JPH0633424Y2 - Input circuit of measuring instrument - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an input circuit such as a measuring instrument called an ohmmeter or an LCR meter.

[Conventional technology]

A digital measuring instrument called an LCR meter for measuring a resistance value, a capacitance value, an inductance value or the like of an object to be measured is generally used, and an outline of its configuration is shown in FIG.

That is, for example, impedance Z from measurement signal source 1
_A constant voltage V is applied to the device under test 2 having _X, and a flowing current I
_X is a voltage V at the current / voltage converter 3 such that V _X = k · _IX
It is converted into _X and added to the phase polarity detector 4. However, k is a known conversion constant. A voltage V of the signal source 1 is also applied to the phase polarity detector 4 via, for example, a buffer amplifier 5, where the conversion voltage V with respect to the signal source voltage V is applied.
The phase polarity of _X is detected. That is, if the impedance Z _X of the DUT 2 is a blunt resistance,
Since the phase polarity is in phase with the signal source voltage V, 0 is detected, and if capacitive, a value of + is detected, and if inductive, a value of − is detected.

The phase polarity information and the two voltages V and V _X are applied to, for example, the measuring unit 6, converted into direct current and then digitally converted to measure the values of V and V _X.

From the measured value of the voltage V _X , the current value I _X is obtained by I _X = V _X / k, and the impedance Z _X of the device under test 2 is calculated by calculating V ÷ I _X.

That is, Z _X = V / I _X = k · V / V _X is obtained. The resistance value R, the capacitance value C, the inductance value L, etc. of the device under test 2 are obtained from the calculated value, the measurement frequency, and the phase polarity information.

[Problems to be solved by the invention]

A general example of an input circuit including the device under test 2 and the current / voltage converter 3 in the conventional apparatus of FIG. 3 is shown in FIG. In this input circuit, a coaxial cord or the like is usually used for connecting the device under test 2 and the current / voltage converter 3 so as not to be affected by external noise or the like.

In such an input circuit, the capacitance component Ci of the coaxial cord enters between the signal path and the ground, which is equivalent to the addition of the impedance 1 / jωCi.
Here, the terminal 7 Add measurement voltage V and the measured body 2 for example, a current I _{X (=} I _R) flows, the terminal 8 side voltage Vi generated by it. The current / voltage converter 3 returns the feedback current I from the output terminal 9 side to the terminal 8 side via the resistor R.
_R (= I _X ) is supplied and the voltage Vi is operated so as to be zero. Due to this feedback current I _R , a voltage V is applied to the terminal 9 side.
_X (= R _X I _R ) occurs.

This operation is performed normally when the frequency is low, but when the frequency is high, the impedance 1 / j is generated due to the capacitance component Ci.
Since ω Ci becomes smaller in proportion to the frequency, the current I _C flowing to the ground side via this impedance becomes larger in proportion to the frequency. The current / voltage converter 3 cancels this current and thus increases the feedback current from I _R to I _R + I _C. Therefore, the voltage V _X appearing on the terminal 9 side is, for example, the fifth
As shown by the dotted line in the figure, V _X = R _X (I _R + I _C ), which rises in proportion to the frequency and cannot be ignored in due time.

In addition, since this capacitance component Ci generally forms a parallel resonance circuit with the wiring of the signal path, etc., it causes resonance at a frequency lower than the upper limit frequency fc that originally starts to attenuate at octave 6 dB,
A phenomenon occurs that the converted voltage V _X has a peak.

Since any increase or peak generation of these conversion voltages V _X causes a measurement error, for example, a correction capacitor Ci having a capacitance substantially equal to the capacitance component Ci in parallel with the load resistance R of the current / voltage converter 3 is provided. ′ Is connected to cancel the increase of the converted voltage V _X and the occurrence of the peak so as to flatten the frequency characteristic.

However, as the measurement frequency is increased, for example, the capacitor Ci ', the resistor R, and the capacitance component Ci interfere with each other via stray capacitance such as wiring, and the desired correction effect cannot be obtained. Therefore, it is generally difficult to flatten the frequency characteristic especially on the high frequency side. Also, as is well known near the resonance frequency, the phase rotation is relatively large, so
There is a risk that the impedance Z _x of the device under test 2 may interfere with the determination as to whether it is inductive or capacitive. Therefore, it is desirable to make the resonance frequency as high as possible and keep its peak voltage low.

The present invention has been made in view of the above points, and its purpose is to eliminate the mutual interference between the correction capacitor Ci ′ and the device under test 2 side, and to flatten the frequency characteristic on the high frequency side, thereby the input circuit of the measuring instrument. To provide.

[Means for solving problems]

Referring to FIG. 1 showing an embodiment of the present invention, in order to solve the above-mentioned problems, in this input circuit, a buffer amplifier 10 having a high input impedance, for example, having a correcting capacitor Ci ' A differential amplifier 11 is provided in parallel with the voltage converter 3 and receives the output of the buffer amplifier 10 and the output of the current / voltage converter 3 respectively.

〔Operating principle〕

In FIG. 1, the impedance of the device under test 2 is Z
_{Let X} be V, the measurement voltage applied to the DUT 2 from the terminal 7 side be V, the current flowing in the DUT 2 be I, and the voltage appearing at the terminal 8 side of the DUT 2 due to this current I be Vi. I
Is expressed by the following equation.

I = (V-Vi) / Z _X (1) _Let I _C and I _R be the currents shunting the current I into the capacitance component Ci and the resistance R of the current / voltage converter 3, respectively, and I = I _C a + _I R ...... (2).

Here, since the impedance of the capacitance component Ci is −j / ωCi, the current I _C is I _C = Vi / (− j / ωCi) = jωCiVi (3)

In addition, the voltage obtained at the output side of the current / voltage converter 3 is V
_{Assuming X} , the current I _R is I _R = (Vi−V _X ) / R (4).

Therefore, by substituting the equations (1), (3), and (4) into the equation (2), (V-Vi) / Z _X = jωCiVi + (Vi-V _X ) / R When the voltage V _X is obtained from the above equation, V _X = Vi− (V−Vi) R / Z _X + jωCiViR (5)

Next, for example, between the − input end of the buffer amplifier 10 and the ground, Ci =
A capacitor Ci ′ having a value of Ci is connected, and a load resistance R having a value equal to the load resistance R of the current / voltage converter 3 is connected between the − input terminal and the output terminal.

When the voltage Vi is applied to the + input terminal in this state, the voltage V _X ′ obtained from the output terminal is V _X ′ = {1 + R / (− j / ωCi ′)} Vi = (1 + jωCi′R) Vi. (6)

Equation (5) and the voltage of (6) _{V X} and _{V X} 'is the differential amplifier 11
Added to. In this case, each resistor R ₁ on the differential amplifier 11 side
The values of R to R ₄ are, for example, R ₁ = R ₃ and R ₂ = R ₄ .

The output voltage obtained at the terminal 9 side of the differential amplifier 11 is V _XO
Then, from the equations (5) and (6), V _XO = V _X −V _X ′ = Vi− (V−Vi) R / Z _X + jωCiViR − (1 + jωCi′R) Vi = −VR / Z _X + ViR / Z _X + jωCiViR −jωCiViR (7)

Here, since the voltage Vi is set to Vi≈0 because the input side of the current / voltage converter 3 is a so-called imaginary short circuit, the second term of the equation (7) can be ignored. The third term can be ignored when the frequency is low, but generally cannot be ignored when the frequency is high.

In this embodiment, as shown in the fourth term of the equation (7), the voltage −jωCiViR having the opposite polarity to the third term is given through the buffer amplifier 10 and Ci = Ci. , The third term is canceled regardless of the frequency level, and V _XO = −V · R / Z _X. That is, the converted voltage V _XO having a flat frequency characteristic can be obtained from the measurement signal voltage V.

〔Example〕

Referring again to FIG. 1, the correction capacitor Ci 'connected between the -input terminal of the buffer amplifier 10 and the ground is represented by a fixed capacitor, but it may be replaced by a variable capacitor (trimmer capacitor). .

FIG. 2 shows another embodiment. That is, the buffer amplifier 10 in FIG. 1 is replaced with, for example, a voltage follower type buffer amplifier 12 and an inverting amplifier 13.
The differential amplifier 11 shown in FIG. 1 is replaced by a summing amplifier 14.

In this embodiment, when the voltage Vi is applied to the voltage follower type buffer amplifier 12, the output becomes the same Vi as the input and is applied to the inverting amplifier 13. Therefore, when the output of the inverting amplifier 13 is V _X ″, V _X ″ = −Vi × R / (− j / ωCi ′) = − jωCi′ViR (8), and this output is, for example, through the resistor R ₃ . Summing amplifier
Added to 14.

In this case, the output V _X of the current / voltage converter 3 is expressed by the equation (5) as in the embodiment of FIG. 1, and is output to the summing amplifier 14 via the resistor R ₁ having a value equal to that of the resistor R _3. Added.

Therefore, assuming that the output of the summing amplifier 14 is V _XO ′, V _XO ′ = V _X + V _X ″ = −VR / Z _X + Vi + ViR / Z _X + jωCiViR−jωCi′ViR. (9) where the amplification factor is 1, that is, R ₃ = R ₁ (=
R ₂ ).

Here, if Ci '= Ci is set as in the case of FIG. 1, the fourth term of the equation (9) is canceled by the fifth term, and V _XO ' =-VR / Z _X + Vi + ViR / Z _X.

However, since Vi≈0, the second and third terms can be ignored, so that V _XO ′ = −V · R / Z _X ≈V _XO , which is substantially the same as the embodiment of FIG. A converted voltage V _XO ′ having a flat frequency characteristic can be obtained.

In each of the above-described embodiments, a constant voltage signal V for measurement is applied to the device under test 2, and the current I flowing according to the magnitude of the impedance Z _X is converted into the voltage V _XO or V _XO ′ to obtain the impedance Z _X. However, these examples are also applicable to the case where a low current signal for measurement is applied to the DUT 2 and the impedance Z _X thereof is obtained.
However, in this case, the current / voltage converter 3 operates as a voltage detector.

[Effect of device]

As described above in detail, the input circuit according to the present invention is a current / voltage conversion device that applies a voltage signal for measurement to a device under test and converts the flowing current into voltage according to the magnitude of the impedance to detect the voltage. And a capacitor having a value equal to the capacitance interposed in the signal path between the device under test and the current / voltage converter, and canceling the unnecessary signal including at least the frequency component among the unnecessary signals generated by the capacitance. An amplifier is provided for forming a signal for use in the operation, and an amplifier for adding or subtracting the correction output of the amplifier to the output of the current / voltage converter is provided.

Therefore, from the output of the amplifier that adds or subtracts,
A signal with flat frequency characteristics can be obtained. Therefore, the measurement error due to the capacitance of the input signal path is extremely small and highly accurate,
In addition, a broadband measuring instrument can be realized.

[Brief description of drawings]

1 and 2 relate to an embodiment of an input circuit of a measuring device according to the present invention, and FIG. 1 is a circuit diagram showing an example of its configuration,
FIG. 2 is a circuit diagram showing a modified embodiment thereof, FIG. 3 is a block diagram showing a configuration of a conventional device, FIG. 4 is a circuit diagram of an input circuit of the conventional device, and FIG. 5 is an input circuit of the conventional device. It is a characteristic explanatory view. In the figure, 1 is a signal source, 2 is a device under test, 3 is a current / voltage converter, 10 and 12 are buffer amplifiers, 11 is a differential amplifier, 13 is an inverting amplifier, and 14 is a summing amplifier.

Claims (1)

[Scope of utility model registration request] 1. A measuring instrument which applies a measuring signal to an object to be measured, detects a response signal generated in relation to the magnitude of its impedance through an input circuit, and performs a predetermined measurement by the detected output. In the input circuit, the input circuit includes a voltage detection means for detecting a current or a voltage of the response signal, and a feedback including a load resistance and a capacitor having a value substantially equal to the capacitance interposed between the DUT and the voltage detection means. A correction signal forming means for forming an canceling signal for an unnecessary signal containing at least a frequency component among the unnecessary signals generated between the device under test and the voltage detecting means by the capacitance, An input circuit of a measuring instrument, comprising: an adder / subtractor for adding or subtracting an output from the correction signal forming means to an output of the voltage detecting means.