JP5030216B2 - Current-voltage conversion circuit - Google Patents

Description

  The present invention relates to a current-voltage conversion circuit used for impedance measurement of an object to be measured, for example, at a remote location, and more particularly to a current-voltage conversion circuit that converts an input current into a voltage by an operational amplifier and outputs the voltage.

FIG. 12 shows an impedance measuring device using a conventional current-voltage conversion circuit, which is a circuit equivalent to those described in Patent Document 1 and Patent Document 2, for example.
In FIG. 12, V _S is a signal source, R _O is an output resistance, DUT is an object to be measured having impedance X, 10 is a current-voltage conversion circuit, A1 is an operational amplifier, R _F is a feedback resistor, B1 is an amplifier, 21 and 22 Is an A / D conversion circuit, 30 is a CPU, and 40 is a display device.
Reference numerals 201 to 204 denote connection cables such as coaxial cables, and C _HC , C _LC , C _LP , and C _HP denote these electrostatic capacities.

In this prior art, an AC voltage is applied from a signal source V _S to the measuring object DUT, as well as A / D conversion to convert the current flowing at that time by the current-voltage conversion circuit 10 into a voltage, both ends of the measurement object DUT The voltage is amplified by the instrumentation amplifier B1 and A / D converted, and the CPU 30 calculates the impedance of the measurement object DUT using the outputs of the A / D conversion circuits 21 and 22.
13 equivalently shows the main part of FIG. 12. In FIG. 13, C _IN is an input of the current-voltage conversion circuit 10 expressed as a combined capacity of the capacitances C _LC and C _LP. Capacity.

Now, in the JIS standard, when measuring a capacitor having an electrostatic capacity of 1 [nF] or less, it is specified that the frequency of the signal source is 1 [MHz]. When measuring a capacitor having a capacitance lower than [pF], the impedance of the measurement object DUT at 1 [MHz] is 160 [kΩ].
In this case, since the current flowing into the inverting input terminal of the operational amplifier A1 is reduced, detection signals necessary to increase considerably the feedback resistor R _F is buried in noise. However, increasing the feedback resistor R _F, the feedback resistor R _F and is divided by the series circuit of the input capacitance C _IN decreases the voltage applied to the inverting input terminal of the operational amplifier A1, sufficient detection accuracy error is large Can't get.

Also, when the input capacitance C _IN is large, the feedback voltage applied to the inverting input terminal from the dividing point between the input capacitance C _IN and the feedback resistor R _F is, the phase is rotated relative to the output voltage of the operational amplifier A1. Generally, the output voltage of the operational amplifier A1 is several [kHz] or more and the phase is delayed by 90 ° with respect to the input voltage. Therefore, when the delay due to the feedback voltage is added to this delay, the phase delay reaches 180 °. Oscillation (positive feedback) is reached, and the circuit operation becomes unstable.

To compensate for the unstable operation of the circuit due to the input capacitance C _IN as described above, be connected to the phase compensation capacitor is conventional in parallel with the feedback resistor R _F. For example, Figure 14 is a comparable prior art to the circuit described in Non-Patent Document 1, 11 is a current-voltage conversion circuit using a voltage feedback operational amplifier A1, C _C is connected in parallel to the feedback resistor R _F Phase compensation capacitor.
By connecting in this manner by adding a phase compensation capacitor C _C in the input capacitance C _IN and the series in the feedback circuit, enabling the stable operation of the circuit with sufficient phase margin.

The circuit shown in FIG. 14 can be applied to a current-voltage conversion circuit using a current feedback type as the operational amplifier A1, but the impedance of the inverting input terminal is very small in the current feedback type operational amplifier. When used in the inverting operation, it is hardly affected by the input capacitance _CIN .

JP 2004-294269 A (paragraphs [0018] to [0020], FIG. 2 etc.) Japanese Patent No. 2960095 (page 2, left column, line 3 to right column, line 25, FIG. 3 etc.) “History of OP Amplifier and Basic Knowledge of Circuit Technology (OP Amplifier Complete Volume 1)”, Chapter 6, 6-4 Influence of High-Speed Current-Voltage Converter and Input Capacitance of Inverting Input Terminal, p.245-p .249, CQ Publisher, issued December 1, 2003

Here, for example, when the feedback resistor R _F = 100 [kΩ], the input capacitance C _IN = 500 [pF], and the operational amplifier A1's GBP (gain bandwidth product) frequency f _GBP = 100 [MHz] in FIG. According to Patent Document 1, the capacitance of the phase compensation capacitor C _C is an operational amplifier A1 is as long as current feedback 2.8 [pF], a 2.4 [pF] when the voltage feedback. In this case, each corner frequency by the feedback resistor _{R F} and the phase compensation capacitor _{C C} is 570 [kHz], there is a problem that 670 [kHz], and the frequency band in any event becomes narrower.
The capacity of the phase compensation capacitor C _C has a large value in accordance with the input capacitance C _IN.

Further, as a result of the divided voltage generated by the phase compensation capacitor C _C and the input capacitance C _IN being input to the inverting input terminal of the operational amplifier A1, when C _IN = 0 [pF] (when there is no connection cable), C _IN = 500 and in the case of [pF] (state where there is connection cable), as shown in FIG. 15, the transimpedance of the current-voltage conversion circuit when the measurement frequency according to the signal source V _S 1 [MHz] are different by about 30% . That is, there is also a problem that the impedance measurement value depends on the length of the connection cable.

Furthermore, when using a current feedback operational amplifier A1, for example 1, but to compensate for the input capacitance C _IN of [nF], it is necessary to use a phase compensation capacitor C _C number [pF], the connection cable is shortened When the input capacitance _CIN becomes small, there is a specific problem that the feedback current increases in the high frequency region of the measurement frequency.
Further, when the measurement target DUT is a capacitor, since the series circuit of this capacitor and a feedback resistor R _F constitute a differentiation circuit, it is necessary to a certain extent increase the phase compensation capacitor C _C in order to limit the band Even in this case, the feedback current increases in the high frequency region. When the feedback current increases in this way, the circuit operation becomes unstable.
In addition, the transimpedance of a current feedback operational amplifier is generally about 100 [kΩ] at 1 [MHz], but when the feedback resistance is 100 [kΩ], only a loop gain of 1 can be obtained, and the accuracy is quite high. There was no problem.

  Note that there are known techniques described in Japanese Patent Application Laid-Open Nos. 2004-317345, 2004-317391, and the like as impedance measuring devices intended to reduce the influence of the input capacity of the connection cable in a high frequency region. However, both require several [ms] to stabilize the feedback system, and are not suitable for high-speed measurement.

  SUMMARY OF THE INVENTION Accordingly, the problem to be solved by the present invention is to provide a current-voltage conversion circuit that stabilizes the operation, eliminates the influence of the input capacitance due to the connection cable, and enables expansion of the frequency band and reduction of detection error. is there.

In order to solve the above problem, the invention according to claim 1 is provided with at least a feedback resistor and a phase compensation capacitor in a feedback circuit between the output terminal and the inverting input terminal, and a reference voltage is applied to the non-inverting input terminal. In the current-voltage conversion circuit that converts the input current into voltage using the operational amplifier
The operational amplifier is a current feedback operational amplifier,
The feedback circuit includes a compensation impedance connected between a circuit input terminal into which the input current flows and the inverting input terminal, and the feedback resistor connected between the circuit input terminal and the output terminal of the operational amplifier. And a parallel circuit of a phase compensation capacitor,
As the compensation impedance, an element having a low impedance value in a low frequency region and a high impedance value in a high frequency region, and an input / output phase difference is substantially zero in a frequency region where the loop gain of the operational amplifier is small. Is used.

The invention according to claim 2 uses an operational amplifier including at least a feedback resistor and a phase compensation capacitor in a feedback circuit between an output terminal and an inverting input terminal, and a reference voltage is applied to a non-inverting input terminal. In the current-voltage conversion circuit that converts the input current into voltage,
The operational amplifier is a current feedback operational amplifier,
The feedback circuit includes a compensation impedance connected between a circuit input terminal into which the input current flows and the inverting input terminal, and the phase compensation connected between the circuit input terminal and the output terminal of the operational amplifier. A capacitor, the feedback resistor connected between the circuit input terminal and the circuit output terminal, and an amplifier connected between the output terminal of the operational amplifier and the circuit output terminal,
As the compensation impedance, an element having a low impedance value in a low frequency region and a high impedance value in a high frequency region, and an input / output phase difference is substantially zero in a frequency region where the loop gain of the operational amplifier is small. Is used.

The invention according to claim 3 uses an operational amplifier including at least a feedback resistor and a phase compensation capacitor in a feedback circuit between an output terminal and an inverting input terminal, and a reference voltage is applied to a non-inverting input terminal. In the current-voltage conversion circuit that converts the input current into voltage,
The operational amplifier is a voltage feedback operational amplifier,
The feedback circuit is
The phase compensation capacitor connected between the circuit input terminal into which the input current flows and the output terminal of the operational amplifier; the feedback resistor connected between the circuit input terminal and the circuit output terminal; Rutotomoni and a amplifier connected between the output terminal and the circuit output terminal,
A DC servo circuit is connected between the circuit input terminal and the inverting input terminal and non-inverting input terminal of the operational amplifier,
The DC servo circuit is
A DC cut capacitor connected between the circuit input terminal and the inverting input terminal;
A resistor connected to the inverting input terminal to pass a bias current of the operational amplifier;
An integrating circuit connected between the circuit input terminal and the non-inverting input terminal and providing the output to the non-inverting input terminal as a bias voltage;
Voltage feedback between the circuit input terminal and the output terminal of the operational amplifier to feed back the output voltage of the operational amplifier to the circuit input terminal side via a diode and maintain the voltage of the circuit input terminal at approximately zero. A circuit is connected .

The invention according to claim 4 is the invention according to claim 1,
A DC servo circuit is connected between the circuit input terminal, the input side of the compensation impedance, and the non-inverting input terminal of the operational amplifier,
The DC servo circuit is
A capacitor for DC cut connected between the circuit input terminal and the input side of the compensation impedance; a resistor connected to the input side of the compensation impedance to pass a bias current of the operational amplifier; and the circuit input And an integrating circuit that is connected between the terminal and the non-inverting input terminal of the operational amplifier and supplies the output to the non-inverting input terminal as a bias voltage.

The invention according to claim 5 is the invention according to claim 2,
A DC servo circuit is connected between the circuit input terminal, the input side of the compensation impedance, and the non-inverting input terminal of the operational amplifier,
The DC servo circuit is
A capacitor for DC cut connected between the circuit input terminal and the input side of the compensation impedance; a resistor connected to the input side of the compensation impedance to pass a bias current of the operational amplifier; and the circuit input And an integrating circuit that is connected between the terminal and the non-inverting input terminal of the operational amplifier and supplies the output to the non-inverting input terminal as a bias voltage.

The invention according to claim 6 is the current-voltage conversion circuit according to claim 5 ,
Voltage feedback between the circuit input terminal and the output terminal of the operational amplifier to feed back the output voltage of the operational amplifier to the circuit input terminal side via a diode and maintain the voltage of the circuit input terminal at approximately zero. A circuit is connected .

Invention Oite the current-voltage conversion circuit according to any one of claims 3-6 according to claim 7,
Compared to the bias current of the operational amplifier, the bias current of the operational amplifier constituting the integration circuit of the DC servo circuit you wherein a sufficiently small.

According to an eighth aspect of the present invention, the current-voltage conversion circuit according to any one of the first to seventh aspects measures the impedance of a remote measurement object connected to the circuit input terminal via a coaxial cable. It is characterized by being configured as an impedance measuring device .

  According to the first aspect of the present invention, by inserting a compensation impedance having a predetermined characteristic into the inverting input terminal of the current feedback operational amplifier, a sufficient phase margin can be secured to enable stable operation of the circuit. Thus, it is possible to realize a current-voltage conversion circuit in which the measurement value is not affected by the length of the connection cable.

  According to the second aspect of the present invention, in addition to the effect of the first aspect of the invention, an equivalent transformer of the current feedback type operational amplifier is inserted by inserting an amplifier having a predetermined gain on the output side of the current feedback type operational amplifier. Impedance can be increased, and detection errors can be reduced even when the feedback resistance is increased.

  According to the third aspect of the present invention, by inserting an amplifier having a predetermined gain on the output side of the voltage feedback operational amplifier, the frequency band can be greatly expanded and detection errors can be reduced.

Further, according to the invention according to claim 3-7, electrodeposition by the input side of the pressure-feedback or current feedback op amp that is connected to a DC servo circuit, excessive DC offset voltage due to the bias current of the operational amplifier Can be prevented from being output.

According to the invention according to claim 3 or 6, by connecting a voltage feedback circuit to the output side of the voltage feedback or current feedback op amp to maintain an input voltage of the DC servo circuit substantially zero, excessive It is possible to keep the reference voltage of the operational amplifier at a predetermined level by preventing abnormal operation of the DC servo circuit due to the input voltage.

In addition, as described in claim 8 , the present invention is preferably used as an impedance measuring device that measures the impedance of a measurement object in a remote place.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram when the first embodiment of the present invention according to claim 1 is applied to an impedance measuring apparatus. In FIG. 1, the configuration other than the current-voltage conversion circuit 101 is the same as that in FIG. 12, and therefore, the same reference numerals are assigned and description thereof is omitted.

That is, in the current-voltage conversion circuit 101, A1 _I is a current feedback operational amplifier, and the feedback circuit between the output terminal and the inverting input terminal includes a compensation impedance Z, a feedback resistor _RF, and a phase compensation capacitor _CC. And a parallel circuit are connected in series. The non-inverting input terminal of the operational amplifier A1 _I, are given reference voltage of ground. The connection point or circuit input terminal 202a of a parallel circuit of a feedback resistor R _F and the phase compensation capacitor C _C and the compensating impedance Z via connection cable 202 such as a coaxial cable having a capacitance C _LC, e.g. It is connected to one end of the measurement object DUT at a remote location.
The output terminal of the operational amplifier A1 _I through the circuit output terminal 202b, and is likewise connected to the A / D conversion circuit 21 and FIG. 12.

Here, as will be described later, the compensation impedance Z becomes low impedance in the low frequency region and high impedance in the high frequency region in order to reduce the influence of the length of the connection cable 202, and the stability of the circuit is improved. purposes, constituted by an element such as a value close to ° phase difference of the input and output is 0 in the frequency domain where the loop gain is reduced of the operational amplifier A1 _I.

The impedance measurement operation according to the first embodiment is basically the same as that of the prior art shown in FIG. That is, the current flowing through the measurement object DUT is converted into a voltage by the current-voltage conversion circuit 101 by the AC voltage from the signal source V _S , while the voltage across the measurement object DUT is amplified by the instrumentation amplifier B1. The impedance of the measurement object DUT is measured by calculating the voltage in the CPU 30 after A / D conversion.
The operation in the case of measuring the impedance using the current-voltage conversion circuit in the second embodiment or later is the same as described above.

Next, the stability of the current-voltage conversion circuit 101 of the first embodiment will be considered.
Figure 16 shows, for comparison, shows the current-voltage conversion circuit 11 in the case of using the current feedback op amps A1 _I as the operational amplifier A1 in FIG. 14 described in the prior art, the frequency band is for example 3 [MHz] Is set.

FIG. 17 shows the frequency characteristics of the current amplification factor and phase when R _F = 1 [kΩ], C _C = 50 [pF] and the input capacitance C _IN is 50 [pF] and 500 [pF] in FIG. Show.
According to FIG. 17, the phase margin at C _IN = 500 [pF] is about 30 °, but the phase margin at C _IN = 50 [pF] is almost 0 °. It can be seen that the circuit becomes unstable when the capacitance _CIN changes.

On the other hand, FIG. 2 is a circuit diagram showing a main part of the embodiment of FIG. 1, and the stability is evaluated for this circuit.
FIG. 3 shows a case where R _F = 1 [kΩ] and C _C = 50 [pF] in FIG. 2 and a parallel circuit of a resistance of 1 [kΩ] and an inductance of 20 [μH] is used as the compensation impedance Z. The frequency characteristics of current amplification factor and phase when the input capacitance C _IN is 50 [pF] and 500 [pF] are shown.
According to FIG. 3, the phase margin at C _IN = 50 [pF] is about 60 °, and the phase margin of about 45 ° is secured even at C _IN = 500 [pF]. Nevertheless, it can be seen that the circuit is stable.

Next, the influence of the magnitude of the compensation impedance Z on the output voltage of the current-voltage conversion circuit 101 will be considered. Here, it is assumed that consider the low-frequency region of the measuring frequency, 1, and shall ignore the effect of the phase compensation capacitor C _C in FIG.
In this case, the output voltage V _O of the current-voltage conversion circuit 101 in FIG. In Equation 1, I _IN is a current (detection current) flowing into the current-voltage conversion circuit 101, T is a transimpedance of the operational amplifier A1 _I , and s is a Laplace operator.

According to Equation 1, it can be seen that if the compensation impedance Z is sufficiently small in the low frequency region, even if the length of the connection cable changes and the input capacitance _CIN changes, the influence on the measured value is small.
Thus, in order to secure the stability of the circuit while reducing the influence of the connection cable, the compensation impedance Z is low impedance in the low frequency region and high impedance in the high frequency region as described above, and the operational amplifier A1 _I In the frequency region where the loop gain becomes small, it is preferable that the input / output phase difference be close to 0 °, for example, a parallel circuit of resistance and inductance, or a ferrite bead.

As described above, according to the first embodiment, the compensation impedance Z having a predetermined characteristic is connected to the inverting input terminal of the current feedback operational amplifier A1 _I , so that a sufficient phase margin is ensured and the circuit can be stably operated. Thus, a current-voltage conversion circuit that is not affected by the length of the connection cable can be realized.

Next, a second embodiment of the present invention according to claim 2 will be described. FIG. 4 is a circuit diagram showing the configuration of this embodiment, and the same reference numerals are assigned to the same components as those in FIG.
Figure 4 and differs from FIG. 1, in the current-voltage conversion circuit 102, a point of connecting the amplifier A2 gain K (K> 1) between the output terminal and the circuit output terminal 202b of the current feedback operational amplifier A1 _I The other configurations are the same as those in FIG.

In the first embodiment of FIG. 1, the as is clear from Equation 1, the first term of the right side denominator of feedback resistor R _F Equation 1 The higher the value of the low frequency range is increased, which increases the detection error There is a fear.
In contrast, in the second embodiment, in the low frequency range where the phase compensation capacitor C _C does not act effectively becomes that transimpedance T of the operational amplifier A1 _I is K times by the gain of the equivalently amplifier A2, the loop The detection error can be reduced by increasing the gain.

Next, a third embodiment showing a main part of the present invention according to claim 3 will be described. FIG. 5 is a circuit diagram showing the configuration of this embodiment.
In this embodiment, a voltage feedback operational amplifier A1 _V is used as the current-voltage conversion circuit 103, the compensation impedance Z in FIG. 4 is removed, and the inverting input terminal of the operational amplifier A1 _V is connected to the circuit input terminal 202a. the configuration of the feedback circuit including the amplifier A2 in the subsequent stage of the operational amplifier A1 _V is the same as FIG.

This embodiment, the amplifier A2 gain K in the feedback circuit of the operational amplifier A1 _V, but common to FIG. 4 in that it includes a phase compensation capacitor C _C and the feedback resistor R _F, the amplifier A2 is mainly connected to the expansion of the frequency band It is inserted to reduce the influence of the input capacity of the cable 202.

5, since the circuit is stable, since the gain of the entire circuit may be secured to the phase margin of more than 45 ° at a frequency of 1, Equation 2 below for the capacitance C _C of the phase compensation capacitor is obtained It is done. Note that in Equation _{2, f GBP} is GBP (gain-bandwidth product) frequency of the operational amplifier A1 _V.

Here, as described with reference to FIG. 14 (prior art without the amplifier A2), feedback resistance R _F = 100 [kΩ], input capacitance C _IN = 500 [pF], GBP frequency f _GBP = 100 [MHz] and], when the gain K = 100 of amplifier A2 in FIG. 5, the capacitance C _C of the phase compensation capacitor for ensuring the stability is determined by equation 3.

Further, the corner frequency f _{C at} this time is expressed by Equation 4.

  From Equation 4, according to the present embodiment, the frequency band is expanded 10 times (= √K times) with respect to 670 [kHz], which is the corner frequency when the voltage feedback type operational amplifier is used in FIG. The frequency band can be expanded as the gain K is increased.

FIG. 6 is a frequency characteristic diagram of the transimpedance when C _IN = 0 [pF] (when there is no connection cable) and when C _IN = 500 [pF] (when there is a connection cable). As is clear from comparison with FIG. 15, in this embodiment, when the measurement frequency is 1 [MHz], the difference in transimpedance is within 3%, and the influence of the length of the connection cable on the measurement value is also affected. It is almost gone.

Next, FIG. 7 is a circuit diagram showing a fourth embodiment of the present invention according to claim 5, which is back and forth.
In general, a bias current of several [μA] flows through the inverting input terminal of the current feedback type operational amplifier. Therefore, choosing a large value such as 100 [kΩ] to the feedback resistor R _F in order to increase the detection sensitivity, the instruction of the measurement value appears a large DC offset voltage at the output of amplifier A2 in the embodiment of FIG. 4 There is a risk that measurement will become impossible due to shaking.

Therefore, in this embodiment, a DC servo circuit 50 is added to the input side of the current-voltage conversion circuit 102 as shown in FIG. 7, and the level of the bias voltage applied to the non-inverting input terminal of the operational amplifier A1 _I is corrected by its output. it was thus to prevent the excessive DC offset voltage from the current-voltage conversion circuit 102 is outputted due to the bias current of the operational amplifier A1 _I.
In this embodiment, the entire circuit combining the current-voltage conversion circuit 102 and the DC servo circuit 50 is also referred to as a current-voltage conversion circuit, and its reference numeral is 104.

In FIG. 7, a DC servo circuit 50 includes a DC cut capacitor C ₁ and a resistor R ₁ connected between a circuit input terminal 202a and one end of a compensation impedance Z, an operational amplifier A3 and a resistor R ₁ constituting an integrating circuit. ₂ and has become a capacitor C ₂ Prefecture, the output terminal of the operational amplifier A3, along with being connected to the non-inverting input terminal of the current feedback operational amplifier A1 _I, the non-inverting input terminal of the operational amplifier A3 is connected to ground.
In such a configuration, the error voltage between the potential and the ground potential of the point a is integrated by the operational amplifier A3, the integrated value will be applied as a bias voltage to the non-inverting input terminal of the operational amplifier A1 _I, the error voltage is zero As a result, the output voltage of the operational amplifier A1 _I , and hence the DC offset voltage output from the amplifier A2, is suppressed.
Here, the operational amplifier A3 of the DC servo circuit 50, the bias current compared to the operational amplifier A1 _I it is necessary to choose a sufficiently small.

The operation of the DC servo circuit 50 in this embodiment will be described in further detail.
(1) by the capacitor _{C 1} of the DC servo circuit 50, the bias current of the operational amplifier A1 _I will not flow through the feedback resistor _{R F.}
(2) Now, the bias current from the inverting input terminal of the operational amplifier A1 _I is assumed to flows into the resistor R _1, the output voltage of the operational amplifier A1 _I swings to the negative side. Since the gain K of the subsequent amplifier A2 of the operational amplifier A1 _I is positive, the output voltage of the amplifier A2, i.e. the voltage at point a in Figure 7 swings to the negative side.

(3) At this time, the output voltage of the operational amplifier A3 of the DC servo circuit 50, that is because the voltage at the non-inverting input terminal of the operational amplifier A1 _I swings to the positive side, also swings to the positive side output voltage of the output voltage and the amplifier A2 .
For this reason, since the output voltage of the amplifier A2 is cancelled, the DC offset voltage becomes almost zero. More specifically, the voltage at point a is Osaekoma the input offset voltage of the operational amplifier A3, to the following values the sum of the voltage drop across the resistor R ₂ by a small bias current of the operational amplifier A3.
Therefore, even if the feedback resistor R _F is large, it is possible to prevent the occurrence of excessive DC offset voltage due to the bias current of the operational amplifier A1 _I.

The concept described above can also be applied to the first embodiment of FIG.
That is, FIG. 8 is a circuit diagram showing a fifth embodiment of the present invention according to claim 4, wherein a DC servo circuit 50 is added to the input side of the current-voltage conversion circuit 101 of the first embodiment, and the entire circuit is shown. This is an example configured as a current-voltage conversion circuit 105.
Operation of this embodiment is basically the same as FIG. 7, it is possible to maintain the voltage of the DC offset voltage and a point of the operational amplifier A1 _I nearly zero.

Next, FIG. 9 is a circuit diagram of a sixth embodiment showing the main part of the present invention according to claim 3 .
In the third embodiment shown in FIG. 5, when increasing the measurement frequency (e.g. to 1 [MHz]), it is necessary to use a broadband and low-noise as a voltage feedback operational amplifier A1 _V. However, since the general flowing bias current of several [μA] This kind of voltage feedback operational amplifier, resulting in a problem similar to the case of the current feedback operational amplifier A1 _I in FIG.

Therefore, in the sixth embodiment shown in FIG. 9, the DC servo circuit 50 is added to the input side of the current-voltage conversion circuit 103 as in FIG. 7, and the entire circuit is configured as the current-voltage conversion circuit 106.
Since the operation of the DC servo circuit 50 in this embodiment is the same as that in FIG. 7, the description thereof is omitted to avoid duplication.
In the present embodiment, it is possible to maintain the voltage of the DC offset voltage and a point due to the bias current of the voltage-feedback operational amplifier A1 _V nearly zero.

Next, FIG. 10 is a circuit diagram showing a seventh embodiment of the present invention according to claim 6 .
This embodiment is for preventing an abnormal operation when an excessive voltage is inputted to the DC servo circuit 50 in the circuit of FIG.

10, differs from FIG. 7, between the output terminal of the circuit input terminal 202a and the operational amplifier A1 _I, antiparallel to a series circuit of a diode _D 1, the series circuit of the _{D 3} and diode _D 2, _{D 4} connected to a point connected to ground via a collectively resistor R ₃ internal connection points of the series circuits. A circuit composed of these diodes D _{1 to} D ₄ and resistor R ₃ is referred to as a voltage feedback circuit here.
In this embodiment, the entire circuit including the current-voltage conversion circuit 107 and the DC servo circuit 50 is used as the current-voltage conversion circuit 108.

Hereinafter, the operation of this embodiment will be described.
First, in the circuit of FIG. 7 described above, when a 1 [pF] capacitor is measured as a measurement object DUT at a frequency of 1 [MHz], the impedance of the capacitor becomes a large value of 160 [kΩ]. As R _F , for example, 100 [kΩ] is used.

At this time, if the capacitor is charged to 10 [mV] by the residual charge, the residual charge is discharged via the current-voltage conversion circuit 102. However, since the discharge current is limited only by the output resistance R ₀ (generally 50 [Ω]) on the signal source V _S side, the output voltage of the amplifier A2 is
10 [mV] x (R _F / R ₀ )
= 10 [mV] × (100 [kΩ] / 50 [Ω]) = 20 [V]
It is clipped at a value near the power supply voltage. That is, the voltage at point a in Figure 7 because it does not become zero, the output voltage of the DC servo circuit 50 (reference voltage of the operational amplifier A1 _I) deviates from the desired DC level. Thus, once the DC level is deviated, a considerable time is required to recover to a predetermined level due to the time constants of the capacitors and resistors in the DC servo circuit 50, thereby hindering the measurement operation. .
Such a problem can also occur when the capacitor that is the measurement object DUT is short-circuited.

Therefore, in this embodiment, as shown in FIG. 10, the operational amplifier A1 inserts a voltage feedback circuit 111 composed of diodes _D 1 to D ₄ and the resistor _{R 3} to the output side of the _I, operational amplifier A1 _I output voltage the diode D-a and to keep the voltage at the point a nearly zero by feeding back the point a via ₁ to D _4.
That is, if the capacitor there is residual charge as the measuring object DUT is connected, the output voltage is the magnitude of the operational amplifier A1 _I in FIG. 10 by the output of the DC servo circuit 50 the voltage of the capacitor is input. However, the series circuit of the diodes _D 1, _{D 3} of the output voltage of the operational amplifier A1 _I is the voltage feedback circuit 111 or exceeds the ON voltage of the series circuit of the diode _D 2, _{D 4,} the output voltage of the operational amplifier A1 _I voltage, The feedback is made to the point a via the feedback circuit 111, and the voltage at the point a is maintained at zero.

For this reason, it is possible to prevent the DC servo circuit 50 from operating abnormally due to an excessive input voltage, and to avoid the disadvantage that the reference voltage of the operational amplifier A1 _I deviates from a predetermined level.
The resistance R ₃ in the voltage feedback circuit 111 is sufficiently smaller than the feedback resistor R _F, and plays a function of flowing a leak current from the diode D ₁ to D ₄ on the ground side.

Finally, FIG. 11 is a circuit diagram showing a main part of an eighth embodiment of the present invention according to claim 3 .
This embodiment is an example in which as to constitute a current-voltage conversion circuit 109 adds a voltage feedback circuit 111 to the circuit with a voltage feedback operational amplifier A1 _V shown in FIG. 9, the current-voltage conversion circuit 109 and a DC servo The entire circuit comprising the circuit 50 is shown as a current-voltage conversion circuit 110.
The operation is the same as that in FIG.
Also in this embodiment, to prevent abnormal operation of the DC servo circuit 50 due to an excessive input voltage, it is possible to maintain the reference voltage of the operational amplifier A1 _V to a predetermined level.

1 is a circuit diagram of an impedance measuring device to which a first embodiment of the present invention is applied. It is a circuit diagram of a 1st embodiment of the present invention. FIG. 3 is a frequency characteristic diagram of current amplification factor and phase in the circuit of FIG. 2. It is a circuit diagram of the impedance measuring apparatus with which 2nd Embodiment of this invention is applied. It is a circuit diagram of the impedance measuring apparatus with which 3rd Embodiment of this invention is applied. FIG. 6 is a frequency characteristic diagram of transimpedance in FIG. 5. It is a circuit diagram which shows 4th Embodiment of this invention. It is a circuit diagram which shows 5th Embodiment of this invention. It is a circuit diagram which shows 6th Embodiment of this invention. It is a circuit diagram which shows 7th Embodiment of this invention. It is a circuit diagram which shows 8th Embodiment of this invention. It is a circuit diagram of the impedance measuring device to which a prior art is applied. FIG. 13 is a circuit diagram equivalently showing the main part of FIG. 12. It is a circuit diagram which shows another prior art. It is a frequency characteristic figure of the transimpedance in FIG. It is a circuit diagram of a current-voltage conversion circuit using a current feedback type operational amplifier. FIG. 17 is a frequency characteristic diagram of current amplification factor and phase in the circuit of FIG. 16.

Explanation of symbols

21, 22: A / D conversion circuit 30: CPU
40: display device 50: DC servo circuit 101-110: current-voltage conversion circuit 111: voltage feedback circuit 201-204: connection cable 202a: circuit input terminal 202b: circuit output terminal V _S : signal source R _O : output resistance R ₁ to R _3: resistance _{C HC, C LC, C HP} , C LP: the capacitance C _IN: input capacitance C _C: phase compensation capacitor R _F: feedback resistor _{C 1,} C _2: capacitor D ₁ to D _4: diode DUT: measurement object X: impedance Z: compensation impedance A1, A1 _I , A1 _V , A3: operational amplifier A2: amplifier B1: instrumentation amplifier

Claims (8)

A current that converts an input current into a voltage using an operational amplifier that has at least a feedback resistor and a phase compensation capacitor in the feedback circuit between the output terminal and the inverting input terminal, and a reference voltage is applied to the non-inverting input terminal. In the voltage conversion circuit,
The operational amplifier is a current feedback operational amplifier,
The feedback circuit is
A compensation impedance connected between the circuit input terminal into which the input current flows and the inverting input terminal; and a feedback resistor and a phase compensation capacitor connected between the circuit input terminal and the output terminal of the operational amplifier. A parallel circuit,
As the compensation impedance,
A device having a low impedance value in a low frequency region, a high impedance value in a high frequency region, and an input / output phase difference being almost zero in a frequency region where the loop gain of the operational amplifier is small is used. A current-voltage conversion circuit. A current that converts an input current into a voltage using an operational amplifier that has at least a feedback resistor and a phase compensation capacitor in the feedback circuit between the output terminal and the inverting input terminal, and a reference voltage is applied to the non-inverting input terminal. In the voltage conversion circuit,
The operational amplifier is a current feedback operational amplifier,
The feedback circuit is
A compensation impedance connected between a circuit input terminal into which the input current flows and the inverting input terminal; a phase compensation capacitor connected between the circuit input terminal and the output terminal of the operational amplifier; The feedback resistor connected between the input terminal and the circuit output terminal, and an amplifier connected between the output terminal of the operational amplifier and the circuit output terminal,
As the compensation impedance,
A device having a low impedance value in a low frequency region, a high impedance value in a high frequency region, and an input / output phase difference being almost zero in a frequency region where the loop gain of the operational amplifier is small is used. A current-voltage conversion circuit. A current that converts an input current into a voltage using an operational amplifier that has at least a feedback resistor and a phase compensation capacitor in the feedback circuit between the output terminal and the inverting input terminal, and a reference voltage is applied to the non-inverting input terminal. In the voltage conversion circuit,
The operational amplifier is a voltage feedback operational amplifier,
The feedback circuit is
The phase compensation capacitor connected between the circuit input terminal into which the input current flows and the output terminal of the operational amplifier; the feedback resistor connected between the circuit input terminal and the circuit output terminal; Rutotomoni and a amplifier connected between the output terminal and the circuit output terminal,
A DC servo circuit is connected between the circuit input terminal and the inverting input terminal and non-inverting input terminal of the operational amplifier,
The DC servo circuit is
A DC cut capacitor connected between the circuit input terminal and the inverting input terminal;
A resistor connected to the inverting input terminal to pass a bias current of the operational amplifier;
An integrating circuit connected between the circuit input terminal and the non-inverting input terminal and providing the output to the non-inverting input terminal as a bias voltage;
Voltage feedback between the circuit input terminal and the output terminal of the operational amplifier to feed back the output voltage of the operational amplifier to the circuit input terminal side via a diode and maintain the voltage of the circuit input terminal at approximately zero. A current-voltage conversion circuit characterized by connecting a circuit. In the current-voltage conversion circuit according to claim 1,
A DC servo circuit is connected between the circuit input terminal, the input side of the compensation impedance, and the non-inverting input terminal of the operational amplifier,
The DC servo circuit is
A DC cut capacitor connected between the circuit input terminal and the compensation impedance input side;
A resistor connected to the input side of the compensation impedance to pass the bias current of the operational amplifier;
And a integrating circuit connected between the circuit input terminal and the non-inverting input terminal of the operational amplifier, and supplying the output to the non-inverting input terminal as a bias voltage. In the current-voltage conversion circuit according to claim 2,
A DC servo circuit is connected between the circuit input terminal, the input side of the compensation impedance, and the non-inverting input terminal of the operational amplifier,
The DC servo circuit is
A DC cut capacitor connected between the circuit input terminal and the compensation impedance input side;
A resistor connected to the input side of the compensation impedance to pass the bias current of the operational amplifier;
And a integrating circuit connected between the circuit input terminal and the non-inverting input terminal of the operational amplifier, and supplying the output to the non-inverting input terminal as a bias voltage. In the current-voltage conversion circuit according to claim 5 ,
Voltage feedback between the circuit input terminal and the output terminal of the operational amplifier to feed back the output voltage of the operational amplifier to the circuit input terminal side via a diode and maintain the voltage of the circuit input terminal at approximately zero. A current-voltage conversion circuit characterized by connecting a circuit. In the current-voltage conversion circuit according to any one of claims 3 to 6,
A current-voltage conversion circuit characterized in that a bias current of an operational amplifier constituting an integration circuit of the DC servo circuit is sufficiently smaller than a bias current of the operational amplifier . The current-voltage conversion circuit according to any one of claims 1 to 7 is configured as an impedance measurement device that measures impedance of a remote measurement object connected to the circuit input terminal via a coaxial cable. current-voltage conversion circuit, characterized in that there.

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