JPH056544Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] This invention uses an RC oscillator circuit whose output frequency is preferably inversely proportional to the capacitance under test to measure low capacitance by eliminating the influence of stray capacitance. This relates to circuits that perform

[Prior Art] Conventionally, as a circuit for measuring capacitance, a method using a capacitance bridge, a leakage method, etc. are known, and these have been implemented in various forms.

[Problem to be solved by the invention] By the way, when using the well-known method, the capacitance of the measurement line and the stray capacitance are about the same size as the low capacitance to be measured, and this will affect the measurement results. ,
Measuring low volumes has been particularly difficult.

The object of this invention is therefore to overcome these drawbacks and provide a circuit for measuring low capacitances without any influence of stray capacitances. Another object of this invention is to provide a low-capacity measuring circuit in which even slight fluctuations in the voltage supplied to the measuring circuit do not affect measurement accuracy at all. Furthermore, another object of the invention is to provide a low capacitance measurement circuit suitable for use remotely, for example in radiosondes.

[Means for solving the problem] In order to achieve the above object, the circuit for measuring low capacitance according to the invention includes an inverting amplifier having an input and an output, and a connection between the input and output of the inverting amplifier. and a bistable oscillator circuit whose input is connected to the output of an inverting amplifier and whose output frequency is inversely proportional to the capacitance to be measured, and the output of this bistable oscillator circuit is connected to the input of the inverting amplifier. It is characterized by being connected so that feedback can be received via.

Embodiments In the following, embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings.

As shown in FIGS. 1, 2 and 3, the measuring circuit of the present invention has an inverting amplifier 1 with a connection point, an input A and an output B. The inverting amplifier 1 is connected to the input C of the bistable oscillator circuit 4 via a resistor R ₁ . As shown, the bistable oscillator circuit 4 comprises two inverting amplifiers 2, 3, the output D of which is fed back to its input via a resistor _R3 . The bistable oscillator circuit 4 has, for example, two voltage levels as a typical feature,
It can be constructed with a Schmitt trigger circuit having hysteresis between these voltage levels.

The capacitance to be measured C _M is connected between the input A and the output B of the inverting amplifier 1 . A frequency f=1/T is obtained from the output D of the bistable oscillation circuit 4. Since the output frequency of this bistable oscillation circuit 4 has a preferable relationship of being inversely proportional to the capacitance to be measured C _M , the capacitance to be measured is measured using this output frequency. The essential feature of the measuring circuit according to this invention is that the output D of the bistable oscillator circuit 4 is fed back to the input A of the inverting amplifier ₁ via a resistor R2. It is also important to note that the input A of the inverting amplifier 1 is capacitively separated (by a small capacitance) from the output B. Also, connection point A
It is also important that the resistive input impedance in the

The operation of the circuit described above is as follows. First, it is assumed that the output D is on the positive side of the input voltage. The current flowing through resistor R ₂ therefore causes the voltage at input A to rise. Since the amplifier 1 is of the inverting type, as the voltage at the input A rises, the voltage at the output B falls and, via the capacitance to be measured C _M , the voltage at the input A also falls. the result,
In an ideal situation, the voltage at input A would not change at all. The capacitance to be measured C _M is output D and input A
charged with a constant current determined by the voltage between and the resistor _R2 . When the voltage at output B falls to the low trigger level of the bistable oscillator circuit, i.e. the Schmitt trigger circuit 4 of the drawing, the state of the bistable oscillator circuit changes and the voltage at output D suddenly drops to the negative side of the supply voltage, at which point From then on the circuit begins to operate as described above, i.e. the current simply flows in the opposite direction.

What is important in the above description is that the voltage at input A does not change at all in all phases during the period. As a result, the stray capacitance C _H1 shown by the dotted line in FIG. 1, which is grounded at the input A, has no change in the output frequency f. The reason is that the voltage at input A does not change, so that the free capacitance C _H2 neither charges nor discharges. The case where the free capacitance C _H2 is connected to the output B is also the same as the above case because a high enough current is obtained in the inverting amplifier 1 to charge and discharge such free capacitance C _H2 .

Therefore, as shown in FIG. 2, it is recognized that the capacitance to be measured C _M can be measured, for example, via a pair of coaxial cables 5a and 5b that are relatively distant from the actual measurement circuit. In fact, this is advantageous when the capacitances to be measured are far from each other, for example in the case of radiosondes, and by using the circuit of this invention there is no loss of measurement accuracy, for example via coaxial cables. Various capacitances can be connected to the measurement circuit. This type of configuration was simply not possible with conventional circuits for measuring low capacitances.

Another important advantage of this invention is that it is possible to change the capacitance to be measured to another capacitance by means of a selector switch, which may be a mechanical or electronic switch, without any risk of stray capacitance effects in the measuring circuit. Possibly, these other capacities can be referenced, for example.

3 and 4 illustrate a preferred electronic selector switch for use for the above purpose. As shown in FIG. 3, the selector switch is provided with a CMOS differential amplifier device, and this device includes six amplifier units 7-7. The outputs b ₁ to b ₇ of each amplifier unit 7 to 7 are connected to the capacitances to be measured C _M1 to C _M6 , and the inputs a ₁ to _{a 6} are connected to a stepping circuit 8, which is connected by arrow 9. An impulse P ₁ is provided alternately to each input terminal a ₁ -a ₆ of the amplifier arrangement 6 via the circuit shown.

Figure 4 shows an example of the configuration of the simplest type of CMOS differential amplifier device, in which case the amplifier device has two transistors T _P and T _N , where the transistor T _P is a P-type MOS transistor. , transistor T _N is an N-type MOS transistor.
The resistance that makes the transistors T _P and _TN conductive is about 500Ω, and the resistance that makes the transistors T _P and _TN conductive is several tens of times higher. As shown in FIG. 3, there are six amplifier units of the type described above in one amplifier arrangement 6, the VDDs of all these amplifier units being interconnected and the VSSs also being interconnected.

In the above selector switch, as shown in Fig. 3, the drive voltage +U is supplied to the selector switch via the resistor _R4 , and this drive voltage is also supplied to the capacitor _C1 of the RC oscillation circuit according to the present invention. .
The resistance R ₄ is preferably on the order of 150 kΩ, and the capacitance C ₁ is preferably on the order of 10 nF, which of course must be sufficiently high so as not to affect the frequency to be measured. When the capacitance C _M to be measured is approximately several pF to several tens of pN, the value of the capacitance C ₁ is suitably approximately 10 nF. The above electronic selector switch has a capacity of C _M1
~C _M6 is connected to a measuring circuit as shown in Fig. 3, and operates to ground the other capacitance.

This invention is not limited to the details described above, which have been explained merely by way of example, but can be modified in various ways within the scope of the concept of the invention as set forth in the claims of the utility model registration.

[Effects of the invention] As explained above, according to this invention,
An RC oscillator circuit whose output frequency is inversely proportional to the capacitance to be measured is used, and the capacitance to be measured is connected between the low impedance generator and the circuit that measures only the current, i.e. between the input and output of the inverting amplifier. Since the measurement is carried out in the same manner, low capacitances can be measured without any influence of stray capacitance and without any small fluctuations in the voltage supplied to the measuring circuit having any effect on the measurement accuracy.

[Brief explanation of the drawing]

Fig. 1 shows an embodiment of the circuit according to this invention, Fig. 2 shows an application example of this invention in which the capacitance to be measured is remotely connected to the measurement circuit via two coaxial cables, and Fig. An application example of this invention is shown in which several capacitances to be measured are connected one by one to a measurement circuit via a special selector switch according to this invention. Figure 4 shows the basics of the CMOS differential amplifier shown in Figure 3. The configuration is shown in detail. In the figure, 1 is an inverting amplifier, 4 is a bistable oscillation circuit,
R ₁ and R ₂ are resistances, _CM is the capacitance to be measured, and C _H1 is the standard capacitance.

Claims (1)

[Claims for Utility Model Registration] 1. In a circuit for measuring low capacitance by eliminating the influence of stray capacitances C _H1 and C _H2 , an inverting amplifier 1 having an input A and an output B
and a capacitance to be measured C _M connected between the input A and output B of this inverting amplifier 1, and a bistable whose input is connected to the output B of the inverting amplifier 1 and whose output frequency is inversely proportional to the capacitance to be measured C _M. 1. An oscillation circuit 4, and an output of the bistable oscillation circuit 4 is connected to an input A of an inverting amplifier 1 so as to be fed back via a resistor _R2 . 2 The input A of the inverting amplifier 1 functions as a low impedance with low internal resistance, and the input A of the inverting amplifier 1
The measuring circuit according to claim 1, wherein the measuring circuit is configured such that the output B of the circuit functions as a circuit that measures only current. 3. The measuring circuit according to claim 1, wherein the bistable oscillation circuit 4 is composed of Schmitt trigger circuits 2, 3, and _R3 . 4 The capacitance to be measured C _M is located at a remote location from the inverting amplifier 1 and the bistable oscillator circuit 4, and the capacitance to be measured is
The measuring circuit according to claim 1, wherein C _M is connected to input A and output B of inverting amplifier 1 by a pair of coaxial cables 5a and 5b. 5 The capacitance to be measured C _M has a pair of spaced apart terminals,
Each of the coaxial cables 5a, 5b has an inner conductor and an outer conductor, the outer conductor of each coaxial cable is grounded, and one inner conductor of each coaxial cable connects one terminal of the capacitance to be measured C _M to the inverting amplifier 1. The measuring circuit according to claim 4, wherein the inner conductor of the other coaxial cable connects the other terminal of the capacitance to be measured C _M to the output of the inverting amplifier 1.