JP2009058290A - Charge amplifier, charge amplifier device, and bias current compensation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a charge amplifier and a charge amplifier device capable of reducing easily measuring drift, and a bias current compensation method capable of compensating easily an influence of a bias current Ib. <P>SOLUTION: The charge amplifier 101 converts input charge from a piezoelectric sensor Sn into an output voltage determined by dividing it by a capacitance C1e of a capacitor C1. When a bias current Ib flows, the sum of a bias charge determined by time-integrating the bias current Ib and the input charge is converted as the output voltage. Since the output voltage is influenced by the bias current Ib, the measuring drift is generated. A compensation current Ia flows wholly as the bias current Ib into an inverting input terminal (-) through a capacitor C2 into which an application voltage Va is applied. The charge amplifier 101 converts only the input charge into the output voltage. Hereby, the measuring drift is reduced. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a charge amplifier that converts an input charge into a voltage, a charge amplifier device that includes a detector that measures various physical quantities, and a bias current compensation method that compensates for the influence of a bias current.

  The charge amplifier is for amplifying a charge signal from direct current to alternating current and measuring various physical quantities using a piezoelectric sensor (detector). In the charge amplifier, a capacitor is connected between the input and output of the operational amplifier, and the charge generated by the piezoelectric sensor is converted into an output voltage.

In addition, a discharge transistor for charge discharge may be connected in parallel to the capacitor. In this discharging transistor, a leakage current flows due to the insulation resistance between the gate and the drain / source. Unless this leakage current is suppressed to several tens [nA] or less, measurement results are affected and measurement drift occurs. As a technique for solving this problem, Patent Document 1 describes a technique for compensating a leakage current depending on a capacitor discharge transistor by using a reverse bias current of a diode and reducing a measurement drift.
JP-A-11-148878 (paragraph number 0012, FIG. 1)

  The reverse bias current of the diode does not depend on the reverse voltage, and a substantially constant reverse saturation current flows. For this reason, it is difficult to select a diode having a reverse saturation current corresponding to the leakage current.

  Furthermore, in order to perform highly accurate measurement, it is necessary to consider the bias current flowing into the inverting input terminal of the operational amplifier. However, since the reverse saturation current is fixed in the technique described in Patent Document 1, the diode cannot be selected in accordance with the bias current, and therefore the measurement drift cannot be reduced.

  It is an object of the present invention to provide a charge amplifier, a charge amplifier device, and a bias current compensation method that can easily compensate for the influence of a bias current that can easily reduce measurement drift.

In order to achieve the object, a charge amplifier according to claim 1 includes an operational amplifier that amplifies a potential difference between an inverting input terminal and a normal input terminal, and an output terminal and an inverting input terminal of the operational amplifier. A charge amplifier that outputs a voltage proportional to the input charge input to the capacitor to the output terminal, and a high resistance element is connected to the connection point between the inverting input terminal and the capacitor. The high resistance element is characterized in that a current corresponding to a leakage current at a connection point between the inverting input terminal and the capacitor flows.
According to the above configuration, a voltage obtained by dividing the input charge by the capacitance of the capacitor is output from the operational amplifier. On the other hand, due to the leakage current at the connection point, the charge input to the capacitor increases or decreases, the output voltage is affected, and measurement drift occurs. By causing a current corresponding to the leakage current to flow through the high resistance element, the amount of increase or decrease in the charge input to the capacitor is reduced, and the influence on the output of the operational amplifier can be suppressed. For this reason, measurement drift is easily reduced.

A charge amplifier according to a second aspect is the charge amplifier according to the first aspect, wherein the high resistance element is a parallel equivalent resistance of a capacitor.
According to the said structure, selection of a high resistance element becomes easy by utilizing the parallel equivalent resistance of a capacitor | condenser. That is, a high resistance element that satisfies the required specifications is readily available.

A charge amplifier according to claim 3 is the charge amplifier according to claim 2, wherein the capacitor is a film capacitor.
According to the said structure, since there are many kinds of film capacitors and selection conditions can be satisfied easily, availability is high.

A charge amplifier according to claim 4 is the charge amplifier according to claim 2, wherein the capacitor is a temperature-compensating ceramic capacitor or a mica capacitor.
According to the above configuration, since the slope of decrease of the insulation resistance value accompanying the temperature rise is smaller than that of other capacitors, the current corresponding to the leakage current at the connection point is less changed with respect to the temperature. For this reason, the measurement drift also has good temperature characteristics.

A charge amplifier according to a fifth aspect is the charge amplifier according to the third or fourth aspect, wherein the operational amplifier has an offset voltage.
According to the said structure, a thing with a large parallel equivalent resistance value (insulation resistance value) of a capacitor | condenser can be selected easily. For this reason, for the operational amplifier with a small bias current, the parallel equivalent resistance value increases, so that the applied voltage becomes larger than the maximum offset voltage, so that it is difficult to be affected by the change in the offset voltage.

The charge amplifier according to claim 6 is the charge amplifier according to claim 1, wherein the leakage current is a bias current of the operational amplifier, and a reference power source that supplies a current substantially equal to the bias current to the high resistance element. Is connected.
According to the above configuration, if the voltage of the reference power supply is adjusted, a compensation current substantially equal to the bias current can be easily flowed, so that the bias current can be compensated. Furthermore, since measurement drift due to bias current can be reduced, highly accurate measurement is possible.

The charge amplifier device according to claim 7 is the charge amplifier according to claim 6, wherein the voltage applied to the high-resistance element is obtained by multiplying the resistance value of the high-resistance element by the bias current and the value of the operational amplifier. It is a value obtained by adding the maximum offset voltage.
According to the above configuration, the maximum offset voltage is the maximum value of the offset voltage, and the voltage applied to the high resistance element can be determined by a value calculated from the maximum value. For this reason, since it is not necessary to measure an offset voltage beforehand, design and manufacture become easy.

A charge amplifier device according to an eighth aspect includes the charge amplifier according to any one of the first to seventh aspects, and a detector that is connected to an inverting input terminal and generates a charge proportional to a physical quantity. It is characterized by that.
According to the said structure, since a measurement drift can be reduced easily, a detector can be selected and various physical quantities can be measured with high precision.

A charge amplifier device according to a ninth aspect is the charge amplifier device according to the eighth aspect, wherein the detector is connected between the inverting input terminal and the non-inverting input terminal.
According to the said structure, the physical quantity from direct current | flow to alternating current can be amplified. For this reason, measurement drift can be reduced and various physical quantities can be measured with high accuracy using a detector.

The charge amplifier device according to claim 10 is the charge amplifier device according to claim 8, wherein the detector is connected between the inverting input terminal and the ground terminal, and the normal rotation input terminal has a predetermined potential. It is characterized by being set to.
According to the above configuration, the fluctuation charge that is the fluctuation of the charge generated by the detector is input to the capacitor, and the voltage across the capacitor is output from the operational amplifier. In addition, since a single power supply operation of the operational amplifier becomes possible, restrictions on the power supply design of the charge amplifier are eased.

The bias current compensation method according to claim 11 includes an operational amplifier that amplifies a potential difference between the inverting input terminal and the normal input terminal, and a capacitor connected between the output terminal and the inverting input terminal of the operational amplifier. A bias current compensation method that compensates for the influence of the bias current of the operational amplifier using a charge amplifier that outputs to the output terminal an output voltage proportional to the input charge input to the capacitor. Set the output voltage to a virtual potential that is approximately the same as the normal input terminal, and then observe the output voltage waveform to measure the elapsed time immediately after opening the capacitor and the voltage change in the output voltage from the virtual ground potential. A waveform observation step to be performed, and a calculation step for calculating the bias current by dividing the elapsed time by the value obtained by multiplying the capacitance of the capacitor by the voltage change amount. Substantially equal compensation current to the value of the bias current calculated in step, characterized in that the flow to the connection point between the inverting input terminal and the capacitor.
According to the above configuration, in the waveform observation step for observing the waveform of the output voltage, the voltage change amount of the output voltage is proportional to the elapsed time, so that measurement data for calculating the bias current can be obtained. In the calculation step for calculating the bias current, the current value of the bias current can be easily calculated by performing a calculation for multiplying the capacitance by the voltage change amount and dividing the elapsed time based on the measurement data. Therefore, if a compensation current equal to the calculated current value is supplied, the influence of the bias current can be easily compensated.

  According to the charge amplifier of the present invention, measurement drift can be easily reduced. Moreover, according to the charge amplifier device of the present invention, various physical quantities can be measured with high accuracy by selecting a detector. Furthermore, according to the bias current compensation method of the present invention, the influence of the bias current can be easily compensated.

(First embodiment)
FIG. 1 is a circuit diagram of the charge amplifier device according to the first embodiment. The charge amplifier device includes a piezoelectric sensor Sn as a detector, a charge amplifier 101, and a sensitivity setting device (not shown). The charge amplifier 101 includes an operational amplifier Ap and capacitors C1 and C2. The capacitor C2 includes a parallel equivalent resistor Ra and a parallel equivalent capacitor Ca, and the parallel equivalent resistor Ra functions as a high resistance element. The piezoelectric sensor Sn is used as an acceleration sensor, a pressure sensor, a force sensor, or the like, and is installed outside to generate an electric charge proportional to a physical quantity to be measured. The charge amplifier 101 inputs all the generated charges to one pole of the capacitor C1, and induces a charge of opposite polarity to the other pole, thereby generating a voltage at both ends of the capacitor C1. Thereby, the charge amplifier 101 converts the charge (hereinafter referred to as input charge) input to the capacitor C1 into a voltage divided by the capacitance C1e.
The two electrodes of the piezoelectric sensor Sn are connected to the external terminals 12 and 13, respectively. The external terminal 12 is connected to the node n1. The external terminal 13 is connected to the ground. For example, one electrode of the piezoelectric sensor Sn and the external terminal 12 are directly connected, and instead of the other electrode of the piezoelectric sensor Sn and the external terminal 13 being directly connected, a ground or housing of a measuring instrument or the like In some cases, they are connected by setting them to the same potential via. This is to cope with conditions such as the measurement environment and the structure of the piezoelectric sensor Sn.

  The operational amplifier Ap includes a normal rotation input terminal (+), an inverting input terminal (−), and an output terminal (OUT), and amplifies a potential difference between the normal rotation input terminal (+) and the inverting input terminal (−). The amplified voltage is output to the output terminal (OUT).

  The node n1 is connected to the inverting input terminal (−) of the operational amplifier Ap, is connected to the output terminal (OUT) through the capacitor C1, and is connected to the reference power source of the applied voltage Va through the capacitor C2. The normal rotation input terminal (+) of the operational amplifier Ap is connected to the ground. The output terminal (OUT) of the operational amplifier Ap is connected to the internal terminal 11. The operational amplifier Ap controls the potential of the node n1 to be kept at the ground potential, thereby making the node n1 a virtual ground. The operational amplifier Ap outputs a voltage (hereinafter referred to as an output voltage) obtained by dividing the input charge by the capacitance C1e of the capacitor C1. A predetermined positive DC voltage and a negative DC voltage are applied to the positive power supply input terminal (V +) and the negative power supply input terminal (V−) of the operational amplifier Ap, respectively.

For example, National Semiconductor's LMC6001 is used for the operational amplifier Ap. On the input side of the LMC 6001, the input resistance value Rin exceeds 1 [TΩ], the maximum bias current Ibm is 25 [fA], and the maximum offset voltage Vosm is 0.7 [mV].
In addition, a three-component force sensor is used as the piezoelectric sensor Sn, and any one of charge signals in directions of orthogonal three components (Fx, Fy, Fz) is input to the charge amplifier 101. The insulation resistance value is 10 [TΩ] or more in each direction, the sensitivity in the Fx and Fy directions is −8.1 [pC / N], and the detector sensitivity in the Fz direction is −3.8 [pC / N]. is there.

  The sensitivity setting unit matches the output voltage output from the internal terminal 11 of the charge amplifier 101 with a clean integer relationship. For example, the output voltage is set to 10 [V] with respect to the physical quantity of force 100 [N]. The charge amplifier device can measure various physical quantities by selecting various sensors in combination with a sensitivity setting device.

The operational amplifier Ap has an input resistance value Rin between the normal input terminal (+) and the inverting input terminal (−). A bias current Ib defined in the direction from the node n1 to the inverting input terminal (−) flows through the inverting input terminal (−) of the operational amplifier Ap, and a charge obtained by time-integrating the bias current Ib (hereinafter referred to as a bias charge). Will occur.
The charge amplifier 101 converts the input charge generated by the piezoelectric sensor Sn into a voltage, converts the bias charge into a voltage, and outputs the sum of both voltages as an output voltage. Since the voltage converted from the bias charge affects the voltage converted from the input charge, measurement drift occurs. As the bias current Ib, a predetermined current continuously flows. The influence of the bias charge is particularly problematic for high-precision measurement.

An offset voltage Vos is generated between the inverting input terminal (−) and the normal rotation input terminal (+) of the operational amplifier Ap due to various causes. The reference power supply applies a positive DC voltage (hereinafter referred to as an applied voltage Va) to the capacitor C2 with respect to the ground. The applied voltage Va is calculated by Equation (1), where Ia (hereinafter referred to as compensation current Ia) is a current flowing through the capacitor C2.
Va = (Ia × Rae) + Vos (1)
However, Rae is the resistance value of the parallel equivalent resistor Ra of the capacitor C2 (resistance value of the high resistance element).

The current value of the bias current Ib is set as the compensation current Ia. That is, the compensation current Ia and the bias current Ib have the same current direction and current value. For this reason, all the compensation current Ia flows into the inverting input terminal (−) of the operational amplifier Ap as the bias current Ib. Therefore, the bias charge is not converted into a voltage by the operational amplifier Ap. The charge amplifier 101 converts only the input charge as an output voltage. Since the voltage converted from the input charge is not affected at all by the voltage converted from the bias charge, the measurement drift is reduced even for high-accuracy measurement.
The charge amplifier device can amplify physical quantities from direct current to alternating current. For this reason, measurement drift can be reduced and various physical quantities can be measured with high accuracy using the piezoelectric sensor Sn.

  The capacitor C2 is selected to have a predetermined parallel equivalent resistance value Rae in order to satisfy the required specifications as a high resistance element. The applied voltage Va is adjusted to a voltage that satisfies Equation (1). For example, when the bias current Ib is 25 [fA] and the offset voltage Vos is 0.7 [mV], the compensation current Ia is made to correspond to the bias current Ib, and the parallel equivalent resistance value Rae is 1 [TΩ]. A nearby capacitor C2 is selected. Further, the applied voltage Va is adjusted to around 25.7 [mV], and a compensation current Ia of 25 [fA] is passed. Therefore, by selecting the operational amplifier Ap having a small bias current Ib and selecting the one having a large parallel equivalent resistance value Rae, the applied voltage Va becomes larger than the offset voltage Vos. It becomes difficult to be influenced by. The insulation resistance value is measured after applying a DC voltage and after a predetermined time has elapsed.

FIG. 2 is a diagram showing temperature characteristics of insulation resistance values (parallel equivalent resistance value Rae) of various capacitors measured by direct current. The vertical axis represents the insulation resistance value [MΩ], and the horizontal axis represents the temperature [° C.]. The insulation resistance value of a capacitor tends to decrease with increasing temperature.
When the dielectric constant ε is constant, the capacitance of the capacitor is proportional to the area S of the electrode plates and inversely proportional to the distance L between the electrode plates. On the other hand, the insulation resistance value of the capacitor is inversely proportional to the area S of the electrode plates and proportional to the distance L between the electrode plates. Therefore, the insulation resistance value of the capacitor tends to increase as the capacitance of the capacitor decreases.

Ceramic capacitors for temperature compensation, mica capacitors, and film capacitors have an insulation resistance value (parallel equivalent resistance value Rae) near 1 [TΩ] near room temperature (20 [° C.]), and have a large insulation resistance value. It is easy to select a capacitor. Considering selection conditions such as required specifications and availability, for example, a ceramic capacitor for temperature compensation, a mica capacitor, or a film capacitor having a large insulation resistance value (small capacitance) is selected as the capacitor C2.
In particular, film capacitors are highly available because they can be easily selected and satisfy the selection conditions. The temperature compensating ceramic capacitor and the mica capacitor are preferable because the slope of the decrease in the insulation resistance value accompanying the temperature rise is small as compared with other capacitors. The compensation current Ia has a small temperature variation and good temperature characteristics.

FIG. 3 is a flowchart showing a method for compensating for the influence of the bias current Ib performed by the manufacturer or the person in charge of inspection. The manufacturer or the like calculates the bias current Ib for the charge amplifier 101 (FIG. 1), selects the capacitor C2, and determines the applied voltage Va, thereby supplying a compensation current Ia that compensates for the influence of the bias current Ib. be able to.
First, the capacitor C2 between the reference power supply and the node n1 is disconnected, and the piezoelectric sensor Sn between the external terminal 12 and the external terminal 13 is disconnected (step S1). Next, in a state where the power supply voltage is supplied to the operational amplifier Ap, both electrodes of the capacitor C1 are short-circuited to discharge the charge of the capacitor C1, and after discharging, both electrodes of the capacitor C1 are opened (step S2).

When the elapsed time ΔT elapses immediately after opening, the output voltage of the output terminal (OUT) of the operational amplifier Ap increases or decreases by a voltage change amount ΔV from the virtual ground potential in proportion to the time. The manufacturer or the like performs waveform observation of the output voltage for obtaining measurement data of ΔT and ΔV (step S3 (waveform observation step)).
Since the output voltage is obtained by converting the bias charge into a voltage, the bias current Ib is calculated by Expression (2).
Ib = C1e × (ΔV / ΔT) (2)
However, C1e is the capacitance of the capacitor C1.
The manufacturer or the like substitutes the measured data of ΔT and ΔV into the equation (2) to calculate the current value of the bias current Ib (step S4 (calculation step)). The current value of the bias current Ib is easily calculated using a calculator. The manufacturer or the like causes a compensation current Ia having a current value equal to the calculated bias current Ib to flow.
Then, an available capacitor satisfying the required specifications as the high resistance element is selected as the capacitor C2 (step S5). Finally, the applied voltage Va is determined by the calculation of equation (1) (step S6), and the processing of the method is terminated. The manufacturer or the like applies the applied voltage Va from the reference power source to the capacitor C2 and causes the compensation current Ia to flow.
Therefore, for the charge amplifier 101 (FIG. 1), the compensation current Ia corresponds to the bias current Ib by flowing the determined compensation current Ia from the node n1 to the inverting input terminal (−) of the operational amplifier Ap. The influence of Ib can be compensated.
In addition, instead of applying the applied voltage Va from the reference power source to the capacitor C2, a circuit design or a printed circuit board layout may be made so that the compensation current Ia flows. In such a case, the effect of compensating for the influence of the bias current Ib is the same.

(Modification)
The present invention is not limited to the embodiments described above, and various modifications such as the following are possible.
The offset voltage Vos and the bias current Ib change due to factors such as manufacturing variations of the operational amplifier Ap and operating temperatures. Even when the applied voltage Va is adjusted to a voltage value satisfying the expression (1) at the time of design, it may change during use and the relationship of Ia = Ib may not be established. Even in such a case, if the relationship of Ia≈Ib is established, the influence of the bias current Ib on the output voltage is small, so that the measurement drift is reduced.
In Equation (1), the value of the maximum bias current Ibm can be set for Ia, and the value of the maximum offset voltage Vosm can be set for Vos.
The maximum offset voltage Vosm is the maximum value of the offset voltage, and the maximum bias current Ibm is the maximum value of the bias current Ib. For example, even when the offset voltage Vos changes greatly, it is below the maximum offset voltage Vosm, and even when the bias current Ib changes greatly, it is below the maximum bias current Ibm. For this reason, the voltage direction of the applied voltage Va and the current direction of the compensation current Ia do not change in the opposite directions.
Even if the current values of the compensation current Ia and the bias current Ib are not equal, it is only necessary to reduce the influence of the bias current Ib. In such a case, the compensation current Ia corresponds to the bias current Ib and the measurement drift is reduced to such an extent that the measurement of the physical quantity is not affected. Further, since it is not necessary to measure the bias current Ib and the offset voltage Vos in advance, the design and manufacture are facilitated.
Furthermore, an element made of an insulator can be used as the high resistance element. This is because it is sufficient to select one having a predetermined parallel equivalent resistance value Rae.

As the applied voltage Va, a negative DC voltage can be applied instead of a positive DC voltage. The compensation current Ia flows from the inverting input terminal (−) of the operational amplifier Ap toward the node n1.
In some cases, the bias current Ib flows from the inverting input terminal (−) of the operational amplifier Ap toward the node n1. In such a case, the compensation current Ia has the same direction and current value as the bias current Ib, and corresponds to the bias current Ib.
All of the bias current Ib flows into the reference power supply via the capacitor C2 as the compensation current Ia. Therefore, since the compensation current Ia acts to cancel the bias current Ib, the bias charge is not converted into a voltage. Since the charge amplifier 101 converts only the input charge as the output voltage, the measurement drift is reduced.

Regarding the operational amplifier Ap, the forward rotation input terminal (+) is set to a predetermined positive potential instead of the ground potential, and the negative power supply input terminal (V−) is connected to the ground instead of the negative DC voltage source. it can. The forward rotation input terminal (+) can be set to a predetermined negative potential instead of the ground potential, and the positive power supply input terminal (V +) can be connected to the ground instead of the positive DC voltage source.
The predetermined positive or negative potential is a potential divided by a series circuit of two resistors connected between a positive or negative DC power source and the ground, respectively. The applied voltage Va is a value obtained by adding a predetermined potential to the equation (1).
In such a case, the fluctuation charge, which is the fluctuation of the charge generated by the piezoelectric sensor Sn, is input to the capacitor C1, and the voltage across the capacitor C1 is output from the operational amplifier Ap. Further, since the single operation of the operational amplifier Ap is possible, the restriction on the power supply design of the charge amplifier 101 is eased.

(Comparative example)
FIG. 4 is a circuit diagram of the charge amplifier 901 of the comparative example. A case where the charge of the capacitor C1 is discharged using the MOS transistor Tr will be described. The charge amplifier 901 may require a circuit that resets the charge of the capacitor C1.
1 is different from FIG. 1 in that a MOS transistor Tr is provided. A diode Di is used. The resistor R1 sets the gate to a negative potential and turns off the MOS transistor Tr. When a positive signal voltage Vg is input to the gate of the MOS transistor Tr, the MOS transistor Tr is turned on. The capacitor C1 is short-circuited at both ends, and the charge is reset.
The MOS transistor Tr has an insulation resistance component between the gate and the drain / source. The leakage current Ir flows from the capacitor C1 through the MOS transistor Tr and the resistor R1 to a voltage source that supplies a negative DC voltage.

The diode Di is reverse-biased by the applied voltage Va which is a positive DC voltage. In the charge amplifier 901, the reverse bias current Id of the diode Di flows as the leakage current Ir. For this reason, a voltage obtained by converting a charge obtained by integrating the leakage current Ir with time (hereinafter referred to as a leakage charge) is not generated.
The charge amplifier 101 converts only the input charge as an output voltage. Leakage charge is not converted to voltage and input charge has no effect on the converted voltage, so no measurement drift occurs.
However, there are many restrictions on circuit design, and it is difficult to select the diode Di that satisfies Id = Ir and to adjust the applied voltage Va.

In this regard, according to the charge amplifier 101 of this embodiment, there are few restrictions on circuit design. Based on the equation (1), it is easy to select the capacitor C2 that satisfies Ia≈Ir and to adjust the applied voltage Va.
It is also easy to measure the sum of the leakage current Ir and the bias current Ib in advance and reduce the measurement drift due to both currents.

1 is a circuit diagram of a charge amplifier device according to a first embodiment. FIG. It is a figure which shows the temperature characteristic of the insulation resistance value of various capacitors measured by direct current | flow. It is a flowchart which shows the method of compensating the influence of the bias current which a manufacturer etc. perform. It is a circuit diagram of the charge amplifier apparatus of a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Internal terminal 12, 13 External terminal 101,901 Charge amplifier C1, C2 Capacitor C1e Capacitance Ca Parallel equivalent capacitor Ra Parallel equivalent resistor Rae Parallel equivalent resistance value Ap Operational amplifier Sn Piezoelectric sensor (detector)
Di diode Tr MOS transistor Va applied voltage Vg signal voltage Vos offset voltage Vosm maximum offset voltage Ib bias current Ibm maximum bias current Ia compensation current Ir leakage current Id reverse bias current

Claims (11)

An operational amplifier for amplifying a potential difference between an inverting input terminal and a normal input terminal, and a capacitor connected between the output terminal of the operational amplifier and the inverting input terminal, and an input input to the capacitor A charge amplifier that outputs to the output terminal a voltage proportional to the charge,
A high resistance element is connected to a connection point between the inverting input terminal and the capacitor,
The charge amplifier according to claim 1, wherein a current corresponding to a leakage current at a connection point between the inverting input terminal and the capacitor flows through the high resistance element.   The charge amplifier according to claim 1, wherein the high resistance element is a parallel equivalent resistance of a capacitor.   The charge amplifier according to claim 2, wherein the capacitor is a film capacitor.   3. The charge amplifier according to claim 2, wherein the capacitor is a temperature compensating ceramic capacitor or a mica capacitor.   The charge amplifier according to claim 3, wherein the operational amplifier has an offset voltage. The leakage current is a bias current of the operational amplifier,
2. The charge amplifier according to claim 1, wherein a reference power source that supplies a current substantially equal to the bias current is connected to the high resistance element.   The voltage applied to the high resistance element is a value obtained by adding a maximum offset voltage of the operational amplifier to a value obtained by multiplying the resistance value of the high resistance element by the bias current. Charge amplifier. A charge amplifier according to any one of claims 1 to 7,
A charge amplifier device comprising: a detector connected to the inverting input terminal and generating a charge proportional to a physical quantity.   The charge amplifier device according to claim 8, wherein the detector is connected between the inverting input terminal and the normal rotation input terminal. The detector is connected between the inverting input terminal and a ground terminal;
9. The charge amplifier device according to claim 8, wherein the normal input terminal is set to a predetermined potential. An operational amplifier for amplifying a potential difference between an inverting input terminal and a normal input terminal, and a capacitor connected between the output terminal of the operational amplifier and the inverting input terminal, and an input input to the capacitor A bias current compensation method for compensating for the influence of a bias current of the operational amplifier using a charge amplifier that outputs an output voltage proportional to an electric charge to the output terminal,
The capacitor is short-circuited so that the output voltage becomes a virtual potential that is substantially the same potential as the normal input terminal, and then the elapsed time immediately after the capacitor is opened, and the voltage change amount of the output voltage from the virtual potential, A waveform observation step for observing the waveform of the output voltage
A calculation step of calculating the bias current by dividing the elapsed time by a value obtained by multiplying the capacitance of the capacitor by the voltage change amount;
A bias current compensation method, wherein a compensation current substantially equal to the value of the bias current calculated in the calculation step is passed through a connection point between the inverting input terminal and the capacitor.

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