JP2000304786A - Device and method for impedance/voltage conversion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of drifts in the output voltage of an impedance/voltage converter corresponding to the value of an impedance to be measured due to environmental changes, the heat generated from the converter itself, and so on. SOLUTION: Immediately after measurement is started, the impedance/voltage converter connects a dummy impedance Zd between the inverted input terminal and output terminal of an operational amplifier 1 by controlling a switch circuit 9 and causes a sample-and-hold circuit 10 to hold the DC voltage value corresponding to the dummy impedance Zd from an AC/DC conversion circuit 2. Then, the converter connects an impedance Zx to be measured between the inverted input terminal and output terminal of the amplifier 1 by switching the switch circuit 9 and, at the same time, causes a subtraction circuit 11 to subtract the voltage held by the 10 from the DC voltage corresponding to the impedance Zx from the conversion circuit 2. Thereby, an output in which drift components are offset can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for converting an 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 drift of an output voltage 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 a configuration of a capacitance-voltage (C / V) converter disclosed in Japanese Patent Application Laid-Open No. 61-14578. In this conventional device, the stray capacitance is superimposed on a signal line for connecting the unknown capacitance Cx to the C / V converter, and the stray capacitance changes due to the movement or bending of the signal line. In order to solve the problem that accurate C / V conversion cannot be performed, the signal line L1 is connected between the AC signal generator OS and the operational amplifier OP as shown in the figure. And L
2 and an unknown capacitor Cx connected through these signal lines L
The effect of stray capacitances Cs1, Cs2, Cs3 is intended to be reduced by shielding 1 and L2 with shielding means s1 and s2. 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. The ends 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 a negative feedback circuit. The two input terminals have substantially the same potential (ground potential), and the stray capacitance Cs2 is not charged. Further, the stray capacitance Cs3 is considered to be a coupling capacitance of the two shield means s1 and s2, but since both of the shield means are grounded, this is also not charged. Therefore, the influence of the stray capacitance of the cable connecting the unknown capacitance Cx is reduced, and a charge equal to the charge induced in the capacitance Cx is induced in the capacitor Cf of the feedback circuit. A voltage Vo = − (Cx / Cf) Vi proportional to Cx is obtained, where Vi is the output voltage of the AC signal generator OS.

[0004]

[Problems to be solved by the present invention] However, FIG.
In the C / V converter of the above, when the unknown capacitance Cx becomes small, the influence of the stray capacitance becomes apparent, and an accurate C / V
There is a problem that conversion cannot be performed. Also, the capacitance Cx
When one of the electrodes is biased to a certain potential, AC cannot be applied to the capacitance,
There is also a problem that C / V conversion is impossible. further,
The output voltage Vo may drift due to environmental changes such as temperature changes or aging of the device. For example, even though the unknown capacitance Cx has not changed, the level of the output voltage Vo differs between when the measurement environment is high and when the temperature is low, and, as a result, it accurately corresponds to the unknown capacitance. Output voltage may not be obtained. The present invention has been proposed to solve such a problem, and an object of the present invention is to reduce the influence of stray capacitance on a line for connecting an impedance to be measured in a Z / V converter. It is another object of the present invention to effectively reduce the drift of the output voltage caused by the surrounding environment or the like, and to obtain an output voltage corresponding to the impedance to be measured with high accuracy.

[0005]

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

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

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

In order to achieve the above object, in the impedance / voltage conversion method for converting an impedance value to be measured into a voltage according to the present invention, a known value of impedance is connected to an inverting input terminal and a non-inverting input terminal is connected to the inverting input terminal. 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 the dummy impedance is connected is subjected to AC / DC conversion to correspond to the dummy impedance. Obtain a DC voltage, sample and hold the DC voltage corresponding to the dummy impedance, connect the measured impedance between the inverting input terminal and the output terminal of the operational amplifier, and connect the output of the operational amplifier with the measured impedance to AC. / DC conversion to obtain a DC voltage corresponding to 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.
Note that instead of sampling and holding the DC voltage corresponding to the dummy impedance, a bias voltage having the same absolute value as the DC voltage and the opposite polarity was generated, and sampled and held from the DC voltage corresponding to the impedance to be measured. Instead of subtracting the voltage corresponding to the dummy impedance, the DC voltage corresponding to the impedance to be measured and the generated bias voltage may be added.

[0009]

FIG. 1 shows the principle configuration of a Z / V converter according to the present invention. In this Z / V converter, an impedance to be measured 6 whose impedance value Zx is to be measured is connected to an inverting input terminal (−) of the operational amplifier 1 via a signal line 5 provided with a 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 (+). An 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, since the inverting and non-inverting input terminals of the operational amplifier 1 are in an imaginary short state due to the nature of the operational amplifier having a negative feedback circuit, they have substantially the same potential. Becomes Therefore, since the signal line 5 and the shield 7 have the same potential, the stray capacitance generated therebetween is not charged, and the input terminal of the operational amplifier 1 is not affected by the stray capacitance. This is true irrespective of the length of the signal line 5, and is true irrespective of the movement or bending of the signal line 5.

More specifically, Z / shown in FIG.
In the V conversion circuit, since the two input terminals of the operational amplifier 1 are in the imaginary short state, the voltage Vm of 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, since the operational amplifier has a high input impedance characteristic due to its property. , I ₁ = i ₂ , so that 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 to be measured Zx can be output from the operational amplifier 1, and the two inputs of the operational amplifier 1 Due to the imaginary short-circuit state of the terminal, no stray capacitance generated in the signal line 5 appears between the input terminals, so that the AC voltage Vo exactly corresponding to the value of the measured impedance Zx can be output. Therefore, it is possible to more accurately output the AC voltage Vo corresponding to the measured impedance Zx as compared with the conventional C / V converter 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 state of the change of the measured impedance Zx can be detected from the change in the level of the DC output voltage Vout, and the value of the measured impedance Zx can be calculated by adding appropriate arithmetic means as needed. Back calculation is possible.

By the way, the impedance 3 inserted in the feedback circuit of the operational amplifier 1 is changed to the impedance to be measured (Z
x), and when 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 given by Vo = Vi (1 + Zx / Zf) from Expression (1). ) (2). As is clear from 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 position between the measured impedance and the known impedance is changed.

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

In FIG. 2, the switch circuit 9 is inserted in a signal line connecting the impedance 6 to be measured and the operational amplifier 1, and the signal lines 5 ₁ and 5 on both sides of the switch circuit 9 are connected.
5 ₂ Faraday shield 7 _1, 7 _2, and in order to reduce the effect of 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 the dummy
Since it is preferable to place the impedance 8 in the same environment as the impedance 6 to be measured, the switch circuit 9 and the dummy
Line 5 _d which connects the impedance 8, are shielded by a shield 7 _d connected to the non-inverting input terminal of the operational amplifier 1 (and oscillators 4) with a signal line 5 ₁ of the same length. It is preferable that the dummy impedance 8 and the measured impedance 6 have the same characteristic such as a resistance or a capacitance. Further, the switch circuit 9 may be arranged near the inverting input terminal of the operational amplifier 1 or near the measured impedance 6 and the dummy impedance 8. In the former case, signal line 5
₁ and the shield 7 ₁ is not required, in the latter case, the signal line 5 _2, 5 _d and the shield 7 _2, 7 _d is not required.

FIG. 3 shows output timings of control signals S ₉ , S ₁₀ , and 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 when the signal is at a high level, and to the measured impedance 6 when the signal is at a low level. Control the switch circuit. Control signal S ₁₀ is supplied to the sample hold circuit 10, by operating the sample-and-hold circuit when the high level, the AC / DC converter 2
Sampling and holding of the output Vout. Control signal S ₁₁ is supplied to the subtraction circuit 11, to perform a subtraction operation if the high level, controls the subtraction circuit 11.

The operation of the first embodiment of the present invention will be described with reference to FIGS. First, when the measurement operation starts at time t ₀ , the control circuit 12 sets the control signal S ₉ to a high level, switches the switch circuit 9 to the dummy impedance 8 side (the position indicated by the dotted line), and controls the control circuit 12 at time t ₁ . and a signal S ₁₀ to the high level, the gate of the sample-hold circuit 10 is turned on 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 stored or held in the capacitor of the sample and hold circuit 10. Next, at time t ₂ , the control signal S ₉ is returned to a low level, and the switch circuit 9 is connected to the side of the impedance 6 to be measured, whereby the DC voltage corresponding to the impedance Zx to be measured is output from the AC / DC conversion circuit 2. Vout (= Vout-x
(t)) is output. At this time, since the control signal S ₁₀ is holding the low level, the gate of the sample-and-hold circuit 10 holds the OFF state, thus holding voltage holding the voltage Vout-d corresponding to the dummy impedance Zd (t) I do.

At time t ₃ , the control circuit 12 sets the control signal S ₁₁ to a high level, whereby the subtraction circuit 11 samples and holds the DC voltage Vout-x (t) corresponding to the measured impedance Zx. The voltage Vout-d (t) held in the circuit 10 is subtracted to output a difference voltage VV = Vout-x (t) -Vout-d (t) (3). Note that 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 time when the switch circuit 9 is switched to the time when the output voltage of the AC / DC conversion circuit 2 is stabilized. It is provided.

Vout-x (t) and Vout-d (t) in equation (3) include a drift component caused by a change in environmental temperature or heat generated by the apparatus itself. Is the same value Δ for both Vout-x (t) and Vout-d (t).
V (t). That is, Vout-x (t) = Vout-x + ΔV (t) Vout-d (t) = Vout-d + ΔV (t) ∴V = Vout-x−Vout-d (4) where Vout-x and Vout-d Is the true measured impedance Z when the output voltage Vout contains no drift component.
Let it represent 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 an environmental change or the like. It is possible to obtain an output voltage V that changes following the change. Incidentally, when it is necessary to continuously monitor the change in the measured impedance Zx is hold voltage Vout-d of the sample-and-hold circuit 10 (t) is considering a decrease by natural discharge or the like, at time t ₄ The switch circuit 9 may be connected again to the dummy impedance Zd side, and the above operation may be repeated at an appropriate cycle. Thereby, the hold voltage Vout-d (t) always exactly corresponds to the dummy impedance Zd.

In the Z / V converter according to the first embodiment shown in FIG. 2, an output voltage Vout-d corresponding to the dummy impedance Zd and containing no drift component is supplied to the reference temperature (for example, (25 ° C.) or the like. Therefore, Vout-x = V-Vout-d (5) is obtained from Expression (4), and a voltage Vout-x corresponding to the measured impedance Zx and containing no drift component is obtained from Expression (5). Can be. It is needless to say that when only the state of change of the measured impedance Zx needs to be monitored, the output V may be monitored without the need for the processing represented by the equation (5).

FIG. 4 shows a Z / V converter according to a 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 instead 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.
_{When 13} is at a high level, a voltage having the same absolute value as the output of the AC / DC converter circuit 2 at that time and having the opposite polarity is generated as a bias voltage. Control signal S _13, the control signal S ₁₀ is generated at the same timing (FIG. 3), a control for signal S ₉ is controlled to be a high level of high-level state, the output corresponding to the dummy impedance Zd A voltage having the same absolute value as the voltage Vout-d (t) but having the opposite sign is set as the bias voltage −Vout-d (t). The adding circuit 14 generates the voltage Vout-x () generated at the same timing as the control signal S ₁₁ (FIG. 3) and performs the adding operation when the control signal S ₁₄ is at a high level. The bias voltage −Vout−d (t) is added to t).

That is, at the start of the measurement, the switch circuit 9 is switched by the control signal S _{9 so} that the signal line 5 _d is grounded, and the corresponding output voltage Vout-d (t) is output from the AC / DC conversion circuit 2. After the output voltage Vout-d (t) is stabilized, the bias voltage generating circuit 13 by the control signal S ₁₃ -
Vout-d (t) is set as a bias voltage. afterwards,
The switch circuit 9 is switched to the impedance 6 to be measured, the voltage Vout-x (t) is output from the AC / DC conversion circuit 2, and the following addition is performed in the addition circuit 14. V = Vout-x (t) + (− Vout-d (t)) ∴V = Vout-x + ΔV (t) − (Vout-d + ΔV (t)) = Vout-x−Vout-d A voltage V corresponding to Zx and containing no drift component can be obtained.

FIG. 5 is a circuit diagram of the Z / V converter according to the first embodiment of the present invention shown in FIG. 2 and the Z / V converter shown in FIG. The measurement result when measuring as an electric capacity is shown. In the graph of FIG. 5, the horizontal axis represents the number of repetitions N with the period t _{0 to} t ₄ in 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. (A) shows the test result by the apparatus of the present invention shown in FIG. 2, and (B) shows the test result by the apparatus of FIG. As is clear from FIG.
According to the present invention, a substantially constant measurement value can be obtained even when the number of repetitions N of the measurement cycle is increased, and an output with reduced drift is obtained as compared with a case where the apparatus does not have a drift compensation function. be able to.

Since the present invention is configured as described above, in the Z / V converter, it is possible to compensate for an output drift due to a change in the measurement environment, heat generation of the device itself, and the like. Can reduce the influence of the stray capacitance on the signal line for connecting the signal lines, thereby making it possible to perform Z / V conversion with extremely high accuracy and extremely high practicality.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a principle configuration of a Z / V converter according to the present invention.

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

FIG. 3 is a timing chart showing a generation timing of a control signal in the Z / V converter shown in FIG. 2;

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

FIG. 5 shows a Z / V converter having the drift compensating means of the present invention shown in FIG. 2 and a Z / V converter shown in FIG.
6 is a graph showing the results of actual machine tests for each of the devices configured as / V converters.

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

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 2G028 AA01 CG07 CG08 DH05 DH21 FK01 FK09 GL01 GL09 5H410 BB08 CC02 DD02 DD09 EA12 FF03 FF09 FF25 GG00 HH00 5J090 AA01 AA51 CA00 CA02 CA12 FA13 FA17 FN09 HA13 N KA01 KA12 KA19 KA26 KA32 KA51 MA13 NN11 TA01 TA02 TA06

Claims (8)

[Claims] An impedance / voltage converter for converting an impedance value to be measured into a voltage, comprising: an operational amplifier; an impedance having a known value connected to an inverting input terminal of the operational amplifier; and a first signal line. A switch circuit connected to the inverting input terminal of the operational amplifier; and a dummy impedance of a known value and a measured impedance 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 portion of the first signal line and is connected to a non-inverting input terminal of the operational amplifier; 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; A third shield for shielding at least a part of a signal line connecting the dummy impedance, and a fourth shield for shielding at least a part of a signal line connecting an impedance having a known value and an inverting input terminal of the operational amplifier. And a shield, wherein at least one of the second to fourth shields is connected to a non-inverting input terminal of the operational amplifier. 3. The impedance according to claim 1 or 2,
An impedance / voltage converter, wherein the impedance to be measured and the dummy impedance both have the same characteristic. 4. The impedance / voltage converter according to claim 1, wherein both the measured impedance and the dummy impedance are exposed to the same condition. . 5. The impedance / voltage converter according to claim 1, further comprising: an AC / DC converter connected to an output of the operational amplifier; and an AC / DC converter connected to an output of the AC / DC converter. A sample-and-hold circuit connected thereto; a subtraction circuit connected to an output of the AC / DC conversion circuit and an output of the sample-and-hold circuit; a switch circuit, a sample-and-hold circuit, and a control circuit for controlling the subtraction circuit. An impedance / voltage conversion device characterized by the above-mentioned. 6. The impedance / voltage conversion device according to claim 1, further comprising: an AC / DC conversion circuit connected to an output of the operational amplifier; and an output of the AC / DC conversion circuit. A bias voltage generating circuit for outputting a bias voltage having a polarity opposite to that of the output voltage of the AC / DC converter and having the same absolute value; and an output of the AC / DC converter and an output of the bias voltage generator. An impedance / voltage conversion device, comprising: an addition circuit; a switch circuit; a bias voltage generation circuit; and a control circuit for controlling the addition circuit. 7. An impedance / voltage conversion method for converting an impedance value to be measured into a voltage, wherein the inverting input terminal of the operational amplifier has a known value of impedance connected to the inverting input terminal and an oscillator connected to the non-inverting input terminal. A dummy impedance is connected between the terminal and the output terminal. The output of the operational amplifier when the dummy impedance is connected is subjected to AC / DC conversion to obtain a DC voltage corresponding to the dummy impedance. Sample and hold the DC voltage, connect the impedance to be measured between the inverting input terminal and the output terminal of the operational amplifier, and convert the output of the operational amplifier when the impedance to be measured is connected to AC / DC to correspond to the impedance to be measured. DC voltage corresponding to the impedance to be measured. An impedance / voltage conversion method, comprising subtracting a voltage corresponding to a filtered dummy impedance to obtain an output from which a drift component has been removed. 8. 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 having an inverting input terminal connected to an impedance of a known value and an oscillator connected to a non-inverting input terminal. A dummy impedance is connected between the terminal and the output terminal. The output of the operational amplifier when the dummy impedance is connected is subjected to AC / DC conversion to obtain a DC voltage corresponding to the dummy impedance. A bias voltage having the same absolute value as the DC voltage and the opposite polarity is generated, the impedance to be measured is connected between the inverting input terminal and the output terminal of the operational amplifier, and the output of the operational amplifier when the impedance to be measured is connected is AC / AC. DC conversion to obtain a DC voltage corresponding to the measured impedance, corresponding to the measured impedance An impedance / voltage conversion method characterized by adding a DC voltage to be generated and a generated bias voltage to obtain an output from which a drift component has been removed.