JPS63108273A - Impedance measuring instrument - Google Patents

Abstract

PURPOSE:To measure impedance stably by supplying a current which flows through an element to be measured to a parallel of a resistance and a capacitor having specific values respectively and calculating the impedance value of the element from measured values of a measured voltage and a voltage for current measurement. CONSTITUTION:A bridge circuit constituted by adding a parallel capacitor 43 to a reference resistance 4 is used to reduce the influence of internal noises of an input amplifier. Further, un stable bridge circuit operation based upon frequency characteristics of an amplifier is eliminated. The output voltage of this bridge circuit is measured while decomposed into a measurement signal and an in-phase and an orthogonal component, and then digitized, and the influence of the added capacitor 3 is removed to find a value of admittance to be measured. Consequently, a stable, high-accuracy measurement is taken even in environment where a measuring instrument needs to be connected to the element to be measured by using a long cable.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention provides an impedance measurement method that improves the method for detecting the current flowing through the device under test and the method for calculating the impedance value to be measured so that impedance measurement can be stably performed using the voltage-current method. Concerning vessels.

[Prior art and its problems]

Precision impedance measuring instruments based on the half-bridge method are commercially available and have been widely praised.

FIG. 2 shows an impedance measuring device according to the prior art in a simplified manner for explaining the operation. A grounded sinusoidal signal source 1 is connected to an inverting input terminal 51 of a high input impedance amplifier 5 via an admittance 2 to be measured. A reference resistor 4 is connected between the inverting input terminal 51 and the output terminal 6 of the amplifier 5 .

Further, a stray capacitance I3 due to a cable or the like exists between the inverting input terminal 51 and the ground, and an equal voltage noise source 7 exists between the non-inverting input terminal 52 of the amplifier 5 and the ground.

(1) Noise source 7. If there is no stray capacity 3.

Assuming that the voltage of the signal source 1 is Vi, its output value v0 is a voltage for current measurement and is expressed by the following equation at the terminal 6.

Vo= R(G-jB) Vz (1)
Here, R: value of reference resistance 4.

G: Conductance of the admittance to be measured.

B: Susceptance of the admittance to be measured.

j: imaginary unit 1”=-1).

An in-phase component V of the voltage v0 with respect to V by a known method.

, and the orthogonal component V. If 2 is measured, the following formula is obtained.

(However, in the measurement, the effect of V□ was multiplied by an appropriate coefficient so that it was a unit.) Therefore, G = Vo
+/RlB =Voz/G as R.

B is required.

(2) When a stray capacity of 3 (value C) exists.

In this case, the following problem occurs.

Considering the frequency characteristics of the amplification degree A of the amplifier 5, it becomes as follows. Here, Ao: DC amplification degree.

ω. :Amplification 3dB decrease angular frequency ω: Output angular frequency of signal source l (measured angular frequency).

Now, ω/ω. 〉〉1, the input admittance of the amplifier 5 viewed from both ends of the capacitor 3 is expressed as follows. Therefore, the admittance combined with the stray capacitance 3 is as follows, the equivalent input admittance of the amplifier 5 becomes small, and the amplification degree becomes unstable.

(3) When there is a stray capacitance 3 and a noise source 7.

The essence remains the same even if the device under test is removed, so if we do so, then V o = (1+j(J) ncR) e
(6) becomes.

Here, e, ω are the magnitude and angular frequency of the noise source 7. Therefore, the noise output increases as the measurement angular wave number ω increases. Conventionally, in order to improve the effect of the frequency change of the amplification degree A, as shown in FIG. 3, the amplifier 5 is replaced with a voltage input current output amplifier 80, and a phase advancing circuit 90 and a buffer 81 are added to the output to perform phase correction. However, the phase advance circuit 90 and the buffer 81 increased the cost, and it was difficult to perform sufficient phase correction. Moreover, even with this, no improvement was made in the increase in noise output.

In the conventional impedance measuring device, as shown in FIG. Stray volume of 41.42 1401.
I was trying to cancel out the effect of 402.

In the configuration of FIG. 4, the effect of the stray capacitance 3 is even more significant, and the operation of the amplifier becomes unstable. In recent years, the measurement cable that connects the measuring instrument and the admittance to be measured has become longer.
This problem is serious as the stray volume ff13 is increasing.

[Object of the present invention]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an impedance measuring instrument that solves the above-mentioned problems by simply adding a parallel capacitance to a reference resistor.

[Summary of the invention]

According to one embodiment of the present invention, the influence of internal noise of the input amplifier is reduced by using a bridge circuit in which a parallel capacitance is added to a reference resistor. Furthermore, unstable bridge circuit operation due to the frequency characteristics of the amplifier is also eliminated.

Using conventional technology, the output voltage of the bridge circuit is measured by decomposing it into an in-phase component and a quadrature component with respect to the measurement signal, and then digitized, and the influence of the added capacitance is removed to determine the value of the admittance to be measured. . The impedance value is calculated from the admittance value.

[Embodiments of the invention]

FIG. 1 shows a bridge circuit according to one embodiment of the present invention. Second
Elements having the same functions and performance as those in FIGS. 3, 3, and 4 are given the same numbers.

Since a parallel capacitance 43 is added to the resistor 4, if its value is set as C, the following equation can be obtained corresponding to the above-mentioned equation (4).

(1+JωC,R) = -(1+AOω.C,R-) Therefore, even if the imaginary part of Yin becomes zero due to the stray field I3, the magnitude of Yin is 1+A0ω.C1R times the conventional value, increasing the stability of the bridge circuit. .

Furthermore, the following equation is obtained corresponding to equation (6).

Therefore, the noise multiplication factor is (1+C/C,
).

Element values in typical examples are shown below.

R=800 and Ω C=200 to 600 pF C,=20 pF ω/2π=100011z A0ω. /2π=3MHz 1+A0ω, C, R=301.6 1 + C/C,=11-31 Next, how to obtain G and B will be shown. Corresponding to equation (11, (2+), therefore, □aυ.

ωC and R are known values. ■. After digitizing I+ vow, it was built into an impedance measuring device (
G and B are calculated by a computer (or externally attached) using the values of ωC and R already stored therein.

〔Effect of the invention〕

As is clear from the detailed description of the embodiments above, by implementing the present invention, even in an environment where a measuring instrument and a device under test must be connected using a long cable,
Stable and highly accurate measurements are possible.

In recent years, automated manufacturing lines and the like have many such demands, so it is useful for practical use.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a bridge portion according to an embodiment of the present invention, Fig. 2 is a circuit diagram of a conventional example corresponding to the bridge portion of Fig. 1, and Fig. 3 uses an amplifier whose phase is corrected by a phase advance circuit. FIG. 4 is a circuit diagram of a conventional bridge section that compensates for the parallel 'am capacitance of a reference resistor. 1: Measurement signal source; 2: Measured admittance; 3.4
01.402: Stray capacitance; 4: Reference resistance; 43: Correction capacitance; 5.80: Amplifier; 6: Output terminal; 7: Noise source; 9
0: Phase advance circuit;

Claims (1)

[Claims] A measurement voltage is applied to the element to be measured, a current flowing through the element is introduced into a parallel circuit of a resistor and a capacitor each having a predetermined value, a voltage for current measurement is generated, and the measurement voltage and the voltage for current measurement are measured. An impedance measuring device that calculates an impedance value of the element from the impedance value.