JPH0421103Y2 - - Google Patents

Description

[Detailed description of the invention] This invention allows two
The present invention relates to a changeover switch for capacitance measurement that is capable of measuring multiple points.

Measuring the capacitance of an object is one of the important measurement means in industrial measurement because changes in the shape, properties, etc. of the object appear as fluctuations in capacitance.

By the way, in industrial capacitance measurements, there are many demands for measuring multiple points (two or more points), and in such cases, it is necessary to use a switch that sequentially switches each point and connects it to one measuring device. It is used. This changeover switch is not only more economically advantageous than preparing a plurality of measuring instruments, but also advantageous in that measurement errors can be made uniform. Further, a changeover switch is also required when measuring the reference capacitance and the object to be measured almost simultaneously and correcting errors in the measured value of the object to be measured using the measured value of the reference capacitance.

The above-mentioned changeover switches include mechanical ones such as relay contacts and semiconductor switching elements, but in industrial measurement, mechanical ones have problems in response speed and lifespan, so semiconductor switching devices are used. The use of elements is considered. However, semiconductor switching devices have a drawback in that the conduction resistance when on (hereinafter referred to as on-resistance) does not become completely zero. This on-resistance causes a phase error and absolute value error of impedance when measuring capacitance. Furthermore, since the on-resistance changes depending on the temperature, the measured value becomes unstable. Therefore, as the above-mentioned semiconductor switching element, one with a small on-resistance is selected. However, due to the essential structure of a semiconductor, the smaller the on-resistance, the larger the capacitance between each electrode (the on-resistance increases). (Some semiconductor switching devices with a resistance of 1 ohm or less have a capacitance of several 100 pF or more.) However, there is a problem in that this capacitance also changes depending on the temperature.

For example, when an N-channel MOS field effect transistor (hereinafter referred to as MOS-FET) is used as a changeover switch for a capacitance measuring instrument (hereinafter referred to as measuring instrument), when the MOS-FET is turned on as shown in Figure 1a, The equivalent circuits in the OFF state are shown in Figures b and c, respectively. In the figure, S is the source, D is the drain,
G is the gate and SB is the substrate.

Here, even if we assume that the on-resistance R _SD of the equivalent circuit when on is negligibly small, the source-gate capacitance C _SG , the drain-gate capacitance C _DG , the source substrate capacitance between
C _SSB , drain-to-substrate capacitance
C _DSB appears as a measurement error caused by this semiconductor switching element. Note that junction field effect transistors without substrate electrodes (hereinafter referred to as
Even in the case of a J-FET (J-FET), measurement errors appear similarly because capacitances C _SG and C _DG exist.

In addition, even if the on-resistance R _SD becomes infinite when it is off, the capacitance C _SD between the source and drain between the measuring instrument and the object to be measured, as shown in Figure 1c, is
The capacitances C _SG and C _SSB become error factors because they are coupled with each other and affect other objects under test.

For this reason, conventionally, in a bridge-type measuring instrument as shown in FIG. 6, a semiconductor switching element SW of the same type _is attached to arm _{a 1 of} bridge B that connects the object to be measured C The error caused by the semiconductor switching element SW can be canceled out, or the current flowing through the device under test _(C
To determine the capacitance of C _XN , the reference capacitor and _the object to be measured _C reference capacitor
Calculations are performed while correcting them using the measured value of CA to reduce the measurement error due to the ON resistance of the semiconductor switching element SW and the capacitance between each terminal, but the absolute error is kept below 1 pF. It is difficult to suppress the measurement stability and to suppress the measurement stability to 0.1 pF or less in an industrial atmosphere of -25°C to +85°C.

However, capacitance measurements used in industrial measurements often require an error of 0.01pF or less, and in such cases, it is necessary to use one measuring device for one measured point. Therefore, there are problems such as being extremely expensive.

This idea was devised in view of the above circumstances, and its purpose is to reduce the on-resistance and eliminate the influence of capacitance between electrodes, thereby reducing the chance of errors caused by switching. An object of the present invention is to provide a changeover switch for capacitance measurement.

In other words, this invention was based on the principle of the guard shield technique used to cancel the capacitance of the cable connecting the measuring device and the object under test when measuring capacitance. Hereinafter, the principle of the guard shield technique will be explained based on FIG.

As shown in FIG. 2, the guard shield technique uses an internal conductor 2a of a shield wire 2 to
The capacitance C of the object to be measured 1 is measured by applying an AC voltage ν from a suitable AC power source 3 via the AC power source 3 and measuring the current i flowing through the object to be measured 1. That is,
The capacitance C is determined as i=jwCν ∴C=i/jwν. However, since a cable capacitance C _S exists between the inner conductor 2a and the outer conductor 2b of the cable 2, when the outer conductor 2b is connected to the ground, a current also flows through the cable capacitance C _S , and the measured current i _S is i _S =jw(C+C _S )ν, and the capacitance C of the object to be measured 1 cannot be measured. Therefore, the outer conductor 2b of the cable 2 is connected to the application point of the AC power source 3 to the inner conductor 2a, and is driven with an AC voltage ν. At this time, if the output impedance of the AC power supply 3 is sufficiently small and the internal impedance of the ammeter is also sufficiently small, the potential difference between the internal conductor 2a and the external conductor 2b of the cable 2 disappears, and current flows through the cable capacitance C _S. figure,
The measurement current im becomes im=i=jwCν, and it becomes possible to directly determine the capacitance C of the object to be measured 1.

That is, as shown in FIG. 3, for example, an AC voltage source V for measurement of a measuring instrument (not shown) is connected to the source S of the input/output terminal of a semiconductor switching element such as an N-channel MOS-FET4, and the output terminal is Connect the object to be measured 1 to the drain D. Then, a driving voltage source V' that outputs an AC voltage having the same phase and the same potential as the measurement AC voltage source V is connected to terminals other than these input/output terminals S and D, that is, the gate G of the control terminal,
Output voltage ν′ of the drive voltage source V′ and opening/closing control signal
A voltage V _G +ν' that is superimposed with V _G is applied. Similarly, the drive voltage source V' is connected to the substrate terminal SB, and a voltage -V _S +ν' in which the output voltage ν' of the drive voltage source V' and the bias voltage -V _S are superimposed is applied. By doing this, an effect similar to the guard shield technique appears, and the capacitances C _SG , C _SSB , C _DG , C _DSB
Almost no alternating current from the measuring alternating current voltage source V flows through, and there is no measurement error directly caused by the capacitance between the electrodes when the MOS-FET 4 is on or off. At this time, the drive voltage source V' is connected to the drain D (sometimes the source S) which is disconnected from the measuring instrument when the MOS-FET 4 is turned off via a switch means 5 that operates only when the MOS-FET 4 is turned off. The output voltage ν' of the voltage source V' is applied to bring the source S and drain D to the same potential, and the measuring instrument and the measured object are connected to each other in the off state by applying the output voltage ν _' of the voltage source V' and making the source S and the drain D the same potential to prevent leakage current or charging/discharging current due to the capacitance CSD. Eliminate the bond with thing 1. The switching means 5 uses a semiconductor switching element such as a MOS-FET, and the gate of this switching element is also connected to the drive voltage source V' in the same way as the gate G of the MOS-FET 4 to reduce the on-resistance. , and the influence of capacitance between each electrode may be eliminated.

Note that the capacitance C of the object to be measured 1 is MOS-
A measuring AC voltage applied to the source S of FET4,
By measuring the current im flowing through the source S,
Although it is determined as C=i/jwν, the capacitance C may be calculated by other means such as determining the absolute value of the current and the phase difference between the voltage and the current.

Next, an embodiment of this invention will be explained based on FIG. 4. SW ₁ and SW ₂ are N-channel MOS-
Main switch and auxiliary switch consisting of FET,
The sources S of each of the switches SW ₁ and SW ₂ are connected to a measurement AC voltage source V of a measuring device (not shown). In addition, the substrate SB of each switch SW ₁ and SW ₂ is supplied with a predetermined bias voltage -V _S (usually the supply voltage -15V) via a resistor R ₁ .
On the other hand, a measuring AC voltage source V is connected as a drive voltage source V' via a capacitor _C1 , and a voltage -V _S +v' is applied. Furthermore, an opening/closing control signal V _G is applied to the gate G of the main switch SW ₁ via a resistor R ₂ , and a measurement AC voltage source V is connected via a capacitor C ₂ as a drive voltage source V'. , voltage V _G +ν' is applied. Further, a switching control signal _G is applied to the gate G of the auxiliary switch SW ₂ via an inverter 6 and a resistor R ₃ , and an AC voltage source V for measurement is connected as a drive voltage source V' via a capacitor C ₃ . and a voltage _G + ν' is applied. Auxiliary switch SW ₂
is the DUT 1 side of main switch SW ₁ , which was turned on and cut off when main switch SW ₁ was off,
That is, the drain D is connected to the measurement AC voltage source V, and the measurement AC voltage is applied to the drain D to eliminate the influence of the capacitance C _SD of the main switch SW ₁ when it is off.

In the circuit diagram shown in Fig. 4, if the impedance of the capacitor C ₁ due to the frequency of the measurement AC voltage source V is sufficiently small compared to the impedance of the resistor R ₁ , the substrate SB will have a voltage approximately equal to the measurement AC voltage source V. Therefore, the capacitance C _SSB・C _DSB
The effect of will disappear. In addition, the impedance of the capacitors C ₂ and C ₃ due to the frequency of the AC voltage source V for measurement is sufficiently small compared to the impedance of the resistors R ₂ and R ₃ , and is selected so as not to distort the waveform of the signal voltage V _G・_G too much. Then, each gate G becomes equal to the voltage mentioned above, and the influence of the capacitances C _SG and C _DG disappears. Furthermore, better characteristics may be exhibited if the AC voltage applied to each electrode is slightly lower than the measurement AC voltage; in such a case, the voltage may be divided appropriately using a capacitor.

Next, FIG. 5 shows another example of the use of this invention. This is a case where it is not preferable to directly apply the measurement AC voltage source V to the object under test 1 when the main switch SW ₁ is off. Adding the AC voltage source V for measurement to all of the points is the AC voltage source V
This may be the case if the burden is too great. In this case, as shown in the figure, the measurement AC voltage source V is connected to the device under test 1 via two switches SW ₁ and SW ₃ , and the sub switch SW ₂ is connected between the switches SW ₁ and SW ₃ . Just set it at a point.

Note that the above embodiment is an N-channel MOS-
Although FET is used in the explanation, other semiconductor switching elements such as J-FET may be used. J-
In the case of a FET, it can be considered that there is no substrate in an N-channel MOS-FET, so for example, the gate of the J-FET is connected to an AC voltage source for measurement, and the AC voltage for measurement is superimposed on the switching control signal voltage. In the case of a three-terminal (one with only three connection points, such as a transistor) semiconductor switching element, the control terminal is connected to the measurement AC voltage source V, similar to the J-FET. , a voltage is applied that is a superposition of the switching control signal and the voltage based on the AC voltage for measurement, and the voltage based on the AC voltage for measurement is applied to the connection terminal of the object to be measured by the switching means 5, which operates only when the semiconductor switching element is turned off. Just apply it. In addition, in the case of a semiconductor switching element with four or more terminals, for example, one composed of a plurality of diodes, the terminals other than the terminal to which the measured object is connected and the terminal to which the AC voltage for measurement is applied are connected in the same way as above. , a voltage based on the measurement AC voltage may be superimposed on the voltage to be applied to each terminal, and a voltage based on the measurement AC voltage may be applied to the terminal to which the object to be measured is connected when the semiconductor switching element is off.

Naturally, in the FET element, the source S and drain D have a symmetrical structure, so the drain D may be used as an input terminal and the source S may be used as an output terminal.

As described above, this invention connects an AC voltage source for measurement of a capacitance measuring instrument and an object to be measured to the input/output terminals of a semiconductor switching element with a control terminal, and connects the semiconductor switching element to which the object to be measured is connected. Either one of the input/output terminals of the input/output terminal and a drive voltage source that outputs an AC voltage of the same phase and voltage as the measurement AC voltage source are connected by an OFF signal of the opening/closing control signal applied to the control terminal of the semiconductor switching element. By connecting the semiconductor switching element through the operating switching means and connecting the driving voltage source to terminals other than the input/output terminals of the semiconductor switching element, it is possible to eliminate the capacitance existing between each electrode of the semiconductor switching element. This has the advantage that a switch with low on-resistance can be obtained, measurement errors caused by the changeover switch can be minimized, and a switch with high speed and high reliability can be obtained.

[Brief explanation of the drawing]

Figure 1a is a symbol diagram of an N-channel MOS-FET, Figures 1b and c are equivalent circuit diagrams of Figure 1a when they are on and off, Figure 2 is a principle diagram of the guard shield technique, and Figure 3 is a diagram of the principle of the guard shield technique. The figure is a principle diagram of a circuit showing an example of this invention, Fig. 4 is a circuit diagram showing a practical example of Fig. 3, and Fig. 5 is a simplified circuit diagram showing another example of this invention.
FIGS. 6 and 7 are schematic diagrams showing conventional examples. 1...Object to be measured, SW ₁ ...MOS-FET, SW ₂
...Switch means, D...Output terminal (drain),
G...Control terminal (gate), S...Input terminal (source), SB...Substrate, V...Measurement AC voltage source, V'...Drive voltage source.

Claims (1)

[Claims for Utility Model Registration] (1) An AC voltage source for measurement of a capacitance measuring device and an object to be measured are connected to input/output terminals of a semiconductor switching element with a control terminal, and the object to be measured is connected one of the input/output terminals of the semiconductor switching element and the same phase as the measurement AC voltage source,
and a drive voltage source that outputs an AC voltage of the same voltage.
It is characterized in that the switching means is connected via a switching means operated by an OFF signal of the opening/closing control signal applied to the control terminal of the semiconductor switching element, and the drive voltage source is connected to a terminal other than the input/output terminal of the semiconductor switching element. Switch for capacitance measurement. (2) A changeover switch for capacitance measurement according to claim 1, in which the semiconductor switching element is an insulated FET element. (3) The changeover switch for capacitance measurement according to claim 1, in which the semiconductor switching element is a junction FET element. (4) A changeover switch for capacitance measurement according to claim 1, wherein the semiconductor switching element is a transistor element.