JP3501401B2 - Impedance detection circuit, impedance detection device, and impedance detection method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impedance detection circuit, an impedance detection device, and an impedance detection method useful for detecting the impedance of an impedance element such as a capacitance sensor.

[0002]

2. Description of the Related Art As a conventional example of the above-mentioned impedance detection circuit, there is one disclosed in Japanese Patent Application Laid-Open No. 9-280806. FIG. 15 is a circuit diagram showing this impedance detection circuit. In this detection circuit, the capacitance sensor 51 formed by the electrodes 54 and 55 is connected to the signal line 5
7 to the inverting input terminal of the operational amplifier 59. A capacitor 60 is connected between the output terminal of the operational amplifier 59 and the inverting input terminal, and an AC voltage Vac is applied to the non-inverting input terminal. Further, the signal line 57 is covered with a shield line 56 to electrically shield the signal line 57 connected to the capacitance sensor 51. The shield line 56 is connected to the non-inverting input terminal of the operational amplifier 59. The output voltage Vd is taken out from the output terminal of the operational amplifier 59 via the transformer 61.

In the above conventional detection circuit, the operational amplifier 59 is used.
The inverting input terminal and the non-inverting input terminal of are in an imaginary short circuit state. Therefore, the signal line 57 connected to the inverting input terminal and the shield line 56 connected to the non-inverting input terminal
And have almost the same potential. Therefore, even if the signal line 57 is covered with the shield line 56, the stray capacitance between the two 56 and 57 is canceled, and it is intended to obtain an accurate output voltage Vd that is not affected by the stray capacitance.

[0004]

According to the above conventional example,
Certainly, when the capacitance of the sensor 51 is large to some extent, an accurate output voltage Vd that is not affected by the stray capacitance between the signal line 57 and the shield line 56 can be obtained. However, in the tests conducted by the inventors, when the capacitance of the sensor 51 is reduced (for example, on the order of 10 ⁻¹⁵ F), the expected accurate output voltage Vd cannot be obtained and the error increases. Therefore, the inventors have repeatedly examined this phenomenon. As a result, we have obtained the following findings.

That is, the operational amplifier 59 in the above-mentioned conventional example has an AC voltage Vac applied to its input terminal. However, particularly when the non-inverting input terminal voltage of the operational amplifier 59 is changed by the high-frequency AC voltage Vac, the actual operation of the operational amplifier 59 is in an imaginary short state due to a tracking error in the operational amplifier 59 or the like. As a result, a slight phase / amplitude deviation occurs between the voltages at the inverting input terminal and the non-inverting input terminal. Due to this deviation or the like, a phenomenon occurs in which the harmonics of the AC voltage Vac are superimposed on the output voltage of the operational amplifier 59. Then, it was found that this harmonic becomes one of the causes of the error. Further, since one operational amplifier 59 in which one terminal is not connected to the DC bias has two functions of imaginary short circuit and gain, fluctuation occurs especially at high frequency operation. The above has been found for the first time in the research leading to the present invention. Therefore, if the signal line 57 and the shield line 56 are made to have the same potential by utilizing the imaginary short circuit of the operational amplifier 59, stray capacitance that cannot be ignored when the capacitance of the capacitance sensor 51 is very small is generated. It has been found that the output voltage Vd is present between the line 56 and the stray capacitance, so that an error occurs in the output voltage Vd.

On the other hand, as the sensor manufacturing technology has improved in recent years, a sensor having a minute impedance of the order of, for example, 10 ⁻¹⁵ F is emerging.
By using such a sensor, it becomes easy to monitor a minute physical phenomenon which has been difficult in the past. Therefore, there is an increasing need for a circuit and a device that can accurately detect a minute impedance as such a sensor has.

The present invention has been made to solve the above-mentioned conventional problems, and an object thereof is to provide an impedance detection circuit and an impedance useful for accurately detecting a minute impedance or a minute change in impedance. It is to provide a detection device and an impedance detection method.

[0008]

An impedance detection circuit according to the present invention includes a voltage follower and a first operational amplifier, a signal line having one end connected to an input terminal of the voltage follower and an impedance element connected to the other end. Provided between the output terminal of the first operational amplifier and the signal line, shield means for electrically shielding at least a part of the signal line, shield voltage applying means for applying a shield voltage to the shield means. And a signal output terminal connected to the output terminal of the first operational amplifier.

The term "impedance element" as used herein means an element as a component and a measurement electrode provided at the end of the signal line.
The impedance formed between the measurement electrode and the measurement target is also included.

Further, in the impedance detecting circuit of the present application, the first operational amplifier, the second operational amplifier whose input terminals are in an imaginary short circuit state, and one end of which is connected to the force terminal of one of the second operational amplifiers. A signal line to which an impedance element can be connected at the other end, a shield means for electrically shielding at least a part of the signal line, and a first line provided between the output terminal of the first operational amplifier and the signal line.
It is characterized by including an impedance and a signal output terminal connected to the output terminal of the first operational amplifier. A shield voltage applying means may be connected to the shield means.

Here, in these, the shield voltage applying means may include a phase amplitude compensating means, and one input terminal of the first operational amplifier may be connected to a predetermined first voltage. The "predetermined first voltage" can be referred to as a predetermined voltage or a constant voltage, and both indicate that the voltage is maintained at a known voltage during impedance detection or adjustment such as a zero point. . Of course, it also includes a connection to the ground or earth, and in such a case, it is equivalent to keeping a constant voltage of zero volt.

Further, in the present invention, a canceling means for removing the output voltage of the voltage follower or the second operational amplifier from the output voltage of the signal output terminal may be provided.
At this time, the canceling means includes an inverting amplifier that inverts one of the output voltages using the third operational amplifier, and another voltage of the two output voltages and an output voltage of the inverting amplifier. May be provided, and an inverting amplifier for inverting one of the output voltages and an output of the inverting amplifier for the other of the output voltages may be provided. An adding unit that adds the voltage and the voltage using the fourth operational amplifier may be provided.
Then, one of the input terminals of the third operational amplifier or the fourth operational amplifier may be connected to a predetermined first voltage. In addition, the inverting amplifier may include a phase amplitude compensator.

Here, the canceling means may be configured to include a subtracting section that receives the both output voltages as input.
At this time, the subtraction unit may include a fifth operational amplifier, and one input terminal of the fifth operational amplifier may be connected to a predetermined first voltage. Further, a phase amplitude compensating means may be provided at the input of the subtracting section.

In the present invention, at least either a DC bias or an AC bias may be superposed on the impedance element connected to the signal line, specifically on the side opposite to the signal line.

Further, the impedance detection circuit of the present invention further includes a switching means having a primary side connection terminal having at least one terminal and a secondary side connection terminal having at least two terminals, and the switching means of the switching means. The primary side connecting terminal is connected to at least the impedance element, the secondary side connecting terminal is connected to at least the signal line and the shield means, and the connection destination of the primary side connecting terminal changes between the secondary side connecting terminals. You may have the switching means to do. In this secondary side connection terminal, a voltage having a predetermined relationship with one terminal voltage is applied to the other terminal. Here, the “predetermined relationship” means a predetermined relationship or a known relationship. As an example, one or both of the phases and amplitudes between both voltages is a constant ratio, a gradual change, a random change, or a constant relationship. It depends on the surrounding environment. Further, it goes without saying that provision of another terminal capable of switching connection with the primary connection terminal is not excluded. At this time, a plurality of the switching means may be provided.

On the other hand, a configuration may be added in which the first impedance includes a plurality of resistors, and a selection unit that selects at least one of the plurality of resistors. The selecting means has a structure similar to that of the switching means, and may be provided either between the first impedance and the signal line or between the first impedance and the signal output terminal or both. Providing for both can reduce the effect of unselected impedance.

The switching means or the selecting means may be made of a multiplexer. Further, these switching means or selection means may be used as the changing means for changing the impedance value of the impedance portion that determines the amplification characteristic of the circuit of the present invention.

When the impedance element is a resistance component, the first impedance is a resistance, and when the impedance element is a capacitive component, the first impedance is a capacitance and the phase or amplitude of the signal is adjusted. It is easy to do and is preferable.

The impedance detecting apparatus of the present invention comprises the above impedance detecting circuit and a terminal to which an impedance element can be connected to the signal line from the outside.
At this time, shield means for electrically shielding at least a part of the substrate on which the impedance detection circuit is mounted may be provided, and the shield voltage may be applied to the shield means. Further, a terminal capable of connecting the shield means to the outside and a terminal connected to the bias means and capable of applying a voltage from the outside may be provided. These terminals can greatly improve the convenience in actual use while maintaining the measurement accuracy.

In the impedance detecting method of the present invention, one input terminal of the voltage follower is connected to the signal line and no current flows in and out, and the shield voltage based on the output voltage of the voltage follower is set to at least one of the signal lines. The impedance of the impedance element is detected by a current applied to the shield means for shielding the part and flowing in the first impedance connected to the signal line.

Further, according to the impedance detecting method of the present invention, both input terminals of the second operational amplifier are imaginarily short-circuited, one input terminal is connected to the signal line and there is no current input and output, and the other input is connected. The impedance of the impedance element is obtained by connecting the terminal to a shield means for shielding at least a part of the signal line and amplifying the voltage applied to the impedance element by the first impedance and the first operational amplifier connected to the signal line. To detect. At this time, more preferably, a shield voltage is applied to the shield means.

In these impedance detection methods, the shield voltage may be phase-amplitude compensated and applied to the shield means.

Further, the output of the voltage follower or the output of the second operational amplifier may be subtracted from the detection signal of the impedance element.

In addition, at least either a DC bias or an AC bias may be applied to the impedance element connected to the signal line.

In the impedance detecting method of the present invention, the connection destination of the impedance element can be switched from the signal line to the shield means for initial setting.

Similarly, the gain can be changed by changing the value of the first impedance to change the potential difference applied to the first impedance.

In the impedance detection circuit, impedance detection device, or impedance detection method described above, the potential of the shield and the signal line are set to the same potential by the voltage follower or the second operational amplifier as described above, and the first operational amplifier is used. Since voltage amplification is performed by the circuit, it is possible to separate the voltage amplification part and the part where the potential between the shield and the signal line is the same in the circuit, and the AC voltage input to the signal output voltage It is possible to almost eliminate the slight phase and amplitude deviation between the inverting input terminal and the non-inverting input terminal due to the superposition of harmonics or the tracking error inside the operational amplifier. As a result, it is very small. Or, it is possible to minimize the influence of parasitic capacitance such as shield when measuring high-precision impedance. To become. In this way, it is possible to obtain a signal that is exactly proportional to the current flowing through the impedance element.

At this time, if one input terminal of the operational amplifier is connected to the predetermined first voltage, the operation of each operational amplifier is stabilized, and the impedance element is controlled while controlling the stray capacitance between the signal line and the shield. It is possible to further suppress the harmonic component of the voltage appearing at the output terminal of the operational amplifier according to the flowing current.

Further, by compensating the phase amplitude of the voltage applied to the shield means, it becomes possible to accurately control the potential difference between the signal line and the shield means to a desired value even if the input signal has a high frequency. For example, the potential difference between the signal line and the shield means can be made almost completely zero, whereby the stray capacitance between them can be canceled almost completely. In the case of a low frequency, the allowable error range may be obtained without using the phase amplitude compensation, so that it may not be used at that time.

On the other hand, if the switching means is used, resetting and initial setting of the circuit of the present invention can be accurately performed either with or without connecting the impedance element. By these, it becomes possible to maintain the potential difference between the impedance element and the signal line in a predetermined relationship,
For example, if this potential difference is set to zero, it is possible to cancel the stray capacitance between the two and further improve the accuracy. This also applies to the multiplexer.

Further, by using the selecting means, the value of the first impedance can be changed, the potential difference applied to the first impedance can be changed, and the gain can be changed while maintaining the measurement accuracy.

In this impedance detection circuit, device, and method, by using the above-mentioned switching means or selection means, while maintaining the potential difference between the selected impedance and the unselected impedance in a predetermined relationship, The amplification characteristics and gain of this circuit can be changed. Therefore, even when the measurement range is switched according to the detection target or the measurement situation, it is possible to accurately change the characteristics and perform highly accurate detection.

In the impedance detecting device, the connection with the external impedance element can be shielded by the shield means in which the potential difference between the connection line through which the measurement signal is transmitted is accurately controlled. For example, if the potential of the shield means is made equal to the potential of the connection line, the stray capacitance between the two can be canceled.

Further, in this impedance detecting device,
Since the board itself on which the impedance detection circuit is mounted is electrically shielded by the shield means, it is possible to accurately control the potential difference between the signal line and the shield means. It is possible to cancel the stray capacitance between the line and the outside of the substrate, and it becomes possible to accurately detect a minute impedance or a minute impedance change.

[0035]

BEST MODE FOR CARRYING OUT THE INVENTION Next, specific embodiments of the impedance detection circuit, impedance detection method, and impedance detection device of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing the core portion 1 of the impedance detection circuit taken out and shown. This core part 1
Is composed of a first operational amplifier 12 and a second operational amplifier 11. The second operational amplifier 11 has a inverting input terminal and an output terminal short-circuited to form a voltage follower. Here, the voltage follower refers to one that has a high input impedance, a low output impedance, an input / output gain of 1, and functions as an impedance converter. A signal line 19 is connected to the non-inverting input terminal of the second operational amplifier 11. A capacitance sensor can be connected to the signal line 19 as an impedance element 18. This capacitive sensor
The capacitance Cs of the physical quantity is changed according to the received physical quantity (acceleration, pressure, gas, light, sound wave, etc.).
The other end of the capacitance sensor connected to the signal line 19 is connected to a DC bias terminal (bias means) 23 or grounded. Although it may be floating, it is possible to measure with higher accuracy by connecting the bias means. The second operational amplifier 12 has its non-inverting input terminal grounded while the inverting input terminal has a first
Resistance (resistance value R1) 15 and second resistance (resistance value R2) 1
One end of each of 6 is connected. Although the non-inverting input terminal is preferably grounded in this way, it is functionally sufficient that the non-inverting input terminal is held at a constant zero potential, for example, even if a bias voltage is applied. Since such a so-called electrical fluctuation can be suppressed, such a method may be adopted. The other end of the first resistor 15 is an AC voltage generator (AC voltage applying means, generated AC voltage Vin,
Each frequency ω) 14 is connected, and the other end of the second resistor 16 is connected to the output terminal of the second operational amplifier 11.

The output terminal of the first operational amplifier 12 is connected to the non-inverting input terminal of the second operational amplifier 11 via a third resistor (first impedance, resistance value R3) 17. Further, the output terminal of the first operational amplifier 12, the second
The signal line 19 connecting the non-inverting input terminal of the operational amplifier 11 and the capacitance sensor to each other is covered with a shield wire 20. The shield line 20 electrically shields the signal line 19 from the outside. The shield wire 20 is a compensation circuit (shield voltage applying means).
It is connected to the output terminal of the second operational amplifier 11 via 13. The signal output terminal 21 is connected to the output terminal of the first operational amplifier 12, and the AC output terminal 22 is connected to the output terminal of the second operational amplifier 11. further,
Although not shown in FIG. 1 for the sake of simplicity, the non-inverting input terminal of the second operational amplifier 11 has an N-type M connected between the positive and negative power supplies, as shown in FIG. 8 as an example.
An OSFET 47a and a P-type MOSFET 47b are provided as the analog buffer 47. By receiving the signal line 19 at the input of the analog buffer 47, the impedance seen from the signal line 19 side is made extremely high. FIG. 2 is another example of the core unit 1 of the impedance detection circuit. AC voltage generator 14
Is the same as that of FIG. 1 except that is not connected to the inverting input terminal of the first operational amplifier 12.
Can be configured.

Further, FIG. 9 shows a capacitance sensor as the impedance element 18 and a signal line 19 in the core section 1.
The connection part with is shown in detail. In this connection part,
A changeover switch (switching means) 24 is provided. The changeover switch 24 is configured so that the connection destination of the primary side connection terminal 24a can be switched between the two secondary side connection terminals 24b and 24c. This changeover switch 2
The signal line 19 is connected to the secondary side connection terminal 24b of No. 4 and the shield line 20 is connected to 24c.
Then, one end of the capacitance sensor can be connected to the primary side connection terminal 24a of the changeover switch 24.

FIG. 7 shows a compensation circuit 1 provided in the core section 1.
3 is a circuit diagram showing an example of No. 3; FIG. The compensating circuit 13 is composed of a phase adjusting section 48 and an amplitude adjusting section 49.
The phase adjusting unit 48 is configured as an all-pass filter using the operational amplifier 71. That is, the input terminal 30
A resistor 73 is provided between the input terminal 30 and the non-inverting input terminal of the operational amplifier 71, and a variable resistor 74 is provided between the input terminal 30 and the non-inverting input terminal. A resistor 75 is connected between the output terminal and the inverting input terminal of the operational amplifier 71.
And a capacitor 76 is connected to the non-inverting input terminal of the operational amplifier 71. Resistor 73 and resistor 7
The resistance values of 5 and 5 are equal to each other. The output side of the phase adjusting section 48 is connected to the input side of the amplitude adjusting section 49. The amplitude adjusting unit 49 is configured as an inverting amplifier using the operational amplifier 72. That is, the resistor 77 is provided between the input side and the inverting input terminal of the operational amplifier 72, and the variable resistor 78 is provided between the output terminal of the operational amplifier 72 and the inverting input terminal. The non-inverting input terminal of the operational amplifier 72 is grounded.

FIG. 3 is a circuit diagram showing the entire impedance detection circuit including the core section 1. In the core unit 1 shown in the circuit diagram of FIG. 1, the input side of the compensation circuit 13 is connected to the output terminal of the second operational amplifier 11, but in the core unit 1 shown in this circuit diagram, the input side of the compensation circuit 13 is It is connected to the AC voltage generator 14. As will be described later, even with such a connection, the output voltage of the second operational amplifier 11 can be phase-amplitude compensated and applied to the shield line 20. The signal output terminal 21 of the core unit 1 is connected to the inverting amplification unit 2 including the third operational amplifier 36. The signal output terminal 2
1 is connected to the inverting input terminal of the third operational amplifier 36 via the fourth resistance (resistance value R4) 32 whose resistance value is variable. Between the inverting input terminal and the output terminal of the third operational amplifier 36, a fifth resistor (resistance value R5) 33 and a variable resistance sixth resistor (resistance value R
6) 34 is connected in series, and a capacitor 35 is connected in parallel with the sixth resistor 34. Also, the third
The non-inverting input terminal of the operational amplifier 36 is grounded.

Further, the AC output terminal 22 of the core section 1 and the inverting output terminal 42 of the inverting amplification section 2 are connected to the addition section 3 having a fourth operational amplifier 40. The AC output terminal 22 is connected via a seventh resistor (resistance value R7) 37,
The inverting output terminal 42 has an eighth resistor (resistance value R8) 3
8 are respectively connected to the inverting input terminals of the fourth operational amplifier 40. The inverting input terminal and the output terminal of the fourth operational amplifier 40 have the ninth resistor (resistance value R9) 3
9 and the output terminal is connected to the addition output terminal 41. The non-inverting input terminal of the fourth operational amplifier 40 is grounded.

FIG. 10 is a schematic transparent perspective view showing an impedance detecting device provided with the above impedance detecting circuit. Board 44 on which impedance detection circuit is mounted
Is provided inside a shield case (shield means) 45 that electrically shields the inside thereof, and is further disposed inside a device casing (shield means) 4 that electrically shields the inside thereof. The device casing 4 includes an impedance connection terminal 5, a shield terminal 6, a bias input terminal 7,
The ground terminal 8 is provided so as to be exposed to the outside of the casing 4. Then, from the substrate 44, the signal line 19 connected to the terminal 24a of the changeover switch 24 is connected.
Extends and is connected to the impedance connection terminal 5.
The signal line 19 is covered with a shield line 20,
The shielded wire 20 is further covered from the outside with a second shielded wire (shielding means) 46, and is electrically shielded by the second shielded wire 46. The shield wire 20 includes the shield case 45 and the shield terminal 6.
It is connected to the. The device casing 4 and the second shield wire 46 are connected to the ground terminal 8, and the device casing 4 is grounded. The bias input terminal 7 is connected to the DC bias terminal 23 of the detection circuit. The bias input terminal 7 may not be provided when the DC bias terminal 23 is in a floating or ground mode.

Next, the operation of the impedance detecting circuit and the impedance detecting device configured as described above will be described. First, a capacitance sensor as an example of the impedance element 18 is connected to the end of the signal line 19 extending from the impedance connection terminal 5. A double shielded cable is used for this connection. Then, the axis of this cable is used for connecting the capacitance sensor, and the inner shield wire is connected to the shield terminal 6. The outer shield wire is connected to the ground terminal 8 or the shield terminal 6 depending on the length of the cable, the cable type, the operating environment, etc.
It may be properly used depending on the case. The bias input terminal 7 is connected to the ground terminal 8 here.

Let Z be the impedance of the capacitance sensor. Then, in the core portion 1, the voltage Vo of the AC output terminal 22 is represented by Vo = − (R2 / R1) · Vin (1). That is, the voltage follower composed of the first operational amplifier 12 and the second operational amplifier 11, the first resistor 1
5. The second resistor 16 constitutes an amplifier circuit which outputs the voltage Vo obtained by amplifying the input voltage Vin from the output terminal of the voltage follower. However, both input terminals of the second operational amplifier 11 are in an imaginary short state. It also means that the amplifier circuit is configured to include the first operational amplifier 12.

When the resistors 32, 33 and 34 are set so that the gain of the inverting amplifier 2 becomes -1, the inverting output terminal 42
The voltage Vb of is represented by Vb = -Vc.

Further, for the purpose of calculation, when attention is paid only to the adder 3, the voltage Va at the addition output terminal 41 is: Va = -R9. ((Vo / R7) + (Vb / R8)) , If each resistance value is made equal to R7 = R8 = R9, Va becomes Va = − (Vo + Vb) = − (Vo−Vc) = Vc−Vo (2), and the inverting amplification unit 2 and the addition unit 3 It can be seen that the canceling means is configured by.

When the current flowing through the third resistor 17 toward the capacitance sensor is i , these voltage followers have a function such as a voltage follower or an imaginary short circuit.
-Input / output of current at the input terminal of the follower or the second operational amplifier
Then , since almost all amount of the current i flows to the capacitance sensor, Vo = Z · i, and the voltage Vc of the detection signal output from the signal output terminal 21 is Vc = i · R3 + Vo = (1 + R3 / Z) · Vo, and it can be seen that the detection signal Vc includes Vo appearing at the output terminal of the second operational amplifier. This is (1),
When it is transformed using the equation (2), Va = Vc−Vo = (1 + R3 / Z) · Vo−Vo = (R3 / Z) · Vo = − (R3 · R2 / (Z · R1)) · Vin . Since Z = 1 / (jωCs), the voltage Va is eventually expressed as Va = − (jωCs · R3 · R2 / R1) · Vin. Therefore, the voltage value Va proportional to the capacitance value Cs of the capacitance sensor is obtained from the addition output terminal 41.
Therefore, the capacitance value Cs can be obtained by taking out the voltage Va from the addition output terminal 41 and then performing various signal processing based on this Va.

The second operational amplifier 11 operates with its inverting input terminal and non-inverting input terminal being in an imaginary short circuit state. However, the second operational amplifier 11
Since the input impedance of is not ideal infinity,
As described above, a minute amplitude difference and phase difference occur between the voltage Vom at the inverting input terminal and the voltage Vop at the non-inverting input terminal. Then, the amplitude difference and the phase difference become more remarkable as the frequency of the input signal becomes higher. If Vin has a high frequency of the order of 10 ⁹ Hz and the capacitance Cs of the capacitive sensor is of the order of 10 ⁻¹⁵ F, the amplitude difference and the phase difference between Vom and Vop cannot be ignored.
Therefore, even when Cs is zero, Vo = -Vb does not hold and a measurement error occurs. Therefore, before measuring the capacitance value Cs of the capacitance sensor, the primary side connection terminal 24a of the changeover switch 24 is connected to the secondary side connection terminal 24c on the shield wire 20 side to connect the non-inverting input of the second operational amplifier 11. The capacitance sensor is removed from the terminal. Then, in this state, the impedance detection circuit is operated. Then, the fourth resistor 32 of the inverting amplification unit 2 is adjusted so that Vo = −Vb, that is, Va = 0, the amplitude of Vb is adjusted to Vo, and the capacitance is connected in parallel.
The phase is rotated by adjusting the resistance 34, and the phase of Vb is-
Set to Vo. That is, the inverting amplification unit 2 also functions as a zero adjustment unit.

As described above, although there is a slight phase difference / amplitude difference between Vom and Vop, both are AC voltage Vin.
Is a signal synchronized with. Therefore, the phase of Vin is adjusted by the variable resistor 74 provided in the phase adjusting unit 48 of the compensating circuit 13 and the amplitude of Vin is adjusted by the variable resistor 78 provided in the amplitude adjusting unit 49, and Vop, phase and amplitude are adjusted. Of equal shield voltage Vos.
Therefore, in this case, the AC voltage generator 14 and the compensation circuit 13 function as a shield voltage generating means. In the circuit shown in FIG. 1, the input side of the compensating circuit 13 is connected to the output terminal of the second operational amplifier 11. This is because the output voltage Vo of the second operational amplifier 11 is the non-inverting input terminal, that is, the signal. This is because the voltage of the line 19 and the imaginary short circuit are almost equal. However, since the AC voltage Vin is also a signal synchronized with the output voltage Vo, applying Vin to the shield line 20 with phase amplitude compensation as shown in FIG. The same thing as that applied to No. 20. Since Vin, which is the input signal of the detection circuit, has a smaller noise component than Vo, which is the output of the voltage follower, Vin is phase-amplitude compensated to form Vos and applied to the shielded wire 20 for more accurate detection. It becomes possible to do. With these, the voltage of the shield line 20 becomes equal to the voltage of the signal line 19 even in an instant, and the signal line 1
The stray capacitance between 9 and the shield wire 20 is reliably canceled.

In the impedance detection circuit and the impedance detection device, the compensating circuit 13 operates to generate the AC voltage Vin.
The amplitude and the phase of (or the output voltage Vo of the voltage follower) are adjusted to form the shield voltage Vos having the same amplitude and phase as the voltage Vop of the signal line 19, and this is applied to the shield line 20. Therefore, not to mention a low frequency where Vin is about several kHz to several hundred kHz, for example, 10 ⁹ H
Signal line 19 and shielded line 2 even at high frequencies above z
It is possible to reliably cancel the stray capacitance between 0 and 0, and accurately detect only the capacitance Cs of the capacitance sensor.

Further, since all terminals of the second operational amplifier 11 are operated in an alternating current, if a large gain is to be obtained only by the second operational amplifier 11, an operational error due to fluctuations and the like will increase. As a result, the capacitance value Cs
There will be an error in the measurement of. So in the above,
The inverting input terminal and the output terminal of the second operational amplifier 11 are short-circuited to function as a gain follower voltage follower. The first operational amplifier 12 is used to obtain a necessary gain by the first operational amplifier 12. Therefore, Cs can be accurately measured.
The first operational amplifier 12 that performs voltage amplification has its non-inverting input terminal grounded. Since grounding the non-inverting input terminal stabilizes the voltage at this terminal, harmonics contained in the output signal can be suppressed especially when the operational amplifier is operating at high speed. As long as the voltage is stable at this terminal, a constant voltage other than ground may be applied.
In any case, when operating at high speed, it is preferable that this terminal be held at a constant voltage value. As a result, the harmonic component of Va, which has been a factor of error in the conventional circuit, is significantly reduced, so that the calculation accuracy can be significantly improved. Therefore, it is possible to measure Cs with higher accuracy.

Further, in the above, by subtracting Vo from Vc, the voltage Va proportional to Cs is added to the addition output terminal 4
I can take it out from 1. That is, only the current flowing through the capacitance sensor can be detected. This makes Cs
It is possible to simplify the subsequent signal processing circuit necessary for calculating the above and suppress the occurrence of error factors as much as possible. When subtracting Vo from Vc, a direct subtractor is often used as shown in FIGS. 4, 5, and 6. in this case,
Vc and Vo are directly applied to the input terminals of the operational amplifiers 50 and 51. However, like the operational amplifier 50, one terminal is kept at ground or a constant voltage for stabilization. May be. In addition, although the phase compensation circuit is provided on the Vc input side in FIG. 6, the phase amplitude compensation circuit may be added to any of the input terminals in this way. On the other hand, as shown in FIG. 3, when subtracting Vo from Vc, it is possible to invert Vc and then add Vo. In order to stabilize the operation of the operational amplifier with high accuracy, it is desirable to hold the non-inverting input terminal of the operational amplifier at ground or at a constant voltage, but if the configuration of the inverting amplifier and the adder is used, The non-inverting input terminals of the operational amplifiers can be configured such that they are grounded or a constant voltage is applied and held. In the case of this example, the third operational amplifier 36 forming the inverting amplifier 2 and the non-inverting input terminal of the fourth operational amplifier 40 forming the adder 3 are both grounded. In any case, when operating at high speed, the operation of the operational amplifier is stable if these non-inverting input terminals are held at a constant voltage value, so it is preferable to connect the terminals to such a predetermined voltage. is there. By doing so, harmonics that may be included in the output voltage Va of the operational amplifier 40 are suppressed, and Cs can be measured with higher accuracy.

The second operational amplifier 11 operates with its inverting input terminal and non-inverting input terminal being in an imaginary short state. However, as described above, an amplitude difference and a phase difference occur between the two input terminal voltages Vom and Vop. Therefore, even if the amplification degrees of the inverting amplification unit 2 and the addition unit 3 are set to be exactly “1”, Cs =
When it is 0, Va does not become 0. Therefore, in the above description, the phase and the amplitude of Vc can be adjusted in the inverting amplification unit 2, and zero adjustment can be performed so that Va = 0 is surely achieved when Cs = 0. Also, the phase adjustment here is not performed by an all-pass filter in which a capacitor is connected to the non-inverting input terminal of the operational amplifier, but by an inverting amplifier in which the non-inverting input terminal of the operational amplifier is grounded and a feedback circuit is provided with a capacitive component. I am trying to do it. Therefore, also in this case, harmonics that may be included in the output signal are suppressed to prevent the measurement accuracy of Cs from decreasing. Again, the terminals are
Similar to the above, a constant voltage other than ground may be applied.

Zero adjustment is performed in the state where the connection of the capacitance sensor is cut off. If this connection is cut off by a normal switch, the cut capacitance sensor and the signal line 1 are disconnected.
Accurate zero adjustment cannot be performed due to the stray capacitance generated between 9 and 9. Therefore, in the impedance detection circuit of this example, the capacitance sensor separated from the signal line 19 is connected to the shield line 20 by the changeover switch 24. Since the voltage Vos applied to the shield line 20 is equal to the voltage Vop of the signal line 19, an accurate stray capacitance is not generated between the signal line 19 and the capacitance sensor separated from the signal line 19. Zero adjustment is possible.

Further, in the above impedance detecting device,
The shield line 20 that shields the signal line 19 is further shielded by the second shield line 46. When the cable is short, ground the second shielded wire 46 to obtain the voltage Vos.
It is possible to prevent disturbance noise from being superimposed on the shielded wire 20 to which is applied. Therefore, it is possible to reliably maintain the same potential of the signal line 19 and the shield line 20. Further, the substrate 44 on which the impedance detection circuit is mounted is put in the shield case 45, and the shield case 45 is set to the same potential as the shield wire 20. Therefore, the substrate 44 can be shielded while preventing the generation of stray capacitance between the shield case 45 and the signal line 19. Further, the device casing 4 accommodating the shield case 45 is grounded. Therefore, it is possible to prevent disturbance noise from being superposed on the shield case 45 to which the voltage Vos is applied, and to reliably maintain the same potential of the signal line 19 and the shield case 45.

The impedance detection circuit of this example is not limited to the above embodiment, but can be implemented with various modifications within the scope of the present invention. In the above, signal line 1
Although the capacitance sensor is attached to the end portion of 9 as the impedance element 18, the measurement electrode is formed at the end portion of the signal line 19, and the capacitance Cs formed between the measurement electrode and the measurement target is used as the detection circuit. Alternatively, it may be detected by a measuring device.

Further, in the above, the signal line 19 and the shield line 20 are made to have the same potential by the compensating circuit 13. However, when it is desired to give a predetermined potential difference between the signal line 19 and the shield line 20 due to the usage of the impedance detection circuit, etc. Compensation circuit 1
In 3, the amplitude and the phase of the AC voltage Vin may be adjusted appropriately.

Further, the specific configuration of the detection circuit shown in FIG. 1 is an example, and it goes without saying that other circuit configurations can be adopted. For example, the first operational amplifier 12 and the second operational amplifier 11 that form the core unit 1 may be circuits that are configured as non-inverting amplifiers, respectively.

In the above description, the compensating circuit 13 is replaced by the phase adjusting unit 4
8 and the amplitude adjusting unit 49, the compensating circuit 13 may be configured by the same circuit as the inverting amplifying unit 2. In this case, the non-inverting input terminal of the operational amplifier is grounded, so that it is possible to further prevent the shield voltage from including harmonics and causing an error.

Although a capacitive sensor is used as the impedance element 18 in the above, an inductive element may be used instead. To detect the CV (capacitance-voltage) characteristic of the element, the capacitance of which changes depending on the voltage, the DC voltage may be applied to the bias input terminal 7 while changing the DC voltage. Since the bias voltage is applied to the side opposite to the signal line of the capacitance sensor, the potential Vop itself of the signal line 19 remains an AC voltage oscillating around a constant voltage. Therefore, Vc can be prevented from becoming unstable and causing an error in the measurement result. Such a DC bias generator may be externally connected to the measuring device, but the DC of the detection circuit may be previously connected.
It may be connected to the bias terminal 23 and provided inside the measuring device. Further, the generator provided inside and the generator connected from the outside may be switched to be connected to the DC bias input terminal 23.

Furthermore, although the case where a single impedance element is connected has been described above, a plurality of impedance elements may be connected so that the elements to be measured can be switched. FIG. 11 shows the signal line 19 in such a case.
It is a figure which shows the edge part. Multiple impedance elements 1
In the case where 8 and 26 are provided, the changeover switch 2 having the same configuration as the changeover switch 24 described above corresponding to each element 18 and 26.
5 is provided. Then, the signal line 19 is connected to one of the secondary side connecting terminals, and the shield line 20 is connected to the other one of the secondary side connecting terminals. Impedance elements 18 and 26 are connected to the primary side connection terminals, respectively. Then, the multiplexers can be constructed by controlling both changeover switches 24 and 25 as follows. That is, when connecting one of the elements 18 to the signal line 19, as shown in FIG.
Both the changeover switches 24 and 25 are controlled so that the primary side connection terminal of the changeover switch 24 to which 8 is connected and the signal line are connected, and the primary side connection terminal of the other changeover switch 25 is connected to the shield. To do. The other element 26 is connected to the signal line 1
9 is connected so that the primary side connection terminal of the changeover switch 25 to which the element 26 is connected and the signal line are connected, and the primary side connection terminal of the other changeover switch 24 and the shield are connected. The changeover switches 24 and 25 are controlled.
In addition, this multiplexer may be controlled so as to disconnect both elements 18 and 26 from the signal line 19 when performing zero adjustment of the detection circuit, or both elements 18 and 26 may be signaled depending on the purpose of detection. It may be controlled to connect to a line. This multiplexer can reduce disturbance factors such as stray capacitance as much as possible by connecting all the changeover switches other than the changeover switch to which the target element for detection or the like is connected to the shield line side.

Further, such a changeover switch 24 can also be used for changing the gain of the amplifier circuit. Figure 1
Instead of the first impedance 17 shown in FIG. 3, a changeover switch 24 is provided between the plurality of resistances (impedances) having different resistance values and a plurality of signal lines as selection means for the respective resistances (impedances). As shown in FIG. 12, one end of each resistor is connected to the output terminal of the first operational amplifier 12, while the other end of each resistor is the changeover switch 24.
Is connected to the primary side connection terminal 24a. Further, the secondary side connection terminal 24b is connected to the signal line, and the other terminal 24c is connected to the shield line 20. Then, the impedance selected by these selecting means becomes a composite impedance, and the connection destination of the resistance not selected is the shielded wire 2
Make it 0. In this way, the multiplexer that controls each of the changeover switches 24 serves as a selection unit. Then, similarly to the above, it is possible to control the stray capacitance generated between the unconnected resistor and the signal line or the shield, and it is possible to change the potential difference applied to the impedance element without hindering the highly accurate detection of Cs. As a result, the gain can be switched. Also, as shown in FIG.
A changeover switch may be provided between the first operational amplifier 12 and the first impedance 17.

Further, such a changeover switch 24 can also be used for changing the gain of the amplifier circuit. Figure 1
Instead of the second resistor 16 shown in FIG. 14, a plurality of resistors 121 and 122 having different resistance values and changeover switches 24 and 25 corresponding to the resistors are provided as shown in FIG. Then, one end of each resistor is connected to the inverting input terminal of the second operational amplifier 12, while the other end of each resistor is set to 1 of the changeover switch 24.
It is connected to the next connection terminal 24a. Further, the secondary side connection terminal 24b is connected to the output terminal of the second operational amplifier 11, and the secondary side connection terminal 24c is connected to the shielded wire 20. Then, except for the resistor connected to the output terminal of the second operational amplifier 11, each selector switch 24 is controlled so that the connection destination is the shielded wire 20 to form a multiplexer. Then, similarly to the above, the resistor not connected and the second operational amplifier 11
The stray capacitance generated between the output terminal of
It is possible to switch the gain of the amplifier circuit without interfering with the highly accurate detection of. Although not illustrated, the same applies to the first resistor 15. These also changeover switch 24
The resistor to be connected to the circuit can be selected from a plurality of resistors having different resistance values by using, and the resistor which is not selected is connected to the voltage applying terminal 24c so that the range can be accurately switched according to the detection target or the measurement situation. May be allowed.

In the above, the case where the changeover switch 24 is configured as a three-terminal changeover switch having the primary side connection terminals 24a, the secondary side connection terminals 24b, and 24c has been shown.
However, this may have two or more primary side connection terminals 24a and three or more secondary side connection terminals to be connected. In such a case, at least one of the secondary side connection terminals to be the connection destination is connected to the shield, and the connection destination of the primary side connection terminal 24a is between this terminal and another secondary side connection terminal. It only needs to be configured to be switched.

Further, the signal line 19 may be shielded in a double or triple layer. Although the outer shield wire is connected to the shield terminal 6 when the length of the double shielded cable is about 50 cm or more in the above description, the first operational amplifier 12 is a power type capable of extracting a sufficient current. In such a case, it is possible to connect the outer shield wire having a length corresponding to the power of 50 cm or more to the ground terminal 8.

[0066]

According to the impedance detecting circuit, impedance detecting device, or impedance detecting method of the present invention, the voltage appearing at the output terminal of the signal is controlled by the harmonics or the like while controlling the stray capacitance between the signal line and the shield means. It is possible to suppress the inclusion of noise. Therefore, the impedance of the impedance element can be detected with high accuracy.

Further, according to the impedance detecting circuit or the impedance detecting method of the present invention, the potential difference between the signal line and the shield means can be accurately set to a desired value. For example, if this potential difference is set to zero, the stray capacitance between the signal line and the shield means can be canceled. Further, in the impedance detection device of the present invention, the connection line to the external impedance element can be shielded by the shield means in which the potential difference between the connection line and the external connection is accurately controlled. If the potential of the shield means is equal to the potential of the connecting line, the stray capacitance between the two can be canceled. Therefore, these detection circuits, detection methods, or detection devices can detect the impedance value of the impedance element more accurately.

Further, according to the impedance detection circuit or the impedance detection method of the present invention, since a signal proportional to the current flowing through the impedance element can be obtained, the subsequent signal processing can be simplified and the error contained in the subsequent stage can be suppressed. It will be possible.

In addition, the impedance detection circuit of the present invention,
In the impedance detection device or the impedance detection method, the impedance can be detected while applying the bias to the impedance element, and therefore the impedance and the CV measurement can be easily performed for the element whose impedance changes depending on the voltage. It will be possible.

Further, by further using the switching means, it is possible to maintain a predetermined potential between the impedance element and the signal line even when the impedance element is separated from the signal line. For example, if this potential difference is set to zero, it is possible to cancel the stray capacitance between the two. Therefore, for example, in an impedance detection circuit or the like, it is possible to accurately calibrate the circuit such as zero adjustment, reset, and initial setting. Here, by using a plurality of switching means or multiplexers, even when only the necessary impedance elements are selected from the plurality of primary sides and connected to the secondary side, the primary side connection terminals and A predetermined potential can be maintained between the secondary side connection terminal and, for example, if this potential difference is set to zero, the stray capacitance between the two can be canceled. Even when the impedance value is detected by appropriately selecting an element (that is, an impedance element), accurate detection can be performed.

Furthermore, in the impedance detection circuit of the present invention, by using the selection means and the multiplexer, when preparing a plurality of impedances in the circuit and selecting the impedances appropriately, the impedances separated from the unselected connections and the impedances It is possible to maintain the predetermined potential between the connection destination. Therefore, it is possible to switch the range according to the detection target by canceling the stray capacitance related to the non-selective impedance, and as a result, it is possible to accurately change the amplification characteristic of the circuit and perform highly accurate detection. Become.

In the impedance detecting device of the present invention, it is possible to prevent disturbance noise from being superposed on the shield means for shielding the signal line and the measurement circuit board. Further, in the impedance detecting device of the present invention, it is possible to accurately control the stray capacitance between the signal line and the outside of the substrate. Therefore, in these devices, impedance detection can be performed with higher accuracy.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a core portion of an impedance detection circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a core portion of an impedance detection circuit according to an embodiment of the present invention.

FIG. 3 is a circuit diagram showing the impedance detection circuit.

FIG. 4 is a circuit diagram showing a part of the impedance detection circuit.

FIG. 5 is a circuit diagram showing a part of the impedance detection circuit.

FIG. 6 is a circuit diagram showing a part of the impedance detection circuit.

FIG. 7 is a circuit diagram showing a compensation circuit provided in the core section.

FIG. 8 is a circuit diagram showing an input side of a first operational amplifier provided in the core section.

FIG. 9 is a circuit diagram showing an end portion of a signal line provided in the core portion.

FIG. 10 is a schematic transparent perspective view showing an impedance detection device according to an embodiment of the present invention.

FIG. 11 is a circuit diagram showing an end portion of a signal line in a modified example of the above embodiment.

FIG. 12 is a circuit diagram showing another example of the core section of the impedance detection circuit of the present invention.

FIG. 13 is a circuit diagram showing another example of the core section of the impedance detection circuit of the present invention.

FIG. 14 is a circuit diagram showing another example of the core section of the impedance detection circuit of the present invention.

FIG. 15 is a circuit diagram showing a conventional impedance detection circuit.

[Explanation of symbols]

1 core part 2 Inversion amplifier 3 adder 4 Equipment casing 5 impedance connection terminal 6 Shield terminal 7 Bias input terminal 8 ground terminals 11 Second operational amplifier 12 First operational amplifier 13 Compensation circuit 14 AC voltage generator 15 First resistance 16 Second resistance 17th resistance 18 Impedance element 19 signal lines 20 shielded wire 21 Signal output terminal 23 DC bias terminal 24, 25, 26 changeover switch 32 Fourth resistance 33 5th resistance 34 6th resistance 35 capacitor 36 Third operational amplifier 37th resistance 38 8th resistance 39 9th resistance 40 Fourth operational amplifier 44 substrate 45 shield case 46 Second shielded wire 50, 51 Fifth operational amplifier

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshiyuki Matsumoto 1-8 Fuso-cho, Amagasaki City, Hyogo Prefecture Sumitomo Metal Industries, Ltd. Electronics Technology Research Institute (72) Yoshihiro Hirota 1-8 Fuso-cho, Amagasaki City, Hyogo Prefecture Sumitomo Metal Industries, Ltd., Electronics Technology Research Institute (72) Inventor, Koichi Nakano, 538, Meishio, Shiose-cho, Nishinomiya-shi, Hyogo Hokuto Electronics Industry Co., Ltd. (56) Reference JP-A-4-303775 (JP, A) JP A 61-82103 (JP, A) Special Table 2000-514200 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01R 27/00-27/32

Claims (28)

(57) [Claims] 1. A signal line including a voltage follower and a first operational amplifier, one end of which is connected to an input terminal of the voltage follower and an impedance element can be connected to the other end, and at least a part of the signal line is electrically connected. Shield means, a shield voltage applying means for applying a shield voltage to the shield means, a first impedance provided between the output terminal of the first operational amplifier and the signal line, and a first operational amplifier. An impedance detection circuit comprising: a signal output terminal connected to an output terminal. 2. A first operational amplifier, a second operational amplifier whose both input terminals are in an imaginary short circuit state, one end of which is connected to one input terminal of the second operational amplifier and an impedance element is connected to the other end. A signal line capable of, and a shield means for electrically shielding at least a part of the signal line,
An impedance detection circuit comprising: a first impedance provided between an output terminal of a first operational amplifier and the signal line; and a signal output terminal connected to an output terminal of the first operational amplifier. 3. The impedance detection circuit according to claim 2, wherein shield voltage applying means is further connected to the shield means. 4. The impedance detecting circuit according to claim 1, wherein the shield voltage applying means includes a phase amplitude compensating means. 5. The impedance detection circuit according to claim 1, wherein one input terminal of the first operational amplifier is connected to a predetermined first voltage. 6. The canceling means for removing the output voltage of the voltage follower or the second operational amplifier from the output voltage of the signal output terminal is provided.
An impedance detection circuit according to any one of to 5. 7. The inverting amplifying section for inverting one of the two output voltages using a third operational amplifier, and the other voltage of the two output voltages and the output of the inverting amplifying section. 7. The impedance detection circuit according to claim 6, further comprising an adder that adds a voltage to the third operational amplifier, wherein one input terminal of the third operational amplifier is connected to a predetermined first voltage. 8. The fourth operational amplifier, wherein the canceling means outputs an inverting amplifier that inverts one of the output voltages and another voltage of the two output voltages and an output voltage of the inverting amplifier. And an adder unit for adding using the fourth operational amplifier, wherein one of the input terminals of the fourth operational amplifier is connected to a predetermined first voltage.
Impedance detection circuit. 9. The impedance detection circuit according to claim 7, wherein the inverting amplification section includes phase amplitude compensation means. 10. The impedance detecting circuit according to claim 6, wherein the canceling unit includes a subtracting unit that receives the both output voltages as inputs. 11. The impedance detection circuit according to claim 10, wherein the subtraction unit includes a fifth operational amplifier, and one input terminal of the fifth operational amplifier is connected to a predetermined first voltage. . 12. The impedance detecting device according to claim 1, further comprising a biasing device capable of superimposing at least one of a DC bias and an AC bias on the impedance element connected to the signal line. circuit. 13. The method according to any one of claims 1 to 12.
An impedance detection circuit, which is a voltage follower or
Is connected between the non-inverting input terminal of the second operational amplifier and the signal line.
Impedance detection circuit characterized by being continued. 14. The impedance detection circuit further includes switching means having a primary side connection terminal having at least one terminal and a secondary side connection terminal having at least two terminals, and the primary side connection of the switching means. The terminal is connected to at least the impedance element, the secondary side connection terminal is connected to at least the signal line and the shield means, and the connection destination of the primary side connection terminal changes between the secondary side connection terminals. The impedance detection circuit according to any one of claims 1 to 14 , which is characterized in that. 15. The impedance detection circuit according to claim 14 , further comprising a plurality of the switching means. 16. The first impedance comprises a plurality of impedances, and a selection means for selecting at least one of the plurality of impedances, the selection means comprising:
It includes a primary side connecting terminal having at least one terminal and a secondary side connecting terminal having at least two terminals, the primary side connecting terminal is connected to at least the impedance, and the secondary side connecting terminal is at least shield means. 16. The connection destination of the primary-side connection terminal changes between the secondary-side connection terminals.
The impedance detection circuit according to any one of 1. 17. The selection means, the impedance of claim 16, wherein further comprising at least either between the between the first impedance and the signal line or the first impedance and the signal output terminal, Detection circuit. 18. The method of claim 17, wherein the first impedance, the impedance detection circuit according to any one of claims 1 to 17, wherein the resistance or the capacitance. 19. The impedance detecting apparatus characterized by comprising any one of the impedance detection circuit according to claim 1 to 18, and a terminal that can connect the impedance element from outside to the signal line. 20. and the impedance detection circuit according to any one of claims 1 to 18, and a shielding means for electrically shielding at least a portion of the substrate mounted with the impedance detection circuit is provided, the shield to the shield means An impedance detection device characterized in that a voltage is applied. 21. Shielding means for connecting at least one input terminal of the voltage follower to a signal line and keeping no current in and out, and shielding at least a part of the signal line with a shield voltage based on the output voltage of the voltage follower. The impedance detection method is characterized in that the impedance of the impedance element connected to the signal line is detected by a current applied to the first impedance connected to the signal line . 22. Both input terminals of the second operational amplifier are imaginarily short-circuited, one input terminal is connected to a signal line and no current flows, and the other input terminal is at least one of the signal lines. The impedance of the impedance element connected to the signal line by the current flowing through the first impedance connected to the signal line. Method. 23. The impedance detection method according to claim 22, wherein a shield voltage is applied to the shield means. 24. The shield voltage is phase-amplitude compensated and applied to the shield means.
21. The impedance detection method according to 21 or 23 . 25. The impedance detection method according to any one of claims 21-24, wherein the subtracting the output of the output or the second operational amplifier of the voltage follower from the detection signal of the impedance element. 26. Any of the impedance detection method according to claim 21-25, characterized in that the addition of at least either the DC bias or AC bias to the connected impedance element to the signal line. 27. The impedance detection method according to any one of claims 21 to 26, characterized in that to perform the initial setting switches the connection destination of the impedance element from the signal line to the shield means. 28. The first impedance is changed by changing the value of the first impedance.
The impedance detection method according to any one of claims 21 to 27 , wherein the potential difference applied to the impedance is changed to change the gain.