JP2007121125A - Current detecting device and capacitance measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a current detector which accurately and easily detects the current value flowing in a measured object even when the frequency of a measuring signal supplied to the measured object is varied. <P>SOLUTION: The current detector comprises: an oscillator 21 for adjusting the frequency of an alternating current signal S1 for measurement; and an amplification circuit 22 for current-voltage converting the current I1 flowing in the measured object DUT in applying the alternating current signal S1 for measurement. In the amplification circuit 22, a feedback impedance for determining conversion gain of current-voltage conversion is constituted by capacitors 22b-22e. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a current detection device that detects a current flowing through a measurement object and a capacitance measurement device that measures the capacitance of the measurement object.

As this type of current detection device, the applicant discloses a current-voltage conversion circuit applied to an LCR meter in Japanese Utility Model Laid-Open No. 5-40888. This current-voltage conversion circuit includes a measurement AC signal generating means for generating a measurement AC signal having a predetermined frequency, and an operational amplifier having a plurality of switchable current detection resistors. In this operational amplifier, the first current detection resistor selected corresponding to the resistance measurement range of the LCR meter, the second current detection resistor selected corresponding to the inductance measurement range, and the capacitance measurement range A corresponding third current detection resistor is provided, and the output voltage of the operational amplifier at the full scale of each measurement range is configured to be substantially constant. In this case, a measurement sample is connected between the input terminal of the operational amplifier and the measurement AC signal generating means, and a measurement AC signal having a predetermined frequency is supplied to the measurement sample, thereby corresponding to the current flowing through the measurement sample. The output voltage is output to the output side of the operational amplifier.
Japanese Utility Model Laid-Open No. 5-40888 (page 4, FIG. 1)

  However, the current-voltage conversion circuit has the following problems to be solved. That is, in this current-voltage conversion circuit, a current detection resistor is connected to the operational amplifier. For this reason, for example, in order to inspect how the capacitance of the measurement sample changes by changing the frequency of the measurement AC signal (frequency dependence of the capacitance), the frequency of the measurement AC signal When measuring the current flowing through the measurement sample while changing the impedance, the impedance of the measurement sample changes depending on the frequency of the AC signal for measurement, whereas the impedance of the current detection resistor is constant regardless of the change in frequency. The conversion gain of the current-voltage conversion of the operational amplifier changes according to the frequency of the AC signal for measurement. Therefore, even if the frequency of the AC signal for measurement is changed, in order to accurately measure the current while maintaining the conversion gain constant, the current detection resistor must be switched, resulting in complicated current detection work. There is a problem to be improved.

  The present invention has been made in view of such problems to be improved, and the value of the current flowing through the measurement object can be accurately and easily even when the frequency of the measurement AC signal supplied to the measurement object is changed. It is a main object of the present invention to provide a current detection device that can detect the capacitance and a capacitance measurement device that can accurately and easily measure the capacitance of a measurement object.

  In order to achieve the above object, the current detection device according to claim 1 is an AC signal supply unit capable of changing a frequency of a measurement AC signal, and a current flowing through the measurement object when the measurement AC signal is applied. An amplifier circuit for voltage conversion, and the amplifier circuit includes a capacitor having a feedback impedance for determining a conversion gain of the current-voltage conversion.

  The current detection device according to claim 2 is the current detection device according to claim 1, wherein the amplifier circuit includes a plurality of capacitors, and the plurality of capacitors have different electrostatic capacities from each other. Two or more are provided as the feedback impedance so that they can be switched and connected.

  According to a third aspect of the present invention, there is provided a capacitance measuring apparatus comprising: the current detecting apparatus according to the first or second aspect; and a voltage detecting unit that detects a voltage between both ends of the measurement object, The capacitance of the measurement object is measured based on the frequency, the waveform of the voltage subjected to current-voltage conversion by the amplifier circuit of the current detection device, and the waveform of the voltage between both ends detected by the voltage detection unit. To do.

  According to the current detection device of claim 1, the AC signal supply unit configured to be able to vary the frequency of the measurement AC signal supplied to the measurement object, and the feedback impedance for determining the conversion gain of the current-voltage conversion are provided. When the frequency of the measurement AC signal is changed, for example, the impedance of the capacitor as the measurement object and the capacitor that forms the feedback impedance of the amplifier are configured by including the amplifier circuit configured by the capacitor. The impedance can be changed equally. Therefore, unlike the conventional current-voltage conversion circuit in which the feedback impedance is formed by a resistor, the conversion gain of the amplifier circuit can be kept constant even if the frequency of the AC signal for measurement is changed. For example, it is possible to avoid a situation where the voltage value of the output signal indicating the current value overflows the input range of the A / D converter connected to the subsequent stage, for example. For this reason, the operation of switching the capacitor for current detection becomes unnecessary, and as a result, the current value for each frequency of the current flowing through the measurement object can be reliably and easily detected.

  According to the current detection device according to claim 2, a plurality of capacitors are disposed so that capacitances thereof are different from each other and any one or more of them can be switched and connected as a feedback impedance of the amplifier. It is possible to switch to a capacitor having an appropriate capacitance according to the capacitance of the capacitor. Therefore, even when the capacitance of the measurement object is in a wide range, the current value of the current flowing through the capacitance can be reliably detected by switching the capacitor.

  According to the capacitance measuring device according to claim 3, the current detecting device according to claim 1 or 2 and a voltage detection unit that detects a voltage value of a voltage across the capacitor, for example, as a measurement object, The capacitance of the measurement object is measured based on the frequency of the AC signal for measurement, the waveform of the voltage that has been converted from current to voltage by the amplifier circuit of the current detection device, and the waveform of the voltage across the terminal that is detected by the voltage detection unit. As a result, the current value for each frequency of the current flowing through the measurement object can be reliably and easily detected, and as a result, the capacitance of the measurement object for each frequency can be measured accurately and easily.

  Hereinafter, the best mode of a current detection device and a capacitance measurement device according to the present invention will be described with reference to the accompanying drawings.

  First, the configuration of the capacitance measuring device 1 will be described with reference to the drawings.

  As shown in FIG. 1, the capacitance measuring device 1 includes a current detection circuit 2, a voltage detection unit 3, two A / D converters 4, 5, a control unit 6, a storage unit 7, an operation unit 8, and a display unit 9. For example, a capacitance Cd of a capacitor as the measurement object DUT can be measured. The current detection circuit 2 constitutes a current detection device according to the present invention, and includes an oscillator 21 and an amplification circuit 22 as an AC signal supply unit according to the present invention, and a measurement object DUT is connected between the oscillator 21 and the amplification circuit 22. It is configured to be possible. In this case, the current detection circuit 2 detects the current value of the current I1 flowing through the connected measurement object DUT. The oscillator 21 is a constant voltage source and is configured to be able to vary the frequency f of the measurement AC signal S1 supplied to the measurement object DUT within a predetermined range (for example, 1 kHz to 1 MHz). On the other hand, the amplifier circuit 22 includes an operational amplifier 22a, four capacitors 22b to 22e, four analog switches 22f to 22i, a resistor 22j, and a buffer 22k. In addition, the amplifier circuit 22 performs current-voltage (I / V) conversion on the current I1 flowing through the measurement object DUT, generates an output signal S2 whose voltage value is proportional to the current value, and supplies the output signal S2 to the A / D converter 5. Output.

  In the operational amplifier 22a, the measurement object DUT is connected to the inverting input terminal (the input portion side of the amplifier circuit in the present invention). Any one of the capacitors 22b to 22e is disposed so as to be switchable and connectable as a feedback impedance for determining a conversion gain of current-voltage conversion by the amplifier circuit 22 in the feedback path of the operational amplifier 22a. Specifically, each of the capacitors 22b to 22e is controlled to be turned on by the control unit 6 in any one of the analog switches 22f to 22i connected in series with each other, whereby the capacitance Cd of the measurement object DUT. It is switched according to. In this case, any of the capacitors 22b to 22e is connected between the inverting input terminal and the output terminal of the operational amplifier 22a to form the feedback impedance of the operational amplifier 22a. Further, the capacitors 22b to 22e have different capacitances. For example, the capacitor 22b is connected to the subsequent stage when a measurement object DUT having a capacitance Cd of 10 pF or more and less than 100 pF is connected. The capacitance C is set to 100 pF, for example, so that the output signal S2 has an appropriate voltage value with respect to the input range of the A / D converter 5. Similarly, the capacitance C of the capacitor 22c is set to, for example, 1 nF for a measurement object DUT of 100 pF or more and less than 1 nF, and the capacitor 22d is used for a measurement object DUT of 1 nF or more and less than 10 nF. The capacitance C is set to, for example, 10 nF, and the capacitor 22e is set to have a capacitance C, for example, 100 nF, for a measurement object DUT that is about 10 nF to less than 100 nF.

  Any one of the analog switches 22f to 22i is selectively turned on according to the control signal S3 output from the control unit 6 to switch the capacitors 22b to 22e forming the feedback impedance of the operational amplifier 22a. The resistor 22j is connected between the inverting input terminal and the output terminal of the operational amplifier 22a in order to feed back the DC component and stably operate the current detection circuit 2, and has a sufficiently high impedance with respect to the capacitor 22b. The feedback impedance of the operational amplifier 22a is formed together with the electric elements 22b to 22e and 22f to 22i. The buffer 22k is connected to the output side of the operational amplifier 22a in order to use the operational amplifier 22a as an ideal operational amplifier, and buffers and amplifies the signal output from the operational amplifier 22a with a predetermined amplification factor.

  The voltage detector 3 detects the voltage value of the voltage across the measurement object DUT and outputs an output signal S4 indicating the voltage value to the A / D converter 4. The A / D converter 4 outputs a digital signal S5 generated by analog-digital (A / D) conversion of the output signal S4 output from the voltage detection unit 3 to the control unit 6. On the other hand, the A / D converter 5 outputs a digital signal S6 generated by A / D converting the output signal S2 output from the amplifier circuit 22 to the control unit 6.

  The control unit 6 includes the frequency f of the measurement AC signal S1, the waveform of the voltage specified by the digital signal S5 (the waveform of the voltage between both ends in the present invention), and the waveform of the current specified by the digital signal S6 (in the present invention). The capacitance Cd of the measurement object DUT is calculated based on the current-voltage converted voltage waveform. Further, the control unit 6 calculates the capacitance Cd of the measurement object DUT for each frequency f, and causes the storage unit 7 to store the measurement data Dc indicating the capacitance Cd for each frequency f. The storage unit 7 stores the digital signals S5 and S6 and the measurement data Dc for each frequency f under the control of the control unit 6. The operation unit 8 outputs an instruction signal S7 corresponding to the operation content operated by the user to the control unit 6. The display unit 9 displays the measurement data Dc of the frequency f as shown in FIG. 2 under the control of the control unit 6.

  Next, a method for measuring the capacitance Cd of the measurement object DUT using the capacitance measuring device 1 will be described with reference to the drawings.

  First, the measurement object DUT is connected between the oscillator 21 and the amplifier circuit 22. Next, for example, when it is known in advance that the capacitance Cd of the measurement object DUT is in a range of 100 pF or more and less than 1 nF, the operation unit 8 is operated to output the instruction signal S7 to the control unit 6. Then, the control signal S3 is output to the amplifier circuit 22. At this time, in the amplifier circuit 22, the analog switch 22g is controlled to be in an ON state, and the feedback impedance of the operational amplifier 22a is formed by a parallel circuit of the 1nF capacitor 22c and the resistor 22j. In this case, the amplification factor (conversion gain of current-voltage conversion) of the amplifier circuit constituted by the measurement object DUT and the amplifier circuit 22 is the impedance of the measurement object DUT and the impedance (more precisely, reactance) of the capacitor 22c. It is defined by the ratio of

  Next, the control unit 6 outputs a control signal S8, and causes the oscillator 21 to change the frequency f of the measurement AC signal S1 within a range of, for example, 1 kHz to 1 MHz. At this time, the control unit 6 repeatedly executes the calculation process of the capacitance Cd of the measurement object DUT at each frequency f while continuously changing the frequency f of the AC signal for measurement S1. Specifically, the calculation process of the capacitance Cd when the measurement AC signal S1 having a predetermined frequency (10 kHz as an example) f is supplied to the measurement object DUT will be described. In this process, first, the oscillator 21 supplies the measurement AC signal S1 having the frequency f to the measurement object DUT. At this time, the amplifier circuit 22 performs I / V conversion on the current I1 flowing through the measurement object DUT, and outputs an output signal S2 indicating the current value to the A / D converter 5. Next, the A / D converter 5 outputs a digital signal S6 generated by A / D converting the output signal S2 to the control unit 6. Subsequently, the control unit 6 causes the storage unit 7 to store at least the digital signal S6 for one cycle of the measurement AC signal S1. On the other hand, the voltage detection unit 3 outputs an output signal S4 indicating the voltage value of the voltage across the measurement object DUT to the A / D converter 4. At this time, the A / D converter 4 outputs a digital signal S5 generated by A / D converting the output signal S4 to the control unit 6. Subsequently, the control unit 6 causes the storage unit 7 to store at least one cycle of the digital signal S5 of the measurement AC signal S1.

  Thereafter, the control unit 6 is specified by the frequency f of the AC signal for measurement S1, the waveform of the voltage specified by the digital signal S5 (that is, the voltage waveform of the voltage across the measurement object DUT), and the digital signal S6. Based on the current waveform (that is, the current waveform of the current I1 flowing through the measurement object DUT), the capacitance Cd of the measurement object DUT at the frequency f is calculated. Specifically, the control unit 6 includes the voltage waveform specified by the digital signal S5 for one cycle of the measurement AC signal S1 stored in the storage unit 7 and the measurement AC signal S1 stored in the storage unit 7. The phase difference between the current waveform identified by the digital signal S6 for one cycle and the effective value of the voltage calculated from the voltage waveform for one cycle and the current waveform for one cycle were calculated. The impedance of the measurement object DUT is calculated based on the effective value of the current. Next, the control unit 6 calculates the capacitance Cd of the measurement object DUT based on the calculated phase difference, the calculated impedance, and the frequency f of the measurement AC signal S1. Subsequently, the control unit 6 calculates the capacitance Cd of the measurement object DUT for each frequency f while continuously increasing the frequency f of the AC signal for measurement S1, and the capacitance C for each frequency f. Is stored in the storage unit 7. Note that when the frequency f of the measurement AC signal S1 is increased, the impedance of the measurement object DUT decreases with a change in the frequency f, but the impedance of the capacitor 22c also decreases equally. Therefore, even if the frequency f of the measurement AC signal S1 is changed, the change in the conversion gain of the current-voltage conversion of the amplifier circuit constituted by the measurement object DUT and the amplifier circuit 22 is sufficiently suppressed, and the conversion is performed. The gain is almost constant. For this reason, the situation where the voltage value of the output signal S2 overflows the input range of the A / D converter 5 during the process of calculating the capacitance Cd is avoided. Subsequently, after the calculation of the capacitance Cd for the frequency f in the above-described range is completed, the control unit 6 inspects the frequency dependence of the capacitance Cd of the measurement object DUT as shown in FIG. Therefore, the value of the capacitance Cd for each frequency f used for the purpose is displayed on the display unit 9.

  Thus, according to the current detection circuit 2, the oscillator 21 configured to be able to vary the frequency f of the measurement AC signal S1 supplied to, for example, the capacitor as the measurement object DUT, and the conversion gain of the current-voltage conversion Since the determination feedback impedance includes the amplification circuit 22 including the capacitors 22b to 22e, when the frequency f of the measurement AC signal S1 is changed, the impedance of the measurement object DUT is calculated. The impedance of any of the capacitors 22b to 22e forming the feedback impedance of the amplifier 22a can be changed equally. Therefore, unlike the conventional current-voltage conversion circuit in which the feedback impedance is formed by a resistor, the conversion gain of the amplifier circuit 22 can be kept constant even if the frequency f of the measurement AC signal S1 is changed. A situation in which the voltage value of the output signal S2 overflows the input range of the A / D converter 5 as the frequency f changes can be avoided. For this reason, according to the current detection circuit 2, the switching operation of the current detection capacitors 22b to 22e becomes unnecessary, and as a result, the current value for each frequency f of the current I1 flowing through the measurement object DUT is reliably and easily detected. can do. Therefore, according to the capacitance measuring apparatus 1, the current detection circuit 2 and the voltage detection unit 3 that detects the voltage value of the voltage across the measurement object DUT are provided, and the frequency f of the AC signal S1 for measurement, The frequency of the current I1 flowing through the measurement object DUT is calculated by calculating the capacitance Cd of the measurement object DUT based on the voltage waveform specified by the digital signal S5 and the current waveform specified by the digital signal S6. As a result of reliably and easily detecting the current value for each f, the capacitance Cd of the measurement object DUT for each frequency f can be accurately and easily measured.

  Further, according to the capacitance measuring apparatus 1, the capacitors 22b to 22e are arranged so that the capacitances C are different from each other and any one of them can be switched and connected as the feedback impedance of the operational amplifier 22a. The DUT can be switched to capacitors 22b to 22e having appropriate capacitance C according to, for example, the capacitance Cd of the capacitor. Therefore, even when the capacitance Cd of the measurement object DUT covers a wide range, the capacitance Cd can be reliably measured by switching the capacitors 22b to 22e.

  In addition, this invention is not limited to said structure. For example, in the above configuration, the configuration in which the feedback impedance of the operational amplifier 22a is formed by any one of the four capacitors 22b to 22e has been described, but the configuration is not limited thereto. For example, the feedback impedance of the operational amplifier 22a may be formed with a single capacitor. Further, a plurality of (two, three, five or more) capacitors other than four may be switchably connected, and the feedback impedance of the operational amplifier 22a may be formed by any of these capacitors. Furthermore, in a configuration in which a plurality of capacitors are connected in a switchable manner, it is needless to say that two or more capacitors may be switched and connected in parallel.

1 is a configuration diagram of a capacitance measuring device 1. FIG. It is a display screen figure which shows the frequency characteristic of the electrostatic capacitance of the measurement object DUT.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Capacitance measuring apparatus 2 Current detection circuit 3 Voltage detection part 4,5 A / D converter 6 Control part 7 Memory | storage part 8 Operation part 9 Display part 21 Oscillator 22 Amplification circuit 22a Operational amplifier 22b-22e Capacitor 22f-22i Analog switch C, Cd Capacitance DUT Measurement object I1 Current f Frequency S1 AC signal for measurement

Claims (3)

An AC signal supply unit capable of changing the frequency of the AC signal for measurement, and an amplifier circuit for current-voltage conversion of the current flowing through the measurement object when the AC signal for measurement is applied,
The amplifying circuit is a current detecting device in which a feedback impedance for determining a conversion gain of the current-voltage conversion is configured by a capacitor.   2. The current according to claim 1, wherein the amplifier circuit includes a plurality of the capacitors, and the plurality of capacitors have different electrostatic capacitances, and any one or more of the capacitors are arranged to be switched and connected as the feedback impedance. Detection device. The current detection device according to claim 1 or 2, and a voltage detection unit that detects a voltage between both ends of the measurement object,
The measurement object based on the frequency of the AC signal for measurement, the waveform of the voltage converted from current to voltage by the amplifier circuit of the current detection device, and the waveform of the voltage across the terminal detected by the voltage detection unit. Capacitance measurement device that measures the capacitance of

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