JP3371847B2 - Impedance / voltage conversion device and its conversion method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD OF THE INVENTION The present invention relates to impedance / impedance conversion of impedance into a corresponding voltage using an operational amplifier.
The present invention relates to a voltage (Z / V) converter, and more particularly, to a Z / V converter capable of preventing output voltage drift due to environmental changes such as temperature changes and heat generation of the device itself.

[0002]

2. Description of the Related Art FIG. 6 is a schematic diagram showing the configuration of a capacitance-voltage (C / V) converter disclosed in Japanese Patent Laid-Open No. 61-14578. In the device of this conventional example, stray capacitance is superimposed on a signal line for connecting an unknown capacitance Cx to a C / V converter, and the stray capacitance changes due to movement or bending of the signal line. This is proposed in order to solve the problem that accurate C / V conversion cannot be performed, and as shown in the figure, the signal line L1 is provided between the AC signal generator OS and the operational amplifier OP. And L
An unknown capacitance Cx is connected via 2 and these signal lines L
By shielding 1 and L2 with the shield means s1 and s2, it is intended to reduce the influence of the stray capacitances Cs1, Cs2, and Cs3. A feedback circuit composed of a parallel circuit of a resistor Rf and a capacitor Cf is connected between the output terminal and the inverting input terminal of the operational amplifier OP, and the non-inverting input terminal of the inverting amplifier and the AC signal generator OS are connected. The end and the shields s1 and s2 are grounded.

In the C / V converter of FIG. 6 having such a configuration, the two input terminals of the operational amplifier OP are in an imaginary short state due to the nature of the operational amplifier having the negative feedback circuit. The two input terminals have almost the same potential (ground potential), and the stray capacitance Cs2 is not charged. Further, the stray capacitance Cs3 is considered to be the coupling capacitance of the two shield means s1 and s2, but since the shield means is both grounded, it is also not charged. Therefore, the influence of the stray capacitance of the cable connecting the unknown capacitance Cx is reduced, so that the same charge as the charge induced in the capacitance Cx is induced in the capacitor Cf of the feedback circuit, and eventually the capacitance is increased. The voltage Vo =-(Cx / Cf) Vi proportional to Cx [where Vi is the output voltage of the AC signal generator OS] is obtained.

[0004]

However, as shown in FIG.
In the C / V converter of, when the unknown capacitance Cx becomes small, the effect of the stray capacitance becomes apparent, and the accurate C / V
There is a problem that conversion is not possible. Also, the capacitance Cx
When one of the electrodes is biased to a certain potential, it is not possible to apply an alternating current to the capacitance,
There is also a problem that C / V conversion is impossible. further,
There is a possibility that the output voltage Vo may drift due to environmental changes such as temperature changes or aged deterioration of the device. For example, although the unknown capacitance Cx does not change, the level of the output voltage Vo is different between when the measurement environment is at high temperature and when it is at low temperature, and in the end, the unknown capacitance is accurately handled. It may not be possible to obtain the output voltage to be used. The present invention has been proposed to solve such a problem, and an object thereof is to reduce the influence of stray capacitance on a line for connecting an impedance to be measured in a Z / V converter. At the same time, it is possible to effectively reduce the drift of the output voltage caused by the surrounding environment or the like, and to obtain the output voltage that corresponds to the impedance to be measured with high accuracy.

[0005]

In order to achieve the above object, the impedance / voltage (Z / V) converter of the present invention comprises an operational amplifier and a known value connected to the inverting input terminal of the operational amplifier. Impedance, a switch circuit connected to the inverting input terminal of the operational amplifier by the first signal line, and a switch circuit selectively connected between the inverting input terminal and the output terminal of the operational amplifier via the switch circuit. The measured impedance and a dummy impedance of known value,
A first shield that shields at least a part of the first signal line and is connected to the non-inverting input terminal of the operational amplifier; and an oscillator connected to the non-inverting input terminal of the operational amplifier and the first shield. It is characterized by including.

In a preferred embodiment of the present invention, the impedance / voltage conversion device includes a second shield for shielding at least a part of a signal line connecting the switch circuit and the impedance to be measured, the switch circuit and the dummy circuit. A third shield that shields at least a part of the signal line that connects the impedance, and a fourth shield that shields at least a part of the signal line that connects the impedance of a known value and the inverting input terminal of the operational amplifier. Including,
At least one of the second to fourth shields is preferably connected to the non-inverting input terminal of the operational amplifier. Further, it is preferable that the measured impedance and the dummy impedance both have the same characteristic and are exposed to the same condition.

Furthermore, in a preferred embodiment of the present invention, the impedance / voltage converter further comprises an AC / DC converter circuit connected to the output of the operational amplifier and a sample connected to the output of the AC / DC converter circuit. It is preferable to include a hold circuit, a subtraction circuit connected to the output of the AC / DC conversion circuit and the output of the sample hold circuit, a switch circuit, a sample hold circuit, and a control circuit for controlling the subtraction circuit. This makes it possible to obtain an output from which the drift component has been removed. Instead of the sampling and holding circuit, a bias voltage generating circuit that outputs a bias voltage whose absolute value is equal to and opposite in polarity to the output of the AC / DC converting circuit may be used, and an adding circuit may be used instead of the subtracting circuit.

In order to achieve the above-mentioned object, in the impedance / voltage conversion method of the present invention for converting the impedance value to be measured into a voltage, an impedance having a known value is connected to the inverting input terminal and the non-inverting input terminal is connected. A dummy impedance is connected between the inverting input terminal and the output terminal of the operational amplifier to which the oscillator is connected, and the output of the operational amplifier when connecting the dummy impedance is converted to AC / DC to correspond to the dummy impedance. Obtain the DC voltage, sample and hold the DC voltage corresponding to the dummy impedance, connect the impedance to be measured between the inverting input terminal and the output terminal of the operational amplifier, and connect the output of the operational amplifier to AC when connecting the impedance to be measured. / DC conversion to obtain the DC voltage corresponding to the impedance to be measured and the impedance to be measured It is characterized in that to be able to from the corresponding DC voltage consists in subtracting the voltage corresponding to the sampled and held dummy impedance, obtain an output drift component has been removed.
Instead of sample-holding the DC voltage corresponding to the dummy impedance, a bias voltage whose absolute value is equal to and opposite in polarity to the DC voltage is generated, and the DC voltage corresponding to the measured impedance is sample-held. Instead of subtracting the voltage corresponding to the dummy impedance, the DC voltage corresponding to the measured impedance and the generated bias voltage may be added.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the principle configuration of a Z / V converter according to the present invention. In this Z / V converter, the measured impedance 6, which is the object whose impedance value Zx is to be measured, is connected to the inverting input terminal (-) of the operational amplifier 1 via the signal line 5 provided with the shield 7. The feedback impedance 3 having a known impedance value Zf is connected between the output terminal and the inverting input terminal of the operational amplifier 1, and the oscillator 4 and the shield 7 are connected to the non-inverting input terminal (+). The output terminal of the operational amplifier 1 has an AC / DC conversion circuit (A
C / DC) 2 is connected. In the Z / V converter shown in FIG. 1, due to the nature of the operational amplifier having the negative feedback circuit, the inverting and non-inverting input terminals of the operational amplifier 1 are in an imaginary short state, so that they are substantially at the same potential. Becomes Therefore, since the signal line 5 and the shield 7 have the same potential, the stray capacitance generated between them is not charged and the input terminal of the operational amplifier 1 is not affected by the stray capacitance. This is established regardless of the length of the signal line 5, and is established regardless of the movement or bending of the signal line 5.

Explaining in more detail, Z / shown in FIG.
In the V conversion circuit, since the two input terminals of the operational amplifier 1 are in the imaginary short-circuit state, the voltage Vm at the inverting input terminal becomes equal to the oscillation output voltage Vi of the oscillator 4. Then, the current i flowing through the measured impedance 6
_The current i ₂ flowing through ₁ and the feedback impedance 3 is expressed as follows, where Vo is the AC voltage output from the operational amplifier 1. i ₁ = −Vm / Zx = −Vi / Zx i ₂ = (Vm−Vo) / Zf = (Vi−Vo) / Zf Here, the operational amplifier has a high input impedance characteristic by its nature. , I ₁ = i ₂ , and thus Vo = Vi (1 + Zf / Zx) (1) is obtained.

Therefore, according to the Z / V converter of FIG. 1, the AC voltage Vo that changes depending on the impedance Zx to be measured can be output from the operational amplifier 1, and the two inputs of the operational amplifier 1 are also provided. Due to the imaginary short-circuited state of the terminals, the stray capacitance generated in the signal line 5 does not appear between the input terminals, so that it is possible to output the AC voltage Vo that accurately corresponds to the value of the measured impedance Zx. Therefore, the AC voltage Vo corresponding to the measured impedance Zx can be more accurately output as compared with the conventional C / V conversion device shown in FIG. And
By passing the AC voltage Vo from the operational amplifier 1 through an AC / DC (AC / DC) conversion circuit 2 such as a rectifying / smoothing circuit, a DC voltage Vout corresponding to the amplitude of the AC voltage Vo is output. The change state of the measured impedance Zx can be detected by the change of the level of the DC output voltage Vout, and further, the value of the measured impedance Zx can be calculated by adding an appropriate calculation means as necessary. Can be calculated backwards.

By the way, the impedance 3 inserted in the feedback circuit of the operational amplifier 1 is converted into the measured impedance (Z
x) and the impedance 6 connected between the inverting input terminal and the ground is a known impedance (Zf), the output Vo of the operational amplifier 1 is Vo = Vi (1 + Zx / Zf) from the equation (1). ) (2) As is clear from the equation (2), the AC voltage Vo and the DC voltage Vout corresponding to the measured impedance value Zx can be obtained even if the connection positions of the measured impedance and the known impedance are exchanged.

FIG. 2 is a schematic diagram showing a first embodiment of the Z / V converter of the present invention. Although this embodiment is constructed based on the Z / V conversion device whose principle configuration is shown in FIG. 1, it is possible to further prevent the output voltage from drifting due to environmental changes or heat generation of the device itself. It is configured. In FIG. 2, the same or similar components as in FIG. 1 are designated by the same reference numerals. In the first embodiment shown in FIG. 2, in the Z / V converter of FIG. 1, the connection position between the measured impedance 6 of the value Zx and the impedance 3 of the known value Zf is changed and the known value is changed. The dummy impedance 8 of Zd, a switch circuit 9 for selectively connecting the measured impedance 6 and the dummy impedance 8 between the inverting input terminal and the output terminal of the operational amplifier 1, and the AC / DC conversion circuit 2 A sample and hold circuit 10 for sampling and holding an output, and an output of the sample and hold circuit and an alternating current /
Subtraction circuit 11 to which output Vout of DC conversion circuit 2 is input
And a switch circuit 9 and a sample hold circuit (SH) 1
0 and a control circuit 12 for controlling the operation of the subtraction circuit 11 are added.

In FIG. 2, the switch circuit 9 is inserted in the signal line connecting the measured impedance 6 and the operational amplifier 1, and the signal lines 5 _{1 on} both sides of the switch circuit 9 are
In order to apply the shields 7 ₁ and 7 ₂ to 5 ₂ and reduce the influence of the stray capacitance between these signal lines and the shield,
The shields 7 ₁ and 7 ₂ are connected to the non-inverting input terminal of the operational amplifier 1 connected to the oscillator 4. And a dummy
Since it is preferable to place the impedance 8 in the same environment as the measured impedance 6, the switch circuit 9 and the dummy
The line 5 _d connecting to the impedance 8 has the same length as the signal line 5 ₁ and is shielded by the shield 7 _d connected to the non-inverting input terminal (and the oscillator 4) of the operational amplifier 1. The dummy impedance 8 and the measured impedance 6 preferably have the same characteristics such as resistance or capacitance. Further, the switch circuit 9 can be arranged near the inverting input terminal of the operational amplifier 1 or near the impedance 6 to be measured and the dummy impedance 8. In the former case, signal line 5
₁ and the shield 7 ₁ are unnecessary, and in the latter case, the signal lines 5 ₂ and 5 _d and the shields 7 ₂ and 7 _d are unnecessary.

FIG. 3 shows the output timing of the control signals S ₉ , S ₁₀ , S ₁₁ output from the control circuit 12 of the Z / V converter shown in FIG. The control signal S ₉ is supplied to the switch circuit 9 so that the inverting input terminal of the operational amplifier 1 is connected to the dummy impedance 8 side when the level is high and to the measured impedance 6 side when the level is low. Control the switch circuit. The control signal S ₁₀ is supplied to the sample hold circuit 10, and when the signal is at a high level, the sample hold circuit is operated to operate the AC / DC conversion circuit 2
The output Vout of is sampled and held. The control signal S ₁₁ is supplied to the subtraction circuit 11 and controls the subtraction circuit 11 so as to execute the subtraction operation when it is at a high level.

The operation of the first embodiment of the present invention will be described with reference to FIGS. First, when the measurement operation is started at the time point t ₀ , the control circuit 12 sets the control signal S ₉ to a high level to switch the switch circuit 9 to the dummy impedance 8 side (the position of the dotted line), and controls at the time point t ₁ . The signal S _{10 is set} to a high level to turn on the gate in the sample hold circuit 10 to execute the sampling and holding operation. As a result, the DC voltage Vout (= Vout-d (t)) corresponding to the dummy impedance Zd is output from the AC / DC conversion circuit 2 and accumulated or held in the capacitor of the sample hold circuit 10. Next, at time t ₂ , the control signal S ₉ is returned to the low level to connect the switch circuit 9 to the measured impedance 6 side, whereby the DC voltage corresponding to the measured impedance Zx from the AC / DC conversion circuit 2 is obtained. Vout (= Vout-x
(t)) is output. At this time, since the control signal S ₁₀ holds the low level, the gate of the sample hold circuit 10 holds the off state, and therefore the hold voltage holds the voltage Vout-d (t) corresponding to the dummy impedance Zd. To do.

Then, the control circuit 12 sets the control signal S ₁₁ to the high level at the time point t ₃ , so that the subtraction circuit 11 samples and holds from the DC voltage Vout-x (t) corresponding to the measured impedance Zx. The voltage Vout-d (t) held in the circuit 10 is subtracted, and the difference voltage VV = Vout-x (t) -Vout-d (t) (3) is output. The time lag between the time points t ₀ and t ₁ and the time lag between the time points t ₂ and t ₃ require time from the switching of the switch circuit 9 until the output voltage of the AC / DC conversion circuit 2 stabilizes. It is provided.

By the way, Vout-x (t) and Vout-d (t) in the equation (3) include a drift component caused by a change in environmental temperature or heat generation of the device itself. Is the same value Δ for both Vout-x (t) and Vout-d (t).
It is V (t). That is, Vout-x (t) = Vout-x + ΔV (t) Vout-d (t) = Vout-d + ΔV (t) ∴V = Vout-x-Vout-d (4) However, Vout-x and Vout-d Is the true measured impedance Z when the output voltage Vout does not include a drift component.
Let us denote the output voltage corresponding to x and Zd. This is true regardless of the value of the measured impedance Zx (and the dummy impedance Zd). Therefore, the voltage V (equation (4)) represented by the difference between Vout-x and Vout-d does not include a drift component caused by a change in the environment, so that the subtraction circuit 11 changes the measured impedance Zx. It is possible to obtain the output voltage V that changes following the change. In addition, when it is necessary to continuously monitor the change in the impedance Zx to be measured, at the time t ₄ , considering that the hold voltage Vout-d (t) of the sample hold circuit 10 decreases due to spontaneous discharge or the like. It suffices to reconnect the switch circuit 9 to the dummy impedance Zd side and repeat the above operation at an appropriate cycle. As a result, the hold voltage Vout-d (t) always accurately corresponds to the dummy impedance Zd.

In the Z / V converter according to the first embodiment shown in FIG. 2, the output voltage Vout-d corresponding to the dummy impedance Zd and containing no drift component has a reference temperature (for example, It can be obtained in advance by using in an environment such as 25 ° C. Therefore, Vout-x = V-Vout-d (5) is obtained from the equation (4), and the voltage Vout-x corresponding to the measured impedance Zx and including no drift component is obtained from the equation (5). You can Needless to say, when it is sufficient to monitor only the state of change in the measured impedance Zx, the output V may be monitored without the process represented by the equation (5).

FIG. 4 shows a Z / V converter according to the second embodiment of the present invention. The second embodiment differs from the first embodiment shown in FIG. 2 in that a bias voltage generating circuit 13 and an adding circuit 14 are used in place of the sample and hold circuit 10 and the subtracting circuit 11. . The bias voltage generation circuit 13 controls the control signal S from the control circuit 12.
_This is to generate a voltage having a polarity equal to that of the output of the AC / DC conversion circuit 2 at that time and having the opposite polarity when ₁₃ becomes a high level as a bias voltage. The control signal S ₁₃ is generated at the same timing as the control signal S ₁₀ (FIG. 3), and the control signal S ₉ is controlled to be at a high level in a high level state, so that an output corresponding to the dummy impedance Zd is output. A voltage whose absolute value is the same as that of the voltage Vout-d (t) but whose sign is opposite is set as the bias voltage -Vout-d (t). The adder circuit 14 is generated at the same timing as the control signal S ₁₁ (FIG. 3), executes the adding operation when the control signal S ₁₄ is at a high level, and outputs the voltage Vout-x (corresponding to the measured impedance Zx. The bias voltage −Vout-d (t) is added to t).

That is, at the start of measurement, the switch circuit 9 is switched so that the signal line 5 _d is grounded by the control signal S ₉ , and the corresponding output voltage Vout-d (t) is output from the AC / DC conversion circuit 2. After the output voltage Vout-d (t) becomes stable, the bias voltage generating circuit 13 receives the − by the control signal S _13.
Vout-d (t) is set as the bias voltage. afterwards,
The switch circuit 9 is switched to the measured impedance 6 side, the voltage Vout-x (t) is output from the AC / DC conversion circuit 2, and the addition circuit 14 performs the following addition. V = Vout-x (t) + (-Vout-d (t)) ∴V = Vout-x + ΔV (t)-(Vout-d + ΔV (t)) = Vout-x-Vout-d By this, the measured impedance It is possible to obtain the voltage V corresponding to Zx and not including the drift component.

FIG. 5 shows the static impedance of the measured impedance Zx of the same fixed value using the Z / V converter of the first embodiment of the present invention shown in FIG. 2 and the Z / V converter shown in FIG. The measurement result when measured as a capacitance is shown. In the graph of FIG. 5, the horizontal axis represents the number of repetitions N of the period t _{0 to} t ₄ of FIG. 3 as one measurement cycle, and the vertical axis represents the capacitance Cx (fF) that is the measured impedance Zx. It represents the amount of change. Further, (A) shows a test result by the apparatus of the present invention shown in FIG. 2, and (B) shows a test result by the apparatus of FIG. As is clear from FIG.
According to the present invention, it is possible to obtain a substantially constant measurement value even if the number of repetitions N of the measurement cycle increases, and to obtain an output with reduced drift as compared with the case where the drift compensation function is not provided. be able to.

Since the present invention is configured as described above, in the Z / V converter, it is possible to compensate for output drift due to changes in the measurement environment, heat generation of the device itself, and the measured impedance Zx. Since it is possible to reduce the influence of stray capacitance or the like on the signal line for connecting to, it is possible to perform Z / V conversion with extremely high accuracy, and it is extremely practical.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a principle configuration of a Z / V conversion device of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a Z / V conversion device according to a first embodiment of the present invention.

FIG. 3 is a timing chart showing a generation timing of a control signal in the Z / V conversion device shown in FIG.

FIG. 4 is a circuit diagram showing a configuration of a Z / V conversion device according to a second embodiment of the present invention.

5 is a schematic diagram showing the Z / V converter having the drift compensation means of the present invention shown in FIG. 2 and the Z / V converter shown in FIG.
It is a graph which shows the result of having constituted as a / V conversion device and actually tested each.

FIG. 6 is a circuit diagram showing a configuration of a conventional C / V conversion device.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI G01D 5/24 K (56) References JP-A-9-280806 (JP, A) JP-A-2-302628 (JP, A) JP-A-61-82103 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01R 27/02 G01R 27/26 G01R 35/00 G01D 5/24

Claims (8)

(57) [Claims] 1. An impedance / voltage converter for converting an impedance value to be measured into a voltage, an operational amplifier, an impedance of a known value connected to an inverting input terminal of the operational amplifier, and an operational amplifier by a first signal line. A switch circuit connected to the inverting input terminal of, and a measured impedance and a dummy impedance of a known value selectively connected between the inverting input terminal and the output terminal of the operational amplifier via the switch circuit, A first shield that shields at least a part of the first signal line and is connected to the non-inverting input terminal of the operational amplifier; and an oscillator connected to the non-inverting input terminal of the operational amplifier and the first shield. An impedance / voltage conversion device comprising: 2. The impedance / voltage conversion device according to claim 1, further comprising a second shield for shielding at least a part of a signal line connecting the switch circuit and the impedance to be measured, and a switch circuit. A third shield that shields at least a part of the signal line connecting the dummy impedance, and a fourth shield that shields at least a part of the signal line connecting the impedance of a known value and the inverting input terminal of the operational amplifier. An impedance / voltage conversion device comprising a shield, and at least one of the second to fourth shields is connected to a non-inverting input terminal of an operational amplifier. 3. The impedance according to claim 1 or 2.
In the voltage converter, the impedance to be measured and the dummy impedance have the same characteristic, and the impedance / voltage converter is characterized. 4. The impedance / voltage conversion device according to claim 1, wherein the impedance to be measured and the dummy impedance are both exposed to the same condition. . 5. The impedance / voltage conversion device according to claim 1, further comprising: an AC / DC conversion circuit connected to the output of the operational amplifier, and an output of the AC / DC conversion circuit. A sample hold circuit to be connected, a subtraction circuit connected to an output of the AC / DC conversion circuit and an output of the sample hold circuit, a switch circuit, a sample hold circuit, and a control circuit for controlling the subtraction circuit An impedance / voltage conversion device characterized in that 6. The impedance / voltage conversion device according to claim 1, further comprising: an AC / DC conversion circuit connected to the output of the operational amplifier; and an output of the AC / DC conversion circuit. Connected to the bias voltage generation circuit that is connected and outputs a bias voltage whose polarity is opposite to the output voltage of the AC / DC conversion circuit and whose absolute value is the same, and the output of the AC / DC conversion circuit and the output of the bias voltage generation circuit. An impedance / voltage conversion device comprising an adder circuit, a switch circuit, a bias voltage generation circuit, and a control circuit for controlling the adder circuit. 7. An impedance / voltage conversion method for converting an impedance value to be measured into a voltage, wherein an inverting input terminal of an operational amplifier has a known impedance connected to the inverting input terminal and an oscillator connected to the non-inverting input terminal. A dummy impedance is connected between the output terminal and the output terminal, and the output of the operational amplifier when the dummy impedance is connected is converted to AC / DC to obtain a DC voltage corresponding to the dummy impedance, which corresponds to the dummy impedance. Corresponds to the measured impedance by sampling and holding the DC voltage, connecting the impedance to be measured between the inverting input terminal and the output terminal of the operational amplifier, and converting the output of the operational amplifier when the measured impedance is connected to AC / DC. The DC voltage corresponding to the measured impedance An impedance / voltage conversion method comprising: subtracting a voltage corresponding to the dummy impedance that has been dropped so that an output from which a drift component is removed can be obtained. 8. An impedance / voltage conversion method for converting an impedance value to be measured into a voltage, wherein an inverting input terminal is connected to a known impedance, and a non-inverting input terminal is connected to an oscillator. A dummy impedance is connected between the output terminal and the output terminal, and the output of the operational amplifier when the dummy impedance is connected is converted to AC / DC to obtain a DC voltage corresponding to the dummy impedance, which corresponds to the dummy impedance. A bias voltage whose absolute value is equal to that of the DC voltage and whose polarity is opposite is generated, and the impedance to be measured is connected between the inverting input terminal and the output terminal of the operational amplifier. Converts DC to obtain a DC voltage corresponding to the measured impedance, which corresponds to the measured impedance The impedance / voltage conversion method is characterized in that an output from which a drift component has been removed can be obtained by adding a DC voltage to be generated and a generated bias voltage.