JP2003156523A - Impedance detector circuit and capacitance detector circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an impedance detector circuit which is capable of accurately detecting a very small impedance and is moreover suited for miniaturization. SOLUTION: A capacitance detector circuit 10 comprises an AC voltage generator 11, an operational amplifier 14 having a non-inverting input terminal connected to a specified potential (grounding, in the present embodiment), an impedance transformer 16, a resistor (R1 ) 12 connected between the AC voltage generator 11 and an inverting input terminal of the operational amplifier 14, a resistor (R2 ) 13 connected between the inverting input terminal of the operational amplifier 14 and an output terminal of the transformer 16, and an impedance element (capacitor) 15, connected between an output terminal of the operational amplifier 14 and an input terminal of the transformer 16. A capacitor 17 under test is connected between the input terminal of the transformer 16 and the specified potential.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for detecting impedance, and more particularly to a circuit for detecting minute impedance 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. 7 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 applied AC voltage Vac, a tracking error inside the operational amplifier 95,
There is also a problem that a slight phase / amplitude deviation will occur between the voltage of the inverting input terminal and the voltage of the non-inverting input terminal in the state of the emergency short due to the calculation error and the detection error will increase. .

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 an impedance detection circuit including an electrostatic capacitance suitable for the capacitance detection of the capacitance sensor.

[0009]

In order to achieve the above object, an impedance detection circuit according to the present invention is an impedance detection circuit that outputs a detection signal corresponding to the impedance of a detected impedance, and has a high input impedance. An impedance converter having a low output impedance, a capacitive first impedance element, a first operational amplifier, and a voltage generator for applying at least one of an AC voltage and a DC voltage to the first operational amplifier,
A signal output terminal connected to the output of the first operational amplifier, one end of the detected impedance and one end of the first impedance element are connected to an input terminal of the impedance converter, and the first operation The negative feedback path of the amplifier includes the first impedance element and the impedance converter.

As a specific example, a voltage generator, an operational amplifier whose non-inverting input terminal is connected to a predetermined potential, an impedance converter, and a resistor connected between the voltage generator and the inverting input terminal of the operational amplifier. And a resistor connected between the inverting input terminal of the operational amplifier and the output terminal of the impedance converter, and a capacitive first impedance element connected between the output terminal of the operational amplifier and the input terminal of the impedance converter. An impedance detection circuit is configured, and the detected impedance is connected between the input terminal of the impedance converter and a predetermined potential. 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.

With such a configuration, a constant voltage is applied to the impedance to be detected, and almost all the current flowing through the impedance to be detected flows through the first impedance element, and the impedance of the impedance to be detected is detected from the signal output terminal. A signal corresponding to the impedance is output.

It should be noted that noise is mixed in the signal line connecting the impedance detection circuit and the detected impedance,
In order to reduce the generation of stray capacitance between the signal line and a predetermined potential, the detected impedance and the signal line of the impedance detection circuit are preferably as short as possible.

For the voltage generator, at least one of AC and DC can be selected. In AC, both the absolute impedance and the amount of change in impedance can be measured, whereas in DC, the amount of change in impedance is measured. While it can be detected, the AC circuit requires an oscillation circuit and the like, which makes the size of the detection circuit a little larger, while the DC circuit has the characteristic that it can be made more compact. In this way, the voltage generator of the present application can be selected in consideration of its purpose and application. In addition, a second impedance element may be provided between the first operational amplifier and the voltage generator. Also,
A resistor may be connected in parallel with the first impedance element.

Since the signal corresponding to the voltage generated by the voltage generator is included in the output signal from the signal output terminal, a canceling means for canceling the signal is provided in addition to the impedance detection circuit. May be.
Examples of the canceling means include an adder and a subtractor. Also, especially when the detected impedance is capacitive,
A circuit with excellent frequency characteristics can be obtained by using a capacitor for the first impedance element.

[0015]

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 impedance detection circuit according to a first embodiment of the present invention.
In the figure, the capacitance detection circuit 10 as the impedance detection circuit uses a detected capacitor 17 as a detection impedance to be detected (here, a change in the capacitance Cs such as a condenser microphone is used. A capacitive sensor for detecting various physical quantities is connected.

The capacitance detection 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 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. It should be noted that the variable A shown in the multiplication by A or the like in the present application is a real number other than zero.

Regarding the connection between the capacitor to be detected 17 and the electrostatic capacitance detection circuit 10, in order to avoid adding unnecessary stray capacitance as a detection error or mixing in disturbance noise, it is as short as possible. It is preferable to connect with an unshielded conductor (cable, wiring pattern of copper foil, connection terminal, etc.). Furthermore, if possible, in order to enhance the shielding against disturbance noise, the detected capacitor 1
7 and the capacitance detection circuit 10 as a whole are preferably covered with a grounded shield member or housed in a shield box.

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 is obtained. =-(1 + Cs / Cf). (R2 / R1). (Vin / A) (Equation 5) is obtained.

As can be seen from the expression 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.

Here, when the impedance converter 16 is a voltage follower, the voltage gain is A = 1, and both input terminals of the voltage follower are in the state of an emergency short, so that the voltage of the inverting input and the output is determined, and the voltage is determined. The voltage at the non-inverting input of the follower is determined. In this case, it can be said that the operational amplifier 14 and the voltage follower are divided into an amplifier for obtaining a sufficient gain and an amplifier for determining the voltage. By doing so, the non-inverting input of the operational amplifier 14 can be connected to a predetermined potential, and the stability of its operation can be improved, and the gain of the gain can be sufficiently increased and the operation error can be greatly reduced. Since it becomes possible, it can be said that this is a preferable embodiment.

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. 7, the operational amplifier 14 can reduce the operational error without depending on the frequency of the input AC signal, etc., and can generate a stable current with less noise. Since it is supplied to the capacitor 15 and the detected capacitor 17, it is possible to detect the minute capacitance of the detected capacitor 17.

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, an electrostatic capacitance detection circuit according to a second embodiment of the present invention will be described.
FIG. 3 is a circuit diagram of a capacitance detection circuit 30 as an impedance detection circuit according to the second embodiment.
The capacitance detection circuit 30 is roughly divided into a core portion 31 corresponding to the capacitance detection circuit 10 as the impedance detection circuit shown in FIG. 1, and a signal voltage at the signal output terminal 20 of the core portion 31. The inverting unit 32 that inverts V01 as an input, the signal voltage V03 at the output terminal 23 of the inverting unit 32, and the signal voltage V02 at the AC output terminal 22 of the core unit 31 are added, and the voltage V04 is output to the output terminal 24. It is comprised from the addition part 33 which outputs the detection signal of.

The core portion 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 unit 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 or the capacity change 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.

Although the impedance detection circuit according to the present invention has been described above based on the two embodiments, the present invention is not limited to these embodiments and application examples.

For example, the canceling means of the signal corresponding to the voltage generated by the voltage generator of the addition method shown in FIG.
As shown in FIG. 4 or FIG. 5, other adding means and subtracting means may be used.

FIG. 4 shows the signal output 20 of the impedance detection circuit 50 which is functionally the same as the capacitance detection circuit 10.
(Vout) is connected to one input of the adder circuit via the resistor 46, and Vin of the impedance detection circuit 50 is connected to the other input of the adder circuit via the resistor 45. Since this signal output 20 is inverted with respect to Vin, it is possible to cancel the signal corresponding to the voltage generated by the voltage generator by adding it. On the other hand, in FIG. 5, the output from the AC signal output terminal and the output from the signal output terminal Vout of the impedance detection circuit 50 are used as they are. Since these two signals are both inverted with respect to Vin, in order to use these signals as they are, a subtraction circuit is required as shown in this figure. The inputs of the respective addition means and subtraction means can be interchanged.

Further, 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 detected capacitor 17. Instead of this, an impedance element such as a resistor or an inductance may be connected. For example, when a resistor having a resistance value R3 is connected instead of the capacitor 15, the voltage V of the detection signal output from the output terminal 20 of the capacitance detection circuit 10
out is replaced by the above equation 5, and Vout = V01 = {(1 + R3.ΔCs.ωc ・ cos (ωc ・ t)) sin (ωin ・ t) + R3 (Cd + ΔCs ・ sin (ωc ・ t)) ωin ・ cos ( ωin · t)} · (Vin / A) (Equation 11) ΔCs: Change in capacitance of detected capacitor ωc: Frequency of detected capacitor Cd: Reference capacitance of unchanged capacitor ωin: Input voltage frequency . Even in this case, the voltage V04 of the detection signal is
It is still proportional to the capacitance value Cs. Therefore, the unknown capacitance value Cs and the capacitance change can be easily specified by performing various signal processes based on the voltage V04.

Further, as shown in FIG. 6, a resistor 18 may be added and connected in parallel with the capacitor 15 in the electrostatic capacitance detection circuits 10 and 30 in the above 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.

What is connected as an impedance to be detected is an unknown capacitance (in a semiconductor chip, on a board wiring, on a package wiring, etc.), a condenser microphone, an acceleration sensor, a seismograph, a pressure sensor, a displacement sensor, Displacement meter, proximity sensor, touch sensor, ion sensor, humidity 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,
It includes all transducers (devices) that detect various physical quantities such as a liquid quantity sensor, a gas sensor, an infrared sensor, a radiation sensor, a water level meter, a freezing sensor, a moisture meter, a vibrometer, a charge sensor, and a printed circuit board inspection machine.

[0042]

As is apparent from the above description, the impedance detecting circuit and the electrostatic capacitance detecting circuit according to the present invention apply an AC voltage to the operational amplifier via a resistor and apply it to the negative feedback path of the operational amplifier. The impedance of the detected impedance is detected by connecting the detected impedance. That is, a capacitive impedance element is connected between the output terminal of the operational amplifier whose non-inverting input terminal is connected to a predetermined potential and the input terminal of the impedance converter, and the input terminal of the impedance converter is connected to the predetermined potential. The detection impedance is connected.

As a result, almost all the current flowing through the impedance to be detected flows through the impedance element, and an accurate signal corresponding to the impedance of the impedance to be detected is output to the output terminal of the operational amplifier.
It becomes possible to detect extremely small impedance. In particular, when each impedance is capacitive, f
It is possible to measure a minute capacitance of F order or less, and it is possible to perform the measurement that does not depend on the frequency of the change in the detected capacitance.

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 detection signal is detected. The contained noise is suppressed.

Further, since the capacitive impedance element is connected between the operational amplifier and the impedance converter, the frequency of the capacitance change of the detected capacitor does not depend on the frequency of the AC voltage applied to the operational amplifier. A detection sensitivity that does not depend on is also 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.

Here, in the impedance 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. As a result, an unnecessary offset component included in the output signal of the impedance detection circuit is removed, and the net signal corresponding to the impedance of the detected impedance can be greatly amplified.

As described above, according to the present invention, the limitation of the use environment is reduced, a minute impedance can be accurately detected, and an impedance detection circuit, a capacitance detection circuit, etc. suitable for miniaturization are provided. It has been realized, and in particular, the voice performance of a lightweight and small-sized voice communication device such as a mobile phone 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 as an impedance 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 as an impedance detection circuit according to a second embodiment of the present invention.

FIG. 4 shows an example in which the canceling means of the signal corresponding to the generated voltage of the voltage generator by the addition method shown in FIG. 3 is configured by another means (adding circuit).

5 shows an example in which the canceling means for the signal corresponding to the voltage generated by the voltage generator according to the addition method shown in FIG. 3 is configured by another means (subtraction circuit).

FIG. 6 is a circuit diagram of a capacitance detection circuit according to another embodiment of the present invention.

FIG. 7 is a circuit diagram of a conventional 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 the capacitance detection circuit 31 Core part 32 Inversion part 33 Addition parts 34, 35 MOSET 40, 42 Variable resistor 43 Capacitor

Continued front page    (72) Inventor Naoki Ikeuchi             Sumitomo Metal Works, No. 8 Fuso-cho, Amagasaki City, Hyogo Prefecture             Industry Electronics Research Laboratory (72) Inventor Toshiyuki Matsumoto             Sumitomo Metal Works, No. 8 Fuso-cho, Amagasaki City, Hyogo Prefecture             Industry Electronics Research Laboratory (72) Inventor Koichi Nakano             Hokutoden 2-16 Meishio Higashikubo Nishinomiya City, Hyogo Prefecture             Child Industry Co., Ltd. F term (reference) 2G028 AA01 BB10 CG07 CG08 DH03                       DH04 EJ10 FK01

Claims (7)

[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 capacitive first impedance element, and a first calculation. An impedance converter, a voltage generator for applying at least one of an AC voltage and a DC voltage to the first operational amplifier, and a signal output terminal connected to the output of the first operational amplifier. One end of the detected impedance and one end of the first impedance element are connected to an input terminal, and the first impedance element and the impedance converter are included in a negative feedback path of the first operational amplifier. Impedance detection circuit. 2. 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, a first operational amplifier, A voltage generator that applies at least one of an AC voltage and a DC voltage to the first operational amplifier, a signal output terminal connected to the output of the first operational amplifier, and the voltage from the signal at the signal output terminal. A canceling means for canceling all or a part of the signal corresponding to the voltage generated by the generator, wherein one end of the detected impedance and one end of the first impedance element are connected to an input terminal of the impedance converter. The first impedance element and the negative feedback path in the negative feedback path of the first operational amplifier, An impedance detection circuit including an impedance converter. 3. The capacitance detection circuit according to claim 1, wherein the detected impedance is a capacitive impedance element. 4. The electrostatic capacitance detection circuit according to claim 1, further comprising a resistance element connected in parallel with the first impedance element. Capacitance detection circuit. 5. The impedance detection circuit according to claim 1, further comprising a second impedance element provided between the first operational amplifier and the voltage generator. . 6. The impedance detection device according to claim 1, wherein one end of the detected impedance and an input terminal of the impedance converter are connected by an unshielded conductor. circuit. 7. The impedance detection circuit according to claim 1, wherein the impedance converter is a voltage follower having a voltage gain of 1.