JP2003075486A - Impedance detection circuit, and capacitance detection circuit and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a detection circuit of impedance or capacitance that can accurately detect minute impedance and is suited for detecting capacity in a capacity-type sensor such as a condenser microphone. SOLUTION: In a capacitance detection circuit 10 comprising a capacity detection section 1 and a capacity cancellation section 2, the capacity detection section 1 comprises an AC voltage generator 11, a first operational amplifier 14 where a non-inverted input terminal is connected to the ground, a second operational amplifier 16 for composing a voltage follower, a detection capacitor 15 connected between the output terminal of the first operational amplifier 14 and the non-inverted input terminal of the second operational amplifier 16, and the like. The capacity cancellation section 2 comprises a third operational amplifier 30 for inverting and amplifying an AC signal from the AC voltage generator 11, and a cancellation condenser 33 connected between the output terminal and the non-inverted input terminal of the second operational amplifier 16. A capacitor 17 under test is connected between the non-inverted input terminal of the second operational amplifier 16 and the ground.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit and method for detecting electrostatic capacitance, and more particularly to a circuit and method 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 one disclosed in Japanese Patent Application Laid-Open No. 9-280806. FIG. 8 is a circuit diagram showing this capacitance detection circuit. In this detection circuit, a capacitance sensor 92 formed of electrodes 90 and 91 is connected to an inverting input terminal of an operational amplifier 95 via a signal line 93. 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. In addition, the signal line 93 is the shield line 9
4 and is electrically shielded against disturbance noise. The shield line 94 is connected to the non-inverting input terminal of the operational amplifier 95. 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 affected by the stray capacitance is obtained.

[0004]

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, in the detection of a minute capacitance of the order of several pF or fF (femto farad),
There is a problem that the error becomes large.

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 immagnari short circuit state may be slightly 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 or the like, a compact amplifier circuit for converting voice detected by a capacitance sensor such as a condenser microphone into an electric signal with high sensitivity and fidelity is required. Has been. A capacitance of such a capacitance sensor, a change capacitance ΔC that changes according to a change in a physical quantity (such as sound) to be detected,
When expressed by the sum with the constant reference capacitance Cd that does not depend on the physical quantity, the reference capacitance Cd that constantly exists is not important, and the detection target is the change capacitance ΔC. That is, in order to detect a physical quantity such as sound with higher sensitivity, it is desired that only the change capacitance ΔC be detected.

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 small and lightweight voice communication device. It is an object of the present invention to provide a capacitance detection circuit and method suitable for capacitance detection of the capacitance sensor.

[0008]

In order to achieve the above object, an impedance detection circuit according to the present invention is an impedance detection circuit which outputs a detection signal corresponding to the impedance of a detected impedance, and whose input impedance is An impedance converter having a high output impedance, a first impedance element, an operational amplifier, a signal output terminal connected to the output of the operational amplifier, and a canceling current applying means for applying a constant current to the detected impedance. The input terminal of the impedance converter is connected to one end of the detected impedance and one end of the first impedance element, and the first impedance element and the impedance converter are provided in a negative feedback path of the operational amplifier. Includes the signal that appears at the signal output terminal and So that the signal appearing at the input terminal of the impedance converter a relationship of phase, characterized in that the value of the current by one of at least the first impedance element or the cancel current applying means is set.

The capacitance detecting method according to the present invention is
A capacitance detection method for outputting a detection signal corresponding to the capacitance of a capacitor to be detected, wherein an AC voltage is applied to an inverting input terminal of an operational amplifier, and a non-inverting input terminal of the operational amplifier is set to a predetermined potential. Along with the connection, a detection capacitor is connected between the output terminal and the input terminal of the impedance converter, a detected capacitor is connected between the input terminal of the impedance converter and a predetermined potential, and the output terminal of the operational amplifier is connected. The voltage that appears is output as a detection signal, a constant cancel current is applied to the detected capacitor, and the signal that appears at the signal output terminal and the signal that appears at the input terminal of the impedance converter have the same phase relationship, It is characterized in that the value of the current by the detecting capacitor and the cancel current applying means is set in advance.
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.

As a specific example, when the first impedance element and the impedance to be detected are capacitors,
An electrostatic capacitance detection circuit including a capacitance detection unit and a capacitance cancellation unit, wherein the capacitance detection unit includes an AC voltage generator, a first operational amplifier whose non-inverting input terminal is connected to ground, and a voltage follower. And a detection capacitor connected between the output terminal of the first operational amplifier and the non-inverting input terminal of the second operational amplifier,
The capacitance canceling unit includes a third operational amplifier that inverts and amplifies the AC signal from the AC voltage generator, and a cancel capacitor that is connected between the output terminal and the non-inverting input terminal of the second operational amplifier. Is connected between the non-inverting input terminal of the second operational amplifier and the ground.

The signal at the connection point where the non-inverting input terminal of the second operational amplifier, the detected capacitor, the detecting capacitor and the cancel capacitor are connected, and the first
The values of the detection capacitor and the cancellation capacitor are set so that the detection signal at the output terminal of the operational amplifier has the same phase relationship.

For example, the cancel current applying means is
A capacitor microphone that is composed of a voltage amplifier that multiplies the voltage at the connection point between one end of the detected capacitor and the impedance converter by A, and a cancel capacitor having a capacitance Cc that is connected between the output terminal of the voltage amplifier and the connection point. Physical quantity (sound) of the capacitance of the detected capacitor such as
Assuming that a constant reference capacitance that does not depend on Cd is Cd and a change capacitance that changes depending on a physical quantity is ΔC, (i) the voltage gain A of the voltage amplifier and the capacitance Cc of the cancel capacitor are Cd = (A −1) · Cc is satisfied, and (ii) when the absolute value of the maximum value of the change capacitance ΔC is ΔCmax, the capacitance Cf of the detecting capacitor is set to a value satisfying ΔCmax ≦ Cf. It should be noted that the variable A shown in A times and the gain A and the like in the present application all represent real numbers other than zero.

As a second measure, the voltage gain A of the voltage amplifier and the capacitance Cc of the cancel capacitor may be set to values satisfying (A-1) .Cc≤Cd-.DELTA.Cmax + Cf.

Further, as a third measure, the voltage gain A of the voltage amplifier and the capacitance Cc of the cancel capacitor may be set to values satisfying (A-1) · Cc ≧ Cd + ΔCmax + Cf.

The same operational amplifier may be realized for the voltage amplifier and the impedance converter.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. 1 to 4 are circuit diagrams of a capacitance detection circuit 10 as an example of an impedance detection circuit according to an exemplary embodiment of the present invention. In the figure, the capacitance detection circuit 10 is provided with a detected capacitor 17 which is an example of a detected impedance to be detected (here, various physical quantities such as a condenser microphone are utilized by utilizing a change in electrostatic capacity). A capacitance type sensor for detecting, in which an electrostatic capacitance Cs as an example of the impedance thereof changes depending on a reference capacitance Cd as an example of a constant reference impedance that does not depend on a physical quantity of a detection target and an impedance change amount. (Represented by the sum of ΔC) is connected.

The capacitance detection circuit 10 is roughly divided into
The capacitance detection unit 1 which is an example of an impedance detection unit which is a basic circuit for detecting the capacitance of the detected capacitor 17, and a part of the capacitance of the detected capacitor 17 (for example, the reference capacitance C
d) and the capacitance canceling unit 2 that cancels the detection by the capacitance detecting unit 1, and the capacitance of the detected capacitor 17 after the cancellation (for example, the variation Δ
The detection signal (voltage Vout) corresponding to C) is output from the signal output terminal 20.

The capacitance detector 1 includes an AC voltage generator 11 for generating an AC voltage, a resistor (R1) 12, a resistor (R2) 13,
First operational amplifier 14, detection capacitor (capacitance Cf) 1
5 and an impedance converter 16a having a high input impedance and a low output impedance and a gain of A times.

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). Output terminal of AC voltage generator 11 and first operational amplifier 1
A resistor (R1) 12 is connected between the inverting input terminal 4 and the inverting input terminal 4.

The first operational amplifier 14 is a voltage amplifier having extremely high input impedance and open loop gain.
Here, the non-inverting input terminal is connected to 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. The negative feedback path of the first operational amplifier 14, that is, between the output terminal and the inverting input terminal of the first operational amplifier 14, the detection capacitor 15, the impedance converter 16a and the resistor (R2).
13 are connected in series in this order.

In the impedance converter shown in FIGS. 2 to 4, the inverting input terminal and the output terminal are connected, the input impedance is extremely high, the output impedance is extremely low, and the voltage follower 16b having a voltage gain of 1b.
Are configured. One end of the detected capacitor 17 is connected to the non-inverting input terminal of the voltage follower 16b, while the other end of the detected capacitor 17 is connected to a predetermined potential (ground in this example). First operational amplifier 1
A 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, is connected to the output terminal of No. 4.

When the impedance converter 16b is a voltage follower, the voltage gain is A = 1,
Since both input terminals of the voltage follower are in an imaginary short state, the voltage of the inverting input and the output is determined,
The voltage at the non-inverting input of the voltage follower is determined.
On the other hand, since the operational amplifier 14 has a sufficient gain, it can be said that the amplifier for obtaining a sufficient gain and the amplifier for determining the voltage are divided here. By doing so, the non-inverting input of the operational amplifier 14 can be connected to a predetermined potential, the stability of its operation can be improved, and a large reduction in the operational error can be realized while gaining sufficient gain. This can be said to be a more preferable aspect because it becomes possible.

FIG. 7 shows a specific circuit example of the impedance converter 16a in the impedance detection circuit 10 shown in FIG. FIG. 7A shows a voltage follower using the operational amplifier 100. Operational amplifier 1
00 inverting input terminal and output terminal are short-circuited. The non-inverting input terminal of the operational amplifier 100 is used as the input of the impedance converter 16a and the output terminal of the operational amplifier 100 is used as the output of the impedance converter 16a, so that the input impedance is extremely high and the voltage gain A becomes 1. The converter 16a is obtained. When this is specifically incorporated, it becomes FIGS.

FIG. 7B shows a non-inverting amplifier circuit 16 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) 111) is connected between the inverting input terminal and the output terminal of the operational amplifier 101. The non-inverting input terminal of this operational amplifier 101 is used as the input of the impedance converter 16a, and the operational amplifier 101
By using the output terminal of as the output of the impedance converter 16a, an impedance converter 16a having an extremely high input impedance and a voltage gain A of (R10 + R11) / R10 can be obtained.

FIG. 7C 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. 7A and 7B. 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 16a and the output terminal of the operational amplifier as the output of the impedance converter 16a,
An impedance converter 16a having an extremely high input impedance can be obtained.

FIG. 7 (d) shows a circuit such as a buffer in the input stage of FIG. 7 (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. 7 (e), the non-inverting input of the operational amplifier 102 is used as the input of the impedance converter 16a, one end of the resistor 114 is connected to the inverting input terminal of the operational amplifier 102, and the output of the operational amplifier 102 and the inverting input are connected. Resistance between 115
It has been connected through. As shown in FIGS. 7D and 7E, by adopting such a configuration, the impedance converter 16a having an extremely high input impedance can be obtained.

The capacitance canceling section 2 is a constant current source for supplying a constant current (cancellation current) to the detected capacitor 17, and a resistor (R3) 31 for inverting and amplifying the AC signal from the AC voltage generator 11. , Resistor (R4) 32 and third
A cancel capacitor 33 connected between the inverting amplifier circuit including the operational amplifier 30 and the output terminal of the third operational amplifier 30 and the non-inverting input terminal of the impedance converter 16a.
Composed of and. The capacitance canceling unit 2 supplies a constant current to the detection target capacitor 17 so that a part (or change amount) of the current flowing through the detection target capacitor 17 is obtained.
Flow through the detection capacitor 15 to cancel the component corresponding to the steady capacitance of the detected capacitor 17 included in the detection signal appearing at the signal output terminal 20 (to prevent the DC component of the capacitance from being detected). )is doing. In other words, the cancel capacitor 33 and the detected capacitor 17 have V3 so that the connection point between the cancel capacitor 33 and the detected capacitor 17 becomes V1.
Is divided. Then, the voltage to be applied to at least a part of the detected capacitors is applied from the output terminal of the third operational amplifier 30 via the cancel capacitor 33.

Here, as a typical example, the cancel current applied from the capacitance canceling unit 2 toward the detected capacitor 17 flows through the reference capacitance Cd of the detected capacitor 17 (the impedance converter 16a has a non-current value). If the voltage at the inverting input terminal is V1, then jωCd · V1)
It is assumed that the capacitances of the resistor (R3) 31, the resistor (R4) 32, and the cancel capacitor 33 that configure the capacitance canceling unit 2 are adjusted in advance so that they become equal to. At this time, a detection signal corresponding to only the change amount ΔC of the electrostatic capacitance Cs of the detected capacitor 17 is generated at the signal output terminal 20 of the capacitance detection unit 1.

The detailed operation of the electrostatic capacitance detection circuit 10 configured as described above is as follows, taking FIG. 2 as an example.

Focusing on the inverting amplifier circuit composed of the resistor (R1) 12, the resistor (R2) 13, the first operational amplifier 14, etc., both input terminals of the first operational amplifier 14 are in the state of an immagnari short. Since they have the same potential (for example, 0 V) and their input impedance is extremely high and no current flows, the current flowing through the resistor (R1) 12 becomes Vin / R1 and all of them flow through the resistor (R2) 13. Therefore, set the output voltage of the voltage follower 16b to V2.
Then, Vin / R1 = -V2 / R2 holds. By organizing this, the output voltage V2 of the voltage follower 16b becomes V2 =-(R2 / R1) .Vin (Equation 1). Further, since both input terminals of the voltage follower 16b are in the state of the immagnari short circuit, the input voltage (voltage at the non-inverting input terminal) V1 and the output voltage (voltage at the inverting input terminal and the output terminal 22) V2 become equal, The input voltage V1 is as follows: V1 = V2 =-(R2 / R1) .Vin (Equation 2).

On the other hand, focusing on the capacity canceling unit 2,
Since the resistance (R3) 31, the resistance (R4) 32 and the third operational amplifier 30 constitute an inverting amplifier circuit, the voltage V3 at the output terminal of the third operational amplifier 30 is V3 =-(R4 / R3)・ Vin. Here, by eliminating Vin using the above equation 2, the voltage V3 becomes: V3 = (R1 / R2). (R4 / R3) .V1 = A.V1 (Equation 3)

It is expressed as follows. However, A is the connection point 21
A = (R1 / R2) · (R4 / R3).

Now, the non-inverting input terminal of the voltage follower 16b, the detected capacitor 17, the detection capacitor 1
5 and the current flowing in and out of the connection point 21 to which the cancel capacitor 33 is connected, the input impedance of the voltage follower 16b is extremely high, and therefore no current flows in the non-inverting input terminal of the voltage follower 16b. Connect the detection capacitor 15 to the connection point 2
The current i1 flowing toward 1 is the cancellation capacitor 3
3 is the current flowing toward the connection point 21 and i2 is the connection point 2
The current flowing from 1 to the detected capacitor 17 is i
When set to 3, i1 = i3-i2 (Equation 4) is expressed.

Since the current i1 is a current flowing through the detecting capacitor 15, it is expressed as follows: i1 = Cf (d / dt)  (Vout-V1) (Equation 5) Since the current i2 is a current flowing through the cancel capacitor 33, it is expressed as i2 = Cc (d / dt)  (V3−V1), and using the relationship of the above equation 3, = Cc ( A-1). (D / dt) .V1 (Equation 6) The current i3 is the detected capacitor 17
Since it is a current flowing through, i3 = (d / dt) .multidot.Cs.multidot.V1 = (d / dt) .multidot. (Cd + .DELTA.C) .multidot.V1 (Equation 7).

The current i expressed by these three equations 5 to 7
Substituting 1 to i3 into the above equation 4, Cf · (d / dt) · (Vout−V1) = (d / dt) · (Cd +
.DELTA.C) .multidot.V1-Cc.multidot. (A-1) .multidot.V1. When the whole is integrated, Cf. (Vout-V1) = (Cd + .DELTA.C) .multidot.V1-Cc. (A
-1). (D / dt) .V1. If this is rearranged with respect to Vout, the output voltage Vout of the detection signal is Vout = (1+ (Cd- (A-1) .Cc + .DELTA.C) / Cf) .V1 (Equation 8).

Here, the above-mentioned typical condition in the capacitance canceling section 2, that is, the current i2 (= Cc. (A-1). (D / dt) .V1) flowing through the canceling capacitor 33 is the detected capacitor 17 Current ((d
/ Dt) · Cd · V1) is taken into consideration. That is, (d / dt) * Cd * V1 = (d / dt) * Cc * (A-1) *
V1 By rearranging this, it is assumed that the relationship of Cd = (A-1) .Cc (Equation 9) is established.

Then, as can be seen by substituting the Cd expressed by the equation 9 into the equation 8, the voltage Vout of the detection signal is Vout = (1 + ΔC / Cf) V1 (equation) under these conditions. 10) is simplified.

On the other hand, paying attention to the voltage at the connection point 21 to which the non-inverting input terminal of the voltage follower 16b, the detected capacitor 17, the detection capacitor 15 and the cancel capacitor 33 are connected, the charge at the connection point 21 is Considering this, since the input impedance of the voltage follower 16b is extremely high, no current flows through the non-inverting input terminal of the voltage follower 16b, so that the detected capacitor 17, the detecting capacitor 15, and the cancel capacitor 33 have the same charge amount. Become. That is, since the charge is stored, it can be expressed as Cc. (V3-V1) + Cf. (Vout-V1) = (Cd + .DELTA.C) .V1 (Equation 11). Then, Vout becomes as follows. Vout = ((Cd + .DELTA.C + Cc) / Cf + 1) .V1- (Cc / Cf) .V3 =-(R2 / R1). (1+ (Cd + .DELTA.C + Cc) /Cf-A.Cc/Cf) .Vin =-(R2 / R1)・ (1+ (Cd + ΔC- (A-1) Cc) / Cf) ・ Vin (Equation 12)

Here, the above-mentioned typical condition in the capacitance canceling unit 2, that is, the condition that the charge amount of the cancel capacitor 33 is set to be equal to the charge amount of the reference capacitance Cd of the detected capacitor 17 is set. Consider. That is, Cc · (V3−V1) = Cd · V1 (Equation 13) By rearranging this, when the relationship of Cd = (A-1) · Cc (Equation 14) is established, under such conditions Then, the voltage Vout of the detection signal is Vout = (1 + .DELTA.C / Cf) .multidot.V1 (Equation 15), which is a simplified one like Equation 10.

As shown in the equations (10) and (15),
The voltage Vout of the detection signal output from the signal output terminal 20 of the electrostatic capacitance detection circuit 10 does not depend on the reference capacitance Cd of the electrostatic capacitance Cs of the detected capacitor 17, and its variation ΔC.
The value depends only on. For example, the detected capacitor 1
When 7 is a condenser microphone, an unnecessary offset amount (the detected condenser 1
7 does not include the signal corresponding to the reference capacitance Cd of 7) but includes only the component depending on the strength of the sound (the signal corresponding to the change amount ΔC of the capacitance of the detected capacitor 17). Therefore, a net signal corresponding to only sound can be greatly amplified, and a highly sensitive microphone can be realized.

Further, as can be seen from the above expression 10 which does not include the angular frequency ω, the voltage V of this detection signal is
out does not depend on the frequency of the AC signal Vin from the AC voltage generator 11 or the changing frequency of the detected capacitor.
As a result, the capacitance (change ΔC) of the detected capacitor 17 can be detected without depending on the frequency of the AC voltage applied to the detected capacitor 17 or the changing frequency of the detected capacitor (frequency-dependent characteristic). Capacitive detection circuit is realized. Therefore,
It is possible to specify the capacitance value directly 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 will be possible.

Further, in the electrostatic capacitance detection circuit 10 of this example, the first operational amplifier 14 which supplies current to the detection capacitor 15 and the detected capacitor 17 has a non-inverting input terminal of a predetermined potential (main (In the example ground) is connected and fixed. Therefore, unlike the operational amplifier 95 in the conventional circuit shown in FIG. 8, the first operational amplifier 1
Reference numeral 4 denotes a detection capacitor 1 that produces a stable current with little noise, without depending on the frequency of the input AC signal.
5 and the capacitor 17 to be detected, and the calculation error is also reduced, so that a minute capacitance of the capacitor 17 to be detected can be detected.

By the way, as described above, the capacitance C of the capacitor to be detected 17 is simply set by canceling all the current flowing through the reference capacitance Cd.
Defects may occur depending on the value of s. For example, the electrostatic capacity Cs of the detected capacitor 17 is
When the value becomes smaller than a certain value, the waveform of the detection signal may have a shape in which the phase is inverted by 180 degrees, which may make post-processing difficult.

FIG. 5 shows an example of the waveform of the detection signal having such inversion (solid line) and an example of the waveform of the detection signal having no inversion (dotted line). As shown in the figure, in the waveform of the solid line, the phase is 180 degrees when the capacitance Cs of the detected capacitor 17 becomes smaller than a certain value, that is, in the time zone shown by the arrow in the figure. It looks like an inverted shape. Therefore, in order to faithfully reproduce the sound from the signal having such a waveform, a complicated post-processing circuit that restores the inverted portion is necessary. In the figure, the high frequency component included in the waveform corresponds to the AC signal generated by the AC voltage generator 11, and the low frequency component appearing in the envelope is the capacitance C of the detected capacitor 17.
It corresponds to the change amount ΔC of s (change in physical quantity such as sound).

Therefore, a method (a concrete circuit constant setting method) for avoiding the inversion of the detection signal and the post-processing associated therewith has been devised, which will be described below. (1) First Measure This measure is a method of adjusting the capacitance Cf of the detection capacitor 15.

Specifically, (i) a condition for canceling all of the reference capacitance Cd (formula 9 and formula 14), that is, Cd = (A-1) Cc (formula 9 and formula 14 (repost)) ) Is satisfied, and (ii) when the absolute value of the maximum value of the variation ΔC of the electrostatic capacitance Cs of the detected capacitor 17 is ΔCmax, the capacitance Cf of the detecting capacitor 15 satisfies ΔCmax ≦ Cf. Adjust to.

Then, from the condition (i), the output voltage Vout of the detection signal is expressed by the above equations 10 and 15, that is, Vout = (1 + ΔC / Cf) · V1 (equation 10, equation 15 (reprinted)). Further, since −1 ≦ ΔC / Cf ≦ 1 is satisfied from the above condition (ii), the voltage Vou on the right side of the above equation 10 is satisfied.
Proportional coefficient (1 + ΔC / Cf) that correlates t with voltage V1
Is a value greater than or equal to zero. Therefore, the detected capacitor 1
The problem that the phase relationship between the voltage Vout and the voltage V1 of the detection signal is inverted depending on the value of the electrostatic capacitance 7 is avoided.

According to the first measure, the larger the capacitance Cf of the detection capacitor 15 is, the smaller ΔC / Cf on the right side of the above equation 10 is, and the signal (change capacitance ΔC) included in the output voltage Vout is reduced. Has a weakness that the detection sensitivity is lowered due to the decrease of the voltage component). (2) Second Measure This measure is a method of adjusting the cancellation amount with respect to the reference capacitance Cd. Specifically, one of the following two methods may be used. (2-1) Pay attention to when the electrostatic capacitance Cs of the detected capacitor 17 has the minimum value, and even at this time, the cancel amount is set so that the voltage Vout of the detection signal does not have a reverse phase with respect to the voltage V1. Is a method of adjusting. Specifically, it is as follows. First, irrespective of the amount of cancellation, in the electrostatic capacitance detection circuit 10 of FIG. Vout = (1+ (Cd- (A-1) .Cc + .DELTA.C) / Cf) .V1 (Formula 8 (repost)) Vout =-(R2 / R1). (1+ (Cd + .DELTA.C- (A-1) .Cc) / Cf) · Vin (Equation 12 (repost))

Here, when the absolute value of the maximum value of the variation ΔC of the electrostatic capacitance Cs of the detected capacitor 17 is ΔCmax,
As can be seen from the above equations 8 and 12, when the capacitance Cs of the detected capacitor 17 has the minimum value, the output voltage Vout is expressed as follows. Vout = (1+ (Cd- (A-1) .Cc-.DELTA.Cmax) / Cf) .V1 (Equation 16) Therefore, the voltage Vout and the voltage V1 shown in this Equation 11 are obtained.
Proportional coefficient (1+ (Cd- (A-1) .Cc
By setting −ΔCmax) / Cf) to a value of zero or more,
Even when the capacitance Cs of the detected capacitor 17 reaches the minimum value, the phase relationship between the voltage Vout and the voltage V1 can be maintained in the same phase. That is, 1+ (Cd- (A-1) .Cc-.DELTA.Cmax) /Cf.gtoreq.0. This is rearranged so that (A-1) .Cc.ltoreq.Cd-.DELTA.Cmax + Cf (Equation 17) is satisfied. At connection point 21
To the gain A and the capacitance C of the cancel capacitor 33
By adjusting c and the detection capacitor Cf, the output voltage Vou may be changed depending on the capacitance value of the detected capacitor 17.
It is possible to avoid the problem that t is inverted and post-processing becomes difficult. Therefore, the capacitance change ΔC of the detected capacitor 17 can be specified by simply performing a simple process such as obtaining a peak point in the peak hold circuit for the output voltage Vout.

(2-2) Pay attention to when the capacitance Cs of the capacitor to be detected 17 has the maximum value, and even at this time, the voltage Vout of the detection signal has a reverse phase with respect to the voltage V1. It is a method to adjust the cancellation amount. That is, in the above (2-1), the condition that the output voltage Vout is not inverted is satisfied. However, even if the condition that the output voltage Vout is always inverted is satisfied, the phase relationship between the voltage Vout and the voltage V1 remains the same. This is a method of determining the circuit constants so that the conditions can be maintained and the problem can be solved similarly. Specifically, it is as follows.

First, irrespective of the amount of cancellation, in the electrostatic capacitance detection circuit 10 of FIG. 1, the above equations 8 and 12 are established. Vout = (1+ (Cd- (A-1) .Cc + .DELTA.C) / Cf) .V1 (Formula 8 (repost)) Vout =-(R2 / R1). (1+ (Cd + .DELTA.C- (A-1) .Cc) / Cf) · Vin (Equation 12 (repost))

Here, when the absolute value of the maximum value of the variation ΔC of the electrostatic capacitance Cs of the detected capacitor 17 is ΔCmax,
As can be seen from the above equations 8 and 12, when the capacitance Cs of the detected capacitor 17 has the maximum value, the output voltage Vout is expressed as follows. Vout = (1+ (Cd- (A-1) .Cc + .DELTA.Cmax) / Cf) .V1 (Equation 18) Therefore, the voltage Vout and the voltage V1 shown in this Equation 13 are obtained.
Proportional coefficient (1+ (Cd- (A-1) .Cc
By setting + ΔCmax) / Cf) to a value below zero,
The phase relationship between the voltage Vout and the voltage V1 can always be kept the same (opposite phase). In other words, 1+ (Cd− (A-1) · Cc + ΔCmax) / Cf ≦ 0 Organizing this, the connection point in the capacity canceling unit 2 is such that (A-1) · Cc ≧ Cd + ΔCmax + Cf (Equation 19) holds. 21
To the gain A and the capacitance C of the cancel capacitor 33
By adjusting c and the detection capacitor Cf, the output voltage V may be changed depending on the capacitance value of the detected capacitor 17.
It is possible to avoid the problem that out is reversed and post-processing becomes difficult. Therefore, the output voltage Vout
On the other hand, the capacitance change ΔC of the detected capacitor 17 can be specified only by performing a simple process such as obtaining a peak point in the peak hold circuit.

As an application of the electrostatic capacitance detection circuit as the impedance detection circuit of the present invention to electronic equipment, the detected capacitor is a capacitance type sensor that detects a physical quantity according to a change in capacitance, and the electrostatic capacitance is It is considered that the detection circuit is formed on a printed circuit board or a silicon substrate, and the capacitive sensor and the circuit board are fixed to each other. Specifically, a condenser microphone is used as the detected capacitor, and the capacitance detection circuit is realized by an IC.
The condenser microphone and the IC may be integrated and housed in one housing (shield box) as a microphone used in a mobile phone or the like.

The impedance detection circuit according to the present invention has been described above based on the embodiment, but the present invention is not limited to this embodiment.

For example, in the electrostatic capacitance detection circuit 10 shown in FIGS. 1 to 4, the impedance converter 16a (voltage follower 16b) included in the capacitance detection unit 1 and the voltage source (first Although the operational amplifiers are separate from the three operational amplifiers 30), they may be configured by a single impedance converter or operational amplifier.

FIG. 6A is a circuit diagram showing an example of a capacitance detection circuit in which such an impedance converter and a voltage source for generating a cancel current are constituted by one impedance converter 16a. Here, instead of the capacitance canceling section 2 shown in FIG. 1, a resistor (R5) 35 is connected to the negative feedback path of the impedance converter 16a, and a cancel capacitor 33 is connected to the positive feedback path. That is, the impedance converter 16 a is used as a voltage source, and the cancel current similar to the above is applied to the connection point 21 via the cancel capacitor 33.

More specifically, as shown in FIG. 6B, an AC output terminal of a non-inverting amplifier circuit composed of a resistor (R2) 13, a resistor (R5) 35 and a voltage follower 16b as an impedance converter. Focusing on the current of (22), the output voltage V2 of the voltage follower 16b is
Becomes V2 = ((R2 + R5) / R2) .multidot.V1 = A'.V1 (Equation 20). However, A '= (R2 + R5) / R2.

Then, this voltage V2 is applied to the cancel capacitor 33 to generate a cancel current i2, so that the current i2 is i2 = (d / dt) .Cc. (V2-V1) = (d / dt) · Cc · (A′−1) · V1 and is expressed in the same form as the above equation 6 showing the current i2 from the capacitance canceling section 2 shown in FIG. On the other hand,
Further, focusing on the voltage of the AC output terminal (22) of the non-inverting amplifier circuit composed of the resistor (R2) 13, the resistor (R5) 35, and the voltage follower 16b in FIG. Therefore, this voltage V2 is applied to the cancel capacitor 33, and the cancel capacitor 3
It can be said that the charge amount at 3 is determined. Then, the charge amount Qc in the cancel capacitor becomes Qc = Cc.multidot. (V2-V1) = (A'-1) .multidot.Cc.multidot.V1, which is expressed in the same form as the above-mentioned formula 11. Therefore,
The electrostatic capacitance detection circuit 40 shown in this figure has the same function as the electrostatic capacitance detection circuit 10 shown in FIG. 1, and the same can be said for the countermeasures against the inversion of the detection signal.

As another embodiment for further improving the accuracy of the present invention, the wiring around the connection point 21 as shown in FIGS. 3 and 4 is covered with a shield, and a guard voltage is applied to the shield. The guard voltage may be applied by the circuits 60 and 61. By doing so, the shield can be held at the same potential as the wiring around the connection point 21, the generation of stray capacitance due to the shield can be suppressed, and further, highly accurate detection that is strong against external noise becomes possible.

Also, what is connected as the impedance to be detected is an unknown capacitance (semiconductor chip or the like) condenser microphone, acceleration sensor, seismograph, pressure sensor,
Displacement sensor, displacement gauge, proximity sensor, touch sensor, ion sensor, humidity sensor, raindrop sensor, snow sensor, lightning sensor, alignment sensor, contact failure sensor, shape sensor,
Various physical quantities such as end point detection sensor, vibration sensor, ultrasonic sensor, angular velocity sensor, liquid amount sensor, gas sensor, infrared sensor, radiation sensor, water level meter, freezing sensor, moisture meter, vibrometer, electrification sensor, printed circuit board inspection machine, etc. Includes all devices to detect.

[0062]

As is apparent from the above description, the impedance and capacitance detection circuit and the impedance and capacitance detection method according to the present invention output a detection signal corresponding to the impedance of the impedance to be detected. Therefore, an AC voltage is applied to the inverting input terminal of the operational amplifier via a resistor, the non-inverting input terminal of the operational amplifier is connected to a predetermined potential, and the output terminal and the input terminal of the impedance converter are connected to each other. 1 impedance element is connected, the detected impedance is connected between the input terminal of the impedance converter and a predetermined potential, the voltage appearing at the output terminal of the operational amplifier is output as a detection signal, and the detected impedance is constant. A signal that appears at the signal output terminal when a cancel current is applied and the input terminal of the impedance converter A signal appearing at the way a relationship of phase, characterized in that setting the value of the current by the first impedance element and the cancel current applying means.

As a result, 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, so that the operational amplifier operates stably and the noise contained in the detection signal is suppressed. Therefore, it is possible to detect an extremely minute impedance, and particularly when the impedance is a capacitor, it is possible to detect a minute capacitance of the order of several pF or fF.

Then, of the current flowing through the impedance to be detected, the remaining current after subtracting the cancel current is the first
Since it flows to the impedance element and is output as a detection signal to the output terminal of the operational amplifier, for example, detection of an unnecessary reference capacitance in a capacitive sensor such as a condenser microphone is canceled, and a variable capacitance that changes according to a physical quantity such as sound. Only detected. Therefore, a highly sensitive microphone or the like that amplifies only a net signal corresponding to a physical quantity such as sound is realized.

Furthermore, the value of the current by the first impedance element and the cancel current applying means is set so that the signal appearing at the signal output terminal and the signal appearing at the input terminal of the impedance converter have the same phase relationship. ,
It is possible to prevent the inconvenience that the phase relationship between both signals is inverted depending on the impedance value of the detected impedance. Therefore, a complicated post-processing circuit for compensating for phase inversion becomes unnecessary, and the change in impedance of the detected impedance can be taken out only by a simple circuit such as a peak hold circuit, and the entire circuit can be made compact.

The capacitance detection circuit according to the present invention is
Capacitance is detected by passing a current through the capacitor to be detected, so there is no need to attach a polymer film to the electrodes of the capacitor to be electret like an electret condenser microphone, and it is not necessary to use normal electrostatic discharge. It can be applied to a capacitive sensor.

As described above, according to the present invention, an impedance detection circuit, an electrostatic capacitance detection circuit, etc., which can accurately detect minute impedances and capacitances and are suitable for downsizing, are realized, and particularly, portable Light weight such as telephones
The voice performance of small voice communication devices has been dramatically improved, and its practical value is extremely high.

[Brief description of drawings]

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

FIG. 2 is an example of a circuit diagram of a capacitance detection circuit when the impedance converter in FIG. 1 is a voltage follower.

FIG. 3 is an example of a circuit diagram of a capacitance detection circuit when a wiring around a connection point in FIG. 1 is shielded.

FIG. 4 is an example of a circuit diagram of another electrostatic capacitance detection circuit in the case where the wiring around the connection point in FIG. 1 is shielded.

FIG. 5 is an example of a waveform of a detection signal causing inversion (solid line).
FIG. 6 is a diagram showing an example (dotted line) of a waveform of a detection signal in which inversion does not occur.

FIG. 6A is an example of another electrostatic capacitance detection circuit configured by an impedance converter. (B) is an example of another electrostatic capacitance detection circuit configured by a voltage follower.

7A to 7E are specific circuit examples of the impedance converter.

FIG. 8 is a circuit diagram of a conventional capacitance detection circuit.

[Explanation of symbols]

10, 40 Capacitance detection circuit 11 AC voltage generators 12, 13, 31, 32, 35, 110-115 Resistors 14, 16a, 16b, 30, 100-102 Operational amplifier 15 Detection capacitor 17 Detected capacitor 20 Signal Output terminal 21 Connection point 22 AC output terminal 33 Canceling capacitors 34, 35 MOSFETs 60, 61 Guard voltage applying circuit

Claims (10)

[Claims] 1. An impedance detection circuit for outputting a detection signal corresponding to the impedance of a detected impedance, the impedance converter having a high input impedance and a low output impedance, a first impedance element, an operational amplifier, and the operation. A signal output terminal connected to the output of the amplifier, and a cancel current applying means for applying a constant current to the detected impedance are provided, and one end of the detected impedance and the first impedance are provided at an input terminal of the impedance converter. One end of an impedance element is connected, the negative impedance path of the operational amplifier includes the first impedance element and the impedance converter, and a signal that appears at the signal output terminal and a signal that appears at the input terminal of the impedance converter So that they are in phase Impedance detection circuit that also characterized that the value of the current by either one of the first impedance element or the cancel current applying means is set. 2. The canceling current applying means includes a voltage amplifier that amplifies a voltage at a connection point between one end of the detected impedance and the impedance converter by A times, an output terminal of the voltage amplifier, and the connection point. The impedance detection circuit according to claim 1, further comprising a cancel impedance connected between the impedance detection circuit and the impedance. 3. The impedance detection circuit according to claim 1, wherein the voltage amplifier and the impedance converter have the same operational amplifier as a constituent element. 4. The impedance to be detected is a capacitor to be detected, the impedance is an electrostatic capacitance, and the first impedance element is an electrostatic capacitance for detection. Capacitance detection circuit. 5. The cancel impedance according to claim 2, wherein the cancel impedance is a cancel capacitor.
The described capacitance detection circuit. 6. The capacitor to be detected is a capacitive sensor that detects a physical quantity based on a change in electrostatic capacity, and the electrostatic capacity changes depending on a fixed reference capacitance Cd that does not depend on the physical quantity and the physical quantity. 6. A value satisfying Cd = (A-1) Cc, where Cc is the capacity of the canceling capacitor when expressed by the sum of the capacity ΔC and the cancel capacitor is Cc.
The described capacitance detection circuit. 7. The absolute value of the maximum value of the change capacity ΔC is Δ
The capacitance Cf of the detection capacitor is set to a value satisfying ΔCmax ≦ Cf when Cmax is set.
The described capacitance detection circuit. 8. The capacitor to be detected is a capacitive sensor that detects a physical quantity based on a change in electrostatic capacitance, and the electrostatic capacitance changes depending on a constant reference capacitance Cd that does not depend on the physical quantity and the physical quantity. When the absolute value of the maximum value of the change capacitance ΔC is ΔCmax, the capacitance of the cancel capacitor is Cc, and (A-1) · Cc ≦ Cd−ΔCmax + Cf is satisfied. 6. The value is set to a value.
The described capacitance detection circuit. 9. The capacitor to be detected is a capacitive sensor that detects a physical quantity based on a change in electrostatic capacitance, and the electrostatic capacitance changes depending on a constant reference capacitance Cd that does not depend on the physical quantity and the physical quantity. When the absolute value of the maximum value of the change capacitance ΔC is ΔCmax, the capacitance of the cancel capacitor is Cc, and (A-1) · Cc ≧ Cd + ΔCmax + Cf is satisfied. It is set, The claim 5 characterized by the above-mentioned.
The described capacitance detection circuit. 10. A capacitance detection method for outputting a detection signal corresponding to the capacitance of a capacitor to be detected, comprising: applying an AC voltage to an inverting input terminal of an operational amplifier, the non-inverting input terminal of the operational amplifier. Is connected to a predetermined potential, a detection capacitor is connected between its output terminal and the input terminal of the impedance converter, and a detected capacitor is connected between the input terminal of the impedance converter and a predetermined potential, The voltage appearing at the output terminal of the amplifier is output as a detection signal, a constant cancel current is applied to the detected capacitor, and the signal appearing at the signal output terminal and the signal appearing at the input terminal of the impedance converter are in phase relationship. So that the value of the current by the detecting capacitor and the cancel current applying means is set in advance. Capacitance detection method.