremely low output impedance, and a voltage gain of A times. One end of the detected capacitor 17 is connected to the input terminal 21 of the impedance converter 16 via a conductor such as a signal line or a wiring pattern on a printed circuit board, while the other end of the detected capacitor 17 is It is connected to a predetermined potential (ground in this example). The output terminal of the operational amplifier 14 is connected to the signal output terminal 20 for outputting the output signal of the electrostatic capacitance detection circuit 10, that is, the detection signal corresponding to the capacitance of the detected capacitor 17. In the present application,
The variable A indicated by A times or the like represents a real number other than zero (0).

Regarding the connection between the capacitor to be detected 17 and the electrostatic capacitance detection circuit 10, as short as possible in order to avoid adding unnecessary stray capacitance as a detection error and mixing in disturbance noise. Conductor (cable,
It is preferable to connect with a copper foil wiring pattern, a connection terminal, etc.). Further, if possible, it is preferable to cover the entire capacitor 17 to be detected and the electrostatic capacitance detection circuit 10 with a grounded shield member or to store the capacitor 17 and the electrostatic capacitance detection circuit 10 in a shield box, in order to enhance shielding against disturbance noise.

The operation of the electrostatic capacitance detection circuit 10 configured as described above is as follows. Focusing on the inverting amplifier circuit composed of the resistor (R1) 12, the resistor (R2) 13, the operational amplifier 14, etc., both input terminals of the operational amplifier 14 are in the state of an emergency short and the same potential (for example, 0V). Since the input impedance is extremely high and no current flows, the resistance (R1) 12
Current flowing through the resistor becomes Vin / R1 and all of it flows through the resistor (R2) 13, so the impedance converter 16
If the output voltage of V is V2, then Vin / R1 = -V2 / R2 holds. By organizing this, the output voltage V2 of the impedance converter 16 becomes V2 =-(R2 / R1) .Vin (Equation 1). Further, since the voltage gain of the impedance converter 16 is A, the input voltage (voltage of the input terminal 21) V1
And the output voltage (voltage at the output terminal 22) V2, the input voltage V1 is: V1 = (1 / A) V2 (Equation 2) Further, when the current flowing through the capacitor 15 toward the detected capacitor 17 is i, the input impedance of the impedance converter 16 is extremely high. Therefore, all of the current i flows into the detected capacitor 17, so that the current i is jωCs · V1 and the voltage Vout of the detection signal output from the signal output terminal 20 is Becomes

When V2 is eliminated from the above equations 1 and 2, V1 =-(R2 / R1). (Vin / A) (equation 4) is obtained, and when this V1 is substituted into the above equation 3, Vout =-(1 + Cs / Cf). (R2 / R1). (Vin / A) (Equation 5) is obtained.

As can be seen from this equation 5, the voltage Vout of the detection signal output from the signal output terminal 20 of the electrostatic capacitance detection circuit 10 has a value depending on the capacitance Cs of the detected capacitor 17. Therefore, the capacitance Cs can be specified by performing various signal processings on the voltage Vout. Further, as can be seen from the expression 5 that does not include the angular frequency ω, the voltage Vout of this detection signal
Does not depend on changes in the frequency of the AC signal Vin from the AC voltage generator 11 and the frequency of the detected capacitor. As a result, a capacitance detection circuit that can detect the capacitance of the detected capacitor 17 (without the frequency-dependent characteristic of the circuit) without depending on the frequency of the AC voltage applied to the detected capacitor 17 is provided. Will be realized. Therefore, the capacitance value is directly specified from the voltage value of the detected capacitor 17 whose capacitance value changes at a certain frequency (voice band), such as a condenser microphone, without frequency correction of the detected signal. It becomes possible.

Further, in the electrostatic capacitance detection circuit 10 of the present embodiment, the non-inverting input terminal of the operational amplifier 14 supplying current to the capacitor 15 and the detected capacitor 17 is connected to a predetermined potential. , Has been fixed. Therefore, unlike the operational amplifier 95 in the conventional circuit shown in FIG. 9, the operational amplifier 14 does not depend on the frequency or the like of the input AC signal and outputs a stable current with less noise to the capacitor 15 and the detected capacitor 17. Since it is supplied to, the minute capacitance of the detected capacitor 17 can be detected.

According to the experiment related to the present invention, FIG.
In the electrostatic capacitance detection circuit of, for example, when the original electrostatic capacitance of Cs (capacitor to be detected: microphone in this embodiment) is 20 pF, the stray capacitance of the signal line is 200 pF.
When it exceeded pF, the detection sensitivity deteriorated considerably. Further, when the Cs was confirmed by several different capacitance values, the same tendency result was obtained. Further, the capacitance Cf, which is the first impedance element, and the detected capacitor Cs are both capacitance elements connected to the signal line in this circuit, and whichever element gives the same result as the above in calculation. it is conceivable that. From these experimental results and experience, the detected capacitor, the first impedance element, and the impedance converter are placed close to each other so that the stray capacitance of the signal line does not exceed the value of the capacitance value of Cs or Cf which is one digit higher. It was found that good detection sensitivity was obtained.

FIG. 2 shows a specific circuit example of the impedance converter 16 in the capacitance detection circuit 10 shown in FIG. FIG. 2A shows a voltage follower using the operational amplifier 100. The inverting input terminal and the output terminal of the operational amplifier 100 are short-circuited. The non-inverting input terminal of the operational amplifier 100 is connected to the impedance converter 1
By inputting 6 and using the output terminal of the operational amplifier 100 as the output of the impedance converter 16, the impedance converter 16 having an extremely high input impedance and a voltage gain A of 1 can be obtained.

FIG. 2B shows a non-inverting amplifier circuit using the operational amplifier 101. A resistor (R10) 110 is connected between the inverting input terminal of the operational amplifier 101 and the ground, and a feedback resistor (resistor (R11) 33) is connected between the inverting input terminal and the output terminal of the operational amplifier 101. By using the non-inverting input terminal of the operational amplifier 101 as the input of the impedance converter 16 and the output terminal of the operational amplifier 101 as the output of the impedance converter 16,
The input impedance is extremely high, and the voltage gain A is (R
An impedance converter 16 with 10 + R11) / R10 is obtained.

FIG. 2C shows a circuit in which a buffer having a CMOS structure is added to the input stage of the operational amplifier as shown in FIGS. 2A and 2B. As shown, an N-type MOSFET 34 and a P-type MOSF are provided between the positive and negative power supplies.
ET35 is connected in series through resistors 112 and 113, and the output of the buffer is the operational amplifier 100 (or 101).
Connected to the input of. By using the input of this buffer as the input of the impedance converter 16 and the output terminal of the operational amplifier as the output of the impedance converter 16, the impedance converter 16 having an extremely high input impedance.
Is obtained.

FIG. 2 (d) shows a circuit such as a buffer in the input stage of FIG. 2 (c). As shown, an N-type MOSFET 34 and a P-type MOSFET are provided between the positive and negative power supplies.
35 and 35 are connected in series, and an output is made from the connection part of both MOSFETs.

In FIG. 2E, the non-inverting input of the operational amplifier 102 is used as the input of the impedance converter, and the operational amplifier 1
One end of a resistor 114 is connected to the inverting input terminal of 02, and the output of the operational amplifier 102 and the inverting input are connected via a resistor 115. 2 (d) and 2 (e)
As shown in (1), the impedance converter 16 having an extremely high input impedance can be obtained by adopting such a configuration.

(Second Embodiment) Next, the second embodiment of the present invention will be described.
The capacitance detection circuit according to the embodiment will be described.

FIG. 3 is a circuit diagram of the electrostatic capacitance detection circuit 30 according to the second embodiment. The electrostatic capacitance detection circuit 30 is roughly divided into a core portion 31 corresponding to the electrostatic capacitance detection circuit 10 shown in FIG. 1, and a signal voltage V01 at the signal output terminal 20 of the core portion 31 as an input and inverted. And the signal voltage V03 at the output terminal 23 of the inverting section 32 and the signal voltage V02 at the AC output terminal 22 of the core section 31 are added, and the detection signal of the voltage V04 is output to the output terminal 24. It is composed of an adding unit 33 for performing.

The core section 31 is the same circuit as the capacitance detection circuit 10 shown in FIG. Therefore, the core part 3
The voltage V01 of the signal output terminal 20 of No. 1 becomes V01 = − (1 + Cs / Cf) · (R2 / R1) · (Vin / A) (Equation 6) from the above expression 5, and the AC output terminal 22 of the core part 31 The voltage V02 of
From the above formula 1, V02 =-(R2 / R1). (Vin / A) (formula 7).

The inverting section 32 includes a variable resistor (R4) 40, a resistor (R5) 41, a variable resistor (R6) 42, and a capacitor 43.
And an operational amplifier 44 so that the voltage gain is -1, and the phase of the signal V03 at its output terminal 23 is the same as the signal V02 at the AC output terminal 22 of the core section 31. Further, the resistance values of the variable resistor (R4) 40 and the variable resistor (R6) 42 are adjusted. Therefore,
The following relationship is ideally established between the input voltage V01 and the output voltage V03 of the inverting section 32. V03 = -V01 (Equation 8)

The adder 33 is an adder in which three resistors (R7) 45, resistors (R8) 46 and resistors (R9) 47 having the same resistance value are connected to an operational amplifier 48. That is, 2
The following relationships are established between the voltages V02 and V03 of the two input signals and the output voltage V04. V04 =-(V02 + V03) (Equation 9) After substituting the above Equation 8 into this Equation 9 to erase V03,
By substituting the equations 6 and 7, V04 = V01-V02 =-(Cs / Cf). (R2 / R1). (Vin / A) (Equation 10) holds. That is, it can be seen that the voltage V04 of the detection signal output from the output terminal 24 of the electrostatic capacitance detection circuit 30 is proportional to the capacitance value Cs. Therefore, an unknown capacitance value Cs or capacitance change can be easily specified by performing various signal processings based on this voltage V04.

As can be seen by comparing Expression 10 with Expression 5 showing the voltage Vout of the detection signal in the first embodiment, the capacitance detection circuit 3 in the second embodiment is shown.
Unlike the first embodiment, the detection signal obtained at 0 includes only a component proportional to the capacitance of the detected capacitor 17, and does not include an unnecessary offset component (voltage that does not depend on the detected capacitor 17). . Therefore, the detected capacitor 17 is detected from the detection signal in the second embodiment.
The signal processing for specifying the capacity of is simple. In addition, in this example, the example in which V03 = −V01 is described, but the present invention is not limited to this. Depending on the type of capacitance sensor, the output voltage V04 is V04 = {k · (Cs / Cf) + (k + 1)} · (R2 / R, with V03 = k · V01 (k is the amplification factor of the inverting amplifier).
1) ・ Vin may be set.

FIG. 4 is a diagram showing an example of application of the electrostatic capacitance detection circuit in the first and second embodiments to electronic equipment. Here, a cross-sectional view of a microphone 50 used in a mobile phone or the like in which a condenser microphone and a capacitance detection circuit are integrated is shown. The microphone 50 has a lid 51 having a sound hole 52, a vibrating film 53 that vibrates due to sound, a ring 54 that fixes the vibrating film 53, a spacer 55a, and a vibrating film 53 that opposes the spacer 55a. Fixed electrode 56 provided by
Insulating plate 55b supporting fixed electrode 56, and insulating plate 55b
An IC chip 58 fixed to the back surface of the IC chip, on which the capacitance detection circuit of the above-described embodiment is formed, an IC package 59 in which the IC chip 58 is molded, and an external device connected to the IC chip 58 by wire bonding or the like. Electrodes 61a, 61
b and the like.

The vibrating film 53, which is one electrode forming the capacitor, is connected to a predetermined potential (ground in this example), and the fixed electrode 56, which is the other electrode, is an aluminum plate, wire bonding, or contact. It is connected to the circuit of the IC chip 58 via a conductor such as a hole. The capacitance of the capacitor including the vibrating film 53 and the fixed electrode 56 or its change is detected by the electrostatic capacitance detection circuit in the IC chip 58 adjacent to the external electrode 61a via the insulating plate 55b. , 61b and the like. The lid 51 is made of a metal such as aluminum, and shields disturbance noise from entering the internal capacitors 53 and 56 and the IC chip 58 together with a conductive film (not shown) formed on the upper surface of the insulating substrate 60. It plays the role of a shield box. Further, in this example, the fixed electrode 56 and the circuit are connected and the vibrating membrane 53 is connected to a predetermined potential. However, the vibrating membrane 53 and the circuit are connected, and the fixed electrode 56 is connected to a predetermined potential. Good. However, empirically, the former is preferable.

FIG. 5 shows the microphone 50 shown in FIG.
FIG. 5 (a) is a plan view, FIG.
5B is a front view and FIG. 5C is a bottom view. Figure 5
The size of the lid 51 shown in (a) and (b) is, for example, approximately φ length 5 mm × height 2 mm. FIG. 5 (c)
The four external electrodes 61a to 61d shown in FIG.
They are two terminals for the power supply of the electrostatic capacitance detection circuit and two terminals for the output signal.

In such an application example, the capacitor to be detected (condenser microphone in this case) and the electrostatic capacitance detection circuit (IC chip in this case) are provided adjacent to each other, the signal line is extremely short, and its floating They are connected by a conductor whose length does not exceed 10 times the capacitance value of the condenser microphone or the first impedance element in the circuit, whichever is larger. Then, those parts are covered with a shield member such as a metallic lid. Therefore, in such an application example, a signal line (conductor) that connects the capacitor to be detected and the capacitance detection circuit
It is considered that the adverse effects of disturbance noise and the like mixed in the can be ignored.

That is, in such a small microphone, the capacitor to be detected and the capacitance detecting circuit are connected by an extremely short conductor, so that a cable with a shield is connected between them and a guard voltage is applied to the shield. Providing a special circuit for applying a voltage rather increases the circuit scale and prevents the circuit from becoming compact. Therefore, it is preferable to connect the shortest path between the detected capacitor and the electrostatic capacitance detection circuit by an unshielded (unshielded) conductive plate, wiring pattern, wire bonding, lead wire, or the like. As an example of another microphone, FIGS. 6 and 7 show a circuit on which a circuit board is mounted.
It is basically the same except that the capacitance detection circuit of the above-described embodiment is mounted on the substrate 62.

Although the capacitance detection circuit according to the present invention has been described above based on the two embodiments and the application to the product, the present invention is limited to these embodiments and the application. It is not something that will be done.

For example, in the capacitance detection circuits 10 and 30, the capacitor 15 is connected between the operational amplifier 14 and the impedance converter 16 in order to detect the current flowing through the capacitor 17 to be detected. It is also possible to consider connecting an impedance element such as or an inductance.

Further, as shown in FIG. 8, a resistor 18 may be added and connected in parallel with the capacitor 15 in the capacitance detection circuits 10 and 30 in the above-mentioned embodiment. As a result, the connection point between the capacitor 15 and the detected capacitor 17 is connected to the first operational amplifier 1 via the resistor 18.
4 is connected to the output terminal, the floating state in the direct current is eliminated, and the potential is fixed.

The capacitance type sensor connected as the detected capacitor 17 is not limited to the condenser microphone, but may be an acceleration sensor, seismograph, pressure sensor, displacement sensor, displacement meter, proximity sensor, touch sensor, ion sensor, or humidity sensor. Sensor, raindrop sensor, snow sensor, lightning sensor, alignment sensor, contact failure sensor, shape sensor, end point detection sensor, vibration sensor, ultrasonic sensor, angular velocity sensor,
Liquid volume sensor, gas sensor, infrared sensor, radiation sensor, water level meter, freezing sensor, moisture meter, vibrometer, electrification sensor, known capacitive sensor such as printed circuit board inspection machine, etc.
It includes all transducers (devices) that detect various physical quantities by utilizing changes in capacitance.

[0047]

As is apparent from the above description, the electrostatic capacitance detection circuit, the electrostatic capacitance detection device, and the microphone device according to the present invention apply an AC voltage to the operational amplifier via a resistor to apply the AC voltage to the signal line. The capacitance of the detected capacitor is detected by connecting the detected capacitor. That is, a capacitor is connected between the output terminal of the operational amplifier and the input terminal of the impedance converter where the non-inverting input terminal is connected to the specified potential, and a detected capacitor is connected between the input terminal of the impedance converter and the specified potential. is doing.

As a result, all the current flowing through the capacitor to be detected flows through the capacitor, and an accurate signal corresponding to the capacitance of the capacitor to be detected is output to the output terminal of the operational amplifier, which is on the order of several pF or fF or less. It is possible to detect the minute capacitance of.

Since the non-inverting input terminal of the operational amplifier is connected to a predetermined potential and one of the input terminals is fixed in potential, the operational amplifier operates stably, the operational error is reduced, and the detected signal is detected. The contained noise is suppressed.

Further, since the capacitor is connected between the operational amplifier and the impedance converter, it does not depend on the frequency of the AC voltage applied to the operational amplifier nor on the frequency of the capacitance change of the capacitor to be detected. Detection sensitivity is secured. Further, when a resistor is connected between the operational amplifier and the impedance converter, there is no problem of deterioration of the S / N ratio due to thermal noise from the resistor.

The electrostatic capacitance detection circuit and the capacitor to be detected are provided at positions adjacent to each other, or circuit elements connected to the signal line are provided close to each other, whereby a shielded cable for connecting the two is provided. Also, a special circuit for canceling the stray capacitance generated by the cable is unnecessary.

Here, an inverting amplifier circuit for inverting the signal at the signal output terminal and an adder circuit for adding the output signal of the impedance converter and the output signal of the inverting amplifier circuit are added to the capacitance detection circuit. You may. As a result, unnecessary offset components contained in the output signal of the electrostatic capacitance detection circuit are removed, and the net signal corresponding to the capacitance of the detected capacitor can be greatly amplified.

Further, the capacitor to be detected is a condenser microphone, the electrostatic capacitance detection circuit is realized by an IC, and the condenser microphone and the IC are integrated,
Since the condenser microphone and the electrostatic capacitance detection circuit are placed very close to each other by being housed in a single casing (shield box) as a microphone used for a mobile phone or the like, the detected condenser and the electrostatic capacitance are detected. A shielded cable with a large diameter for connecting to a circuit and a special circuit for applying a guard voltage are not required.

Further, since the capacitance detection circuit according to the present invention detects the capacitance by passing a current through the detected capacitor, a polymer film is attached to the electrode of the detected capacitor like an electret condenser microphone. There is no need to paste it into an electret,
It can be applied to an ordinary capacitance type sensor.

As described above, according to the present invention, the limitation of the use environment is reduced, a minute capacitance can be accurately detected, and a capacitance detection circuit suitable for miniaturization is realized. The voice performance of lightweight and small voice communication devices such as mobile phones is dramatically improved, and its practical value is extremely high.

[Brief description of drawings]

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

2A to 2E are diagrams showing examples of impedance converters usable in the present invention.

FIG. 3 is a circuit diagram of a capacitance detection circuit according to a second embodiment of the present invention.

FIG. 4 is a diagram (cross-sectional view of a microphone) showing an application example of the capacitance detection circuit of the present invention to an electronic device.

5A and 5B are schematic external views of the microphone shown in FIG. 3, in which FIG. 5A is a plan view, FIG. 5B is a front view, and FIG.

FIG. 6 is a cross-sectional view of another example of a microphone.

7A and 7B are schematic external views of the microphone shown in FIG. 5, where FIG. 7A is a plan view and FIG. 7B is a front view.

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

FIG. 9 is a circuit diagram of a conventional electrostatic capacitance detection circuit.

[Explanation of symbols]

10, 30 Capacitance detection circuit 11 AC voltage generators 12, 13, 18, 41, 45-47, 110-115
Resistors 14, 44, 48, 100-102 Operational amplifier 15 Capacitor (impedance element) 16 Impedance converter 17 Detected capacitor 20 Signal output terminal 21 Impedance converter input terminal 22 AC output terminal 23 Inversion section output terminal 24 Electrostatic Output terminal 31 of capacitance detection circuit 31 Core section 32 Inversion section 33 Addition section 34, 35 MOSET 40, 42 Variable resistance 43 Capacitor 50 Microphone 51 Lid body 52 Sound hole 53 Vibrating membrane 54 Ring 55a Spacer 56 Fixed electrode 55b Insulation plate 58 IC chip 59 IC package 60 Insulating boards 61a to 61d External electrodes 62 Circuit board

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Claims (9)

[Claims] 1. An electrostatic capacitance detection circuit that outputs a detection signal corresponding to the electrostatic capacitance of a detected capacitor, wherein the impedance converter has a high input impedance and a low output impedance, and a capacitive first impedance element. An operational amplifier, an AC voltage generator that applies an AC voltage to the operational amplifier, and a signal output terminal connected to the output of the operational amplifier, and the input terminal of the impedance converter includes the detected capacitor One end and one end of the first impedance element are connected, the negative impedance path of the operational amplifier includes the first impedance element and the impedance converter, and the detected capacitor and the capacitance detection circuit are adjacent to each other. An electrostatic capacitance detection circuit characterized by being provided as follows. 2. An electrostatic capacitance detection circuit that outputs a detection signal corresponding to the electrostatic capacitance of a capacitor to be detected, the impedance converter having a high input impedance and a low output impedance, and a capacitive first impedance element. An operational amplifier, an AC voltage generator that applies an AC voltage to the operational amplifier, and a signal output terminal connected to the output of the operational amplifier, and the input terminal of the impedance converter includes the detected capacitor One end and one end of the first impedance element are connected, the negative impedance path of the operational amplifier includes the first impedance element and the impedance converter, the detected capacitor, the first impedance element, and the impedance conversion A capacitance detection circuit, which is provided in close proximity to a container. 3. The capacitance detection circuit according to claim 1, wherein the capacitance detection circuit further includes a resistance element connected in parallel with the first impedance element. 4. The electrostatic capacitance detection according to claim 1, further comprising a second impedance provided between the AC voltage generator and the operational amplifier. circuit. 5. The capacitance detection circuit further includes an inverting amplifier circuit that inverts the signal at the signal output terminal, and an addition that adds the output signal of the impedance converter and the output signal of the inverting amplifier circuit. A circuit is provided, The electrostatic capacitance detection circuit of any one of Claims 1-4 characterized by the above-mentioned. 6. The one end of the capacitor to be detected and the input terminal of the impedance converter are connected by an unshielded conductor.
Capacitance detection circuit according to item. 7. The capacitance detection device according to claim 1, wherein the detected capacitor and the capacitance detection circuit are housed in one shield box. circuit. 8. A capacitive sensor as the detected capacitor for detecting a physical quantity according to a change in capacitance, and a capacitive sensor formed on a printed circuit board or a silicon substrate and fixed to the capacitive sensor. An electrostatic capacitance detection device, comprising: the electrostatic capacitance detection circuit according to claim 1. 9. A microphone device comprising: a condenser microphone as the detected capacitor; and the capacitance detection circuit according to claim 1.