JP2002022785A - Impedance detecting circuit and impedance detecting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an impedance detecting circuit and an impedance detecting method, allowing accurate detection of a small change in impedance with high sensitivity. SOLUTION: An amplifying circuit is constituted by a first operational amplifier 1 and a second operational amplifier 2. A feedback circuit network is formed by a second resistance 6 and a third resistance 11. The output terminal of the second operational amplifier 2 is connected to a non-reverse input terminal of the first operational amplifier 1 via a first capacitor 7. A second capacitor 12 is connected between the non-reverse input terminal and the output terminal of the first operational amplifier 1. The output terminal of the second operational amplifier 2 is connected to a signal output terminal 13. A capacity sensor 8 is connected to the nonreverse input terminal of the first operational amplifier 1. A current Id flowing in the component of a basic capacity value Cd out of the capacity sensor 8 with the input of AC voltage Vin is supplied from the first operational amplifier 1 via the second capacitor 12. Voltage Vout is taken out of the signal output terminal 13.

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 and an impedance detection method for detecting the impedance of an impedance element such as a capacitive sensor.

[0002]

2. Description of the Related Art As a conventional example of the above-described impedance detection circuit, there is one described in Japanese Patent Application Laid-Open No. 9-280806. FIG. 4 is a circuit diagram showing the impedance detection circuit. In this detection circuit, the capacitance sensor 51 formed by the electrodes 54 and 55 is connected to a signal line 57.
To the inverting input terminal of the operational amplifier 59. The capacitor 60 is connected between the output terminal of the operational amplifier 59 and the inverting input terminal, and the AC voltage Vac is applied to the non-inverting input terminal. The signal line 57 is covered with a shield line 56 to electrically shield the signal line 57 to which the capacitance sensor 51 is connected. This shield line 56 is connected to the non-inverting input terminal of the operational amplifier 59. The output voltage Vd is extracted from the output terminal of the operational amplifier 59 via the transformer 61.

[0003]

In the above conventional example, the total capacitance Cs of the capacitance sensor 51 is a sum of a constant basic capacitance value Cd and a capacitance change ΔC that changes due to sound, pressure, light or the like received from outside. Can be represented. Cs = Cd + ΔC However, the output voltage Vd depends on the ratio of the capacitance of the capacitor 60 to the capacitance Cs of the capacitance sensor 51.
This is a voltage obtained by amplifying c, and changes according to a change in the capacitance value Cs of the entire capacitance sensor 51. Therefore, when the capacitance change ΔC is smaller than the basic capacitance value Cd, there is a problem that the detection sensitivity of the capacitance change ΔC becomes extremely low. If the degree of amplification of the circuit is increased to solve this problem, the output voltage Vd will be saturated this time, causing a problem that the capacitance change ΔC cannot be detected accurately.

An object of the present invention is to solve the above-mentioned conventional problems, and an object of the present invention is to provide an impedance detection circuit and an impedance detection circuit capable of accurately detecting a minute change in impedance with high sensitivity. It is to provide a method.

[0005]

SUMMARY OF THE INVENTION An impedance detection circuit according to the present invention comprises a first operational amplifier, a second operational amplifier, one input terminal of the first operational amplifier, and an output of the second operational amplifier. A first impedance provided between the input terminal of the first operational amplifier, a signal line having one end connected to the input terminal of the first operational amplifier and an impedance element connected to the other end, and an output terminal of the first operational amplifier. A second impedance provided between the second operational amplifier and a signal output terminal connected to an output terminal of the second operational amplifier, wherein the impedance element connected to the signal line includes a first impedance of the first operational amplifier. A predetermined current is supplied from the output terminal via the second impedance.

Further, the impedance detection method of the present invention
A first operational amplifier, a second operational amplifier, a first impedance provided between the input terminal of the first operational amplifier and an output terminal of the second operational amplifier, and a first operational amplifier A first step of preparing a circuit including a signal line having one end connected to one input terminal of the input terminal; a second step of connecting an impedance element to the other end of the signal line;
A third step of supplying a predetermined current from the output terminal of the operational amplifier to the impedance element via the second impedance, and a fourth step of extracting a detection voltage from the output terminal of the second operational amplifier. It is characterized by having.

Further, the impedance detection circuit according to the present invention comprises a first operational amplifier, a second operational amplifier, and one of the first operational amplifiers provided with a feedback network so as to amplify a signal between input and output terminals. A first impedance provided between the input terminal and the output terminal of the second operational amplifier;
A signal line having one end connected to the input terminal of the operational amplifier and an impedance element connected to the other end, a current path having one end connected to the signal line, and an output terminal of the second operational amplifier. A signal output terminal, and a predetermined current is supplied from the current path to the impedance element connected to the signal line.

The impedance detecting method according to the present invention includes a first operational amplifier, a second operational amplifier, and one of the first operational amplifier provided with a feedback network so as to amplify a signal between input and output terminals. A first impedance provided between the input terminal and the output terminal of the second operational amplifier;
A signal line having one end connected to the input terminal of the operational amplifier and an impedance element connected to the other end, a current path having one end connected to the signal line, and an output terminal of the second operational amplifier. A signal output terminal, and a predetermined current is supplied from the current path to the impedance element connected to the signal line.

[0009] The term "connecting an impedance element" here means that an element as a component is attached, a measurement electrode is provided at an end of a signal line, and impedance is formed between the measurement electrode and an object to be measured. Including.

In the above-described impedance detection circuit or impedance detection method, a predetermined current supplied to the impedance element causes a part of the current flowing through the impedance element, which does not change according to the impedance change, to flow to the first impedance. It becomes possible to reduce from the current. In addition, since the current for this reduction is supplied from the first operational amplifier via the second impedance, it is possible to reduce the size of the circuit.

In the impedance detection circuit, a shield means for electrically shielding at least a part of the signal line and a shield voltage based on the other input terminal voltage of the first operational amplifier are applied to the shield means. When the shield voltage applying means is provided, it is possible to control the stray capacitance between the shield means and the signal line and detect the impedance change of the impedance element with high accuracy.

In the above-described impedance detection circuit or impedance detection method, if the impedance element and the first impedance have the same kind of property with respect to alternating current, the influence due to the phase rotation can be suppressed. It is possible to reliably reduce a current that does not change in response to a change in impedance from a current flowing through one impedance.

Here, "having the same kind of property with respect to alternating current" means that, for example, when one of the impedance elements and the first impedance is capacitive (capacitive reactance), the other is capacitive. Is capacitive, and if one is inductive (inductive reactance), the other is also inductive, and if one is a resistor, the other is also a resistor.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, specific embodiments of the impedance detection circuit and the impedance detection method according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram of an impedance detection circuit according to a first embodiment. This impedance detection circuit includes a first operational amplifier 1 and a second operational amplifier 2
Is provided. One end of a shielded signal line 9 is connected to a non-inverting input terminal of the first operational amplifier 1. The other end of the signal line 9 can be connected to a capacitance sensor as the impedance element 8. This capacitance sensor changes the capacitance of the capacitance sensor according to the received physical quantity (acceleration, pressure, gas, light, sound wave, etc.).

The second operational amplifier 2 has a non-inverting input terminal grounded, and an inverting input terminal having a first resistor (resistance value R).
1) One end of 5 and a second resistor (resistance value R2) 6 are connected to each other. The other end of the first resistor 5 is connected to an AC voltage generator (generated AC voltage Vin, angular frequency ω) 4, and the other end of the second resistor 6 is connected to the inverting input terminal of the first operational amplifier 1. . The output terminal of the second operational amplifier 2 is connected to a first capacitor (first impedance, capacitance value Cf) 7
To the non-inverting input terminal of the first operational amplifier 1. A third resistor (resistance value R3) 11 is connected between the inverting input terminal and the output terminal of the first operational amplifier 1,
A second capacitor (second impedance, capacitance value Cc) is connected to the electric circuit 17 provided between the non-inverting input terminal and the output terminal.
12 are interposed. The output terminal of the second operational amplifier 2 is connected to the signal output terminal 13.

Next, the operation of the impedance detection circuit configured as described above will be described. First, an amplifier circuit configured as described above is prepared. Then, a capacitance sensor whose other end is grounded is connected to one end of the signal line 9 as an example of the impedance element 8. The total capacitance Cs of this capacitance sensor can be expressed as follows: Cs = Cd + ΔC (1) where Cd is the basic capacitance value and ΔC is the capacitance change. Then, an AC input voltage Vin having an angular frequency ω is supplied from the AC voltage generator 4. Since the input terminals of the operational amplifiers 1 and 2 are in an imaginary short state, the inverted input terminal voltage V1 of the second operational amplifier 2 is
0. Thus, the inverted input terminal voltage V2 of the first operational amplifier 1 becomes V2 =-(R2 / R1) .Vin --- (2) = Vp, and the output terminal voltage V3 of the first operational amplifier 1 becomes V3 = − ((R2 + R3) / R1) · Vin ――― (3) That is, the non-inverting input terminal voltage Vp and the output terminal voltage V3 of the first operational amplifier 1 are equal to the input voltage Vi, respectively.
That is, the voltage becomes proportional to n. And V3
Since / V2 = (R2 + R3) / R2, signal amplification is performed between the input and output terminals of the first operational amplifier 1, and a feedback network therefor is configured using the second resistor 6 and the third resistor 11. It can be said that there is.

The current value Is flowing through the impedance element 8
Is expressed as: Is = jωCs · Vp = jωCs · V2 (4) The current Ic flowing through the second capacitor 12 is Ic = jωCc · (V3−V2) (5), and the current If flowing through the first capacitor 7 is If = Is−Ic (6) ). That is, the electric circuit 17 functions as a current path for supplying the predetermined current Ic to the impedance element 8 connected to the signal line 9.

On the other hand, the signal output terminal voltage (detection voltage) Vo
ut is expressed by: Vout = If / jωCf + V2 (7) Therefore, when the above equations (2) to (6) are substituted into the equation (7), the following equation is obtained: Vout = − (R2 + Cs · R2 / Cf−Cc · R3 / Cf) · Vin / R1 Can be Further, by substituting the equation (1) into the equation (8), Vout = − (R2 + Cd · R2 / Cf + ΔC · R2 / Cf−Cc · R3 / Cf) · Vin / R1 (9) it can.

The basic capacitance value Cd of the impedance element 8 is known. Therefore, in the above impedance detection circuit,
The resistors 5 and 6 and the capacitor 12 are selected so that Cd.R2 = Cc.R3. Then, the voltage Vout
Is as follows: Vout = − (1 + ΔC / Cf) · Vin · R2 / R1 (10) Then, by detecting Vout from the signal output terminal 13, the capacitance change ΔC of the impedance element 8 is obtained.
Can be known.

In the impedance detection circuit and the detection method, a voltage Vout having a value corresponding to the current If flowing through the first capacitor 7 is obtained. The current Is flowing through the impedance element 8 is represented by the sum of the current If and the current Ic flowing through the second capacitor 12, as in Expression (6). Is = If + Ic (11) On the other hand, the current Is is the capacitance C of the impedance element 8.
current I flowing in a portion corresponding to the basic capacitance value Cd of the s
d and a current ΔI flowing in a portion corresponding to the capacitance change ΔC.
And can be decomposed into Is = Id + ΔI (12) All of the current Id is supplied from the output terminal of the first operational amplifier 1 via the second capacitor 12. Id = Ic (13) Therefore, the current If flowing through the first capacitor 7 becomes equal to the current ΔI, and If = ΔI (14) It is possible to obtain a voltage Vout independent of the magnitude of the basic capacitance value Cd. You can. Accordingly, even when the capacitance change ΔC is extremely small compared to the basic capacitance value Cd, the capacitance change ΔC can be detected with high sensitivity. Also the first
Even if the detection sensitivity is increased by reducing the capacitance Cf of the capacitor 7, the current If does not include the current Id.
Regardless of the magnitude of the basic capacitance value Cd, it is possible to prevent the voltage Vout from being easily saturated. The current Ic is supplied from the first operational amplifier 1 in the amplifier circuit. Therefore, the size of the detection circuit can be reduced.

(Embodiment 2) FIG. 2 is a circuit diagram showing an impedance detection circuit of Embodiment 2. This detection circuit is different from that of the first embodiment in that the first circuit is connected via a voltage follower or a buffer (shield voltage applying means) 3.
The point is that the inverting input terminal voltage of the operational amplifier 1 is applied to the shield line (shield means) 10. Shield wire 10
Is for electrically shielding the signal line 9. Since both input terminals of the first operational amplifier 1 are imaginarily short-circuited, a shield voltage equal to the non-inverting input terminal voltage can be applied to the shield line 10 in this manner.
As a result, the voltage of the signal line 9 and the voltage of the shield line 10 become equal, and it is possible to avoid the generation of stray capacitance between the two. Further, since the shield voltage is applied using the voltage follower 3 having a high input impedance, it is possible to prevent a current from flowing from the feedback network of the amplifier circuit to the voltage follower 3 side, and to reduce the output voltage V3 of the first operational amplifier 1. It is possible to prevent an error from occurring in the signal output terminal voltage Vout due to fluctuation.

Here, the voltage follower 3 is used as the shield voltage applying means to simplify the circuit configuration. However, if the circuit scale is not limited, a circuit capable of compensating the phase and amplitude of the inverting input terminal voltage of the first operational amplifier 1 may be used as the shield voltage applying means. With such a circuit,
Even when the input voltage frequency ω is extremely high, the potential between the signal line 9 and the shield line 10 can be reliably set to the same potential to prevent generation of stray capacitance. And this phase
When an amplitude compensation circuit having an input impedance as high as the input impedance of the operational amplifier or the gate input impedance of the MOSFET is used, it is possible to reliably prevent an error from occurring in the voltage Vout. Further, in the case where a phase amplitude compensation circuit is used as the shield voltage applying means, this is connected between the shield wire 10 and the AC voltage generator 4, and the voltage Vin of the AC voltage generator 4 is subjected to phase amplitude compensation. You may make it apply to the shield wire 10. Since the inverting input terminal voltage V2 of the first operational amplifier 1 is formed based on the voltage Vin, the shield voltage based on the inverting input terminal voltage V2 of the first operational amplifier 1 is also applied to the shield line 10 in this case. This is because they can be applied.

Although the specific embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and can be implemented with various modifications within the scope of the present invention. In the above description, the capacitance sensor is attached to the end of the signal line 9 as the impedance element 8. However, a measurement electrode is formed at the end of the signal line 9, and the capacitance Cs formed between the measurement electrode and the object to be measured. May be detected by the detection circuit.

Although a capacitive sensor is used as the impedance element 8 in the above description, an inductive sensor or a resistive sensor may be used. The impedances 7 and 12 are not limited to the above-described capacitance elements, but may be resistance elements or inductive elements. The reason why the capacitive element (second capacitor 12) is used as the impedance for supplying Ic in the above-described embodiment is that a capacitive element is used as the impedance element 8, so that Ic equal to Id is reduced to the first operational amplifier. This is because the voltage can be reliably supplied from 1 to the impedance element 8. Therefore, when an inductive sensor is used as the impedance element 8, it is desirable that the impedance for supplying Ic is also an inductive element. When a resistive sensor is used as the impedance element 8, the impedance is also equal to the resistance element. It is desirable to do.

The reason why the capacitance element (the first capacitor 7) is used as the impedance for flowing the current If is that the capacitance element is used as the impedance element 8, whereby the phase between If and Id (or Is) is increased. This is because the influence of the surroundings can be suppressed, and Id can be reliably reduced from If. Therefore, when an inductive sensor is used as the impedance element 8, it is desirable that the impedance for flowing If is also an inductive element.
When a resistance type sensor is used as the impedance element 8, it is desirable that the impedance is also a resistance element.

Further, the impedance 12 may be constituted by a plurality of kinds of impedances. For example,
The capacitance element and the resistance element can be connected in parallel. If the current Id flowing to the basic capacitance value Cd is supplied by the current flowing through the capacitive element, and the current flowing into the first operational amplifier 1 from the non-inverting input terminal is supplied by the current flowing through the resistive element, the first The current If flowing through the capacitor 7 can be made very equal to the current ΔI.

If at least part of the current Id is covered by the current Ic, the contribution of Id to the detection voltage Vout can be reduced, and the effect of the present invention can be obtained. However, in order to detect the capacitance change ΔC with high sensitivity, it is preferable to cover almost the entire amount of the current Id with the current Ic and make If = ΔI as close as possible.

The voltage Vout from the signal output terminal voltage Vout
A differential circuit for subtracting and outputting 2 may be constituted by an operational amplifier or the like, and this differential circuit may be included in the detection circuit. By doing so, a signal not including V2 can be obtained from the output of the difference circuit, so that the subsequent signal processing circuit can be simplified.

Further, in the above, the current I is supplied via the electric path 17 provided between the output terminal of the first operational amplifier 1 and the signal line 9.
Although “c” is supplied to the impedance element 8, another predetermined current source may be provided, and the current Ic may be supplied via an electric path provided between the current source and the signal line 9. The current source may be, for example, an AC voltage generator and an impedance connected thereto, or, for example, an operational amplifier and an impedance connected to an output terminal thereof.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the circuit shown in FIG. 1, R1 = R2 = 1k
Ω, R3 = 10 kΩ, and Cc = 2 pF. As the impedance element 8, a capacitance sensor having a basic capacitance value of Cd = 20 pF was used, and the capacitance Cs was changed from 17 pF to 23 pF. The case where the current Ic was not supplied to the capacitance sensor was set as a comparison value, and the signal output voltage Vout was measured for each of Cf = 5 pF and Cf = 1 pF.

FIG. 3A shows the result when Cf = 5 pF, and FIG. 3B shows the result when Cf = 1 pF. When Cf = 5 pF, Vout according to the change of the capacitance Cs in both the example of the present invention and the comparative example.
Has been detected. However, since the change width of the voltage Vout is about 0.15 V, it can be said that it is not easy to accurately capture a minute change in the capacitance Cs. On the other hand, Cf = 1
In the case of pF, in the comparative example, the output voltage Vout approaches the power supply voltage and saturates, and it is extremely difficult to detect the capacitance Cs. In the embodiment of the present invention, the change in the capacitance Cs can be detected even in this case, and the change width of the voltage Vout at this time is about 0.65V. Therefore, it is clear that a minute change in the capacitance Cs can be accurately detected with high sensitivity.

[0033]

According to the impedance detection circuit or the impedance detection method of the present invention, of the current flowing through the impedance element, the current that does not change according to the impedance change is reduced from the current flowing through the first impedance. be able to. Therefore, it is possible to accurately detect a minute change in impedance with high sensitivity. Further, since the circuit can be made compact, it is suitable for making a chip or the like.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram illustrating an impedance detection circuit according to a second embodiment.

FIG. 3 is a graph showing a relationship between a signal output voltage and a change in capacitance in an example in comparison with a comparative example.

FIG. 4 is a circuit diagram showing a conventional impedance detection circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st operational amplifier 2 2nd operational amplifier 7 1st capacitor 8 Impedance element 9 Signal line 10 Shield line 12 2nd capacitor 13 Signal output terminal

Claims (8)

[Claims] 1. A first operational amplifier, a second operational amplifier, and a first impedance provided between one input terminal of the first operational amplifier and an output terminal of the second operational amplifier. A signal line having one end connected to the input terminal of the first operational amplifier and an impedance element connected to the other end, and a second line provided between the output terminal of the first operational amplifier and the signal line. An impedance, and a signal output terminal connected to the output terminal of the second operational amplifier, wherein a predetermined current is supplied to the impedance element connected to the signal line from the output terminal of the first operational amplifier via the second impedance. And an impedance detection circuit. A shield means for electrically shielding at least a part of the signal line; a shield voltage applying means for applying a shield voltage based on a voltage of the other input terminal of the first operational amplifier to the shield means; The impedance detection circuit according to claim 1, further comprising: 3. A first operational amplifier, a second operational amplifier, and a first impedance provided between one input terminal of the first operational amplifier and an output terminal of the second operational amplifier. A first step of preparing a circuit comprising: a signal line having one end connected to the input terminal of the first operational amplifier;
A second connecting an impedance element to the other end of the signal line;
And a third step of supplying a predetermined current from the output terminal of the first operational amplifier to the impedance element via the second impedance, and extracting a detection voltage from the output terminal of the second operational amplifier. And a fourth step. 4. A first operational amplifier, a second operational amplifier, and one input terminal of the first operational amplifier having a feedback network for amplifying signals between input and output terminals, and a second operational amplifier. A first impedance provided between the output terminal of the amplifier, a signal line having one end connected to the input terminal of the first operational amplifier and an impedance element connected to the other end, and one end connected to the signal line. And the second current path
And a signal output terminal connected to the output terminal of the operational amplifier, wherein a predetermined current is supplied from the current path to the impedance element connected to the signal line. 5. A first operational amplifier having a feedback network for amplifying a signal between input and output terminals, a second operational amplifier, one input terminal of the first operational amplifier and a second operational amplifier. A first impedance provided between the output terminal of the amplifier, a signal line having one end connected to the input terminal of the first operational amplifier, and a current path having one end connected to the signal line. A first step of preparing a circuit, a second step of connecting an impedance element to the other end of the signal line, a third step of supplying a predetermined current to the impedance element via the current path, And a fourth step of extracting a detection voltage from an output terminal of the second operational amplifier. 6. The impedance detection circuit according to claim 1, wherein the impedance element and the first impedance have the same kind of property with respect to alternating current. 7. The impedance detecting method according to claim 3, wherein the impedance element and the first impedance have the same kind of property with respect to alternating current. 8. The impedance detection circuit according to claim 6, wherein both the impedance element and the first impedance are capacitive.