JPH02130014A - Temperature compensation circuit for capacity - Google Patents

Abstract

PURPOSE:To attain highly precise temperature compensation for capacity through simple circuit constitution by compensating the temperature characteristic of a capacity element by the temperature characteristic of the operating characteristic of a transistor(TR), and adjusting the driving voltage of a complementary MOS TR so as to prevent the existence of temperature dependency in oscillation frequency. CONSTITUTION:The temperature coefficient of the drain current of the MOSFET has a characteristic shown in a figure, and it can be changed optionally so as to be positive or negative according to the size of (gate voltage - threshold voltage). Here, since the gate voltage VG is nearly equal to the driving voltage VDD, the temperature coefficient of the drain current which compensates the temperature coefficient of the size of pressure detecting capacity CX and whose CR time constant has no temperature dependency can be set by adjusting the driving voltage VDD. Thus, the temperature dependency of the oscillation frequency can be removed by only adjusting the driving voltage.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a circuit for temperature compensation of a capacitive element,
It can be applied to capacitive pressure detectors, other detectors that detect physical quantities by changes in capacitance, and oscillation circuits whose period is determined by the capacitance of a capacitive element.

[Prior art]

Among various applications, when it comes to pressure sensors, there are two types of small silicon pressure sensors: piezoresistive type and capacitive type. The piezoresistive type constitutes a bridge circuit and detects pressure changes as changes in resistance value. On the other hand, the capacitive type detects pressure changes as capacitance changes. As a method to detect capacity, CR
One is to detect a capacitance change as a frequency change using an oscillation circuit, and the other is to apply an alternating current to a series connection circuit of a pressure sensing capacitor and a fixed capacitor, and detect the capacitance of the pressure sensing capacitor as the voltage across it.

[Problem to be solved by the invention]

Whether the force is a piezoresistance type or a capacitance type,
Resistance or capacitance has temperature dependence, and a circuit is required to compensate for this temperature characteristic. An object of the present invention is to achieve accurate capacitance temperature compensation with a simple circuit configuration and to realize a temperature compensation circuit that is easy to integrate.

[Means for Solving the 5 Problems] The structure of the invention for solving the above problems consists of a capacitive element and a complementary MOS type in which when one is in a conducting or blocking state, the other is in a blocking or conducting state. It is configured using a field effect transistor, the capacitive element and the complementary MOS field effect transistor, and the capacitive element is charged through one transistor of the complementary MOS field effect transistor. , and an oscillation circuit that oscillates at a frequency corresponding to the capacitance of the capacitive element by discharging through another transistor, the temperature characteristic of the capacitive element being compensated by the temperature characteristic of the operating characteristic of the transistor. Accordingly, the drive voltage of the complementary MOS transistor is adjusted so that the oscillation frequency does not have temperature dependence.

[Effect]

phase? ifi type MOS (Matal Oxide 5
ilicon) type field effect transistor (rC-
When one of the MOSPETs is in a conductive or cut-off state with respect to a common gate voltage, the other becomes a cut-off or conductive state. The capacitive element is C-MOSPET and this C-MOSPET.
An oscillation circuit that is connected so that when one transistor of the SFET is conductive, the capacitive element is charged, and when the other transistor is conductive, the charge of the capacitive element is discharged, and the capacitive element is continuously charged and discharged. It is formed. and,
The oscillation frequency at this time is determined by the CR time constants of the charging circuit and the discharging circuit, so the oscillation frequency is inversely proportional to the capacitance of the capacitive element. However, the CR time constant has temperature dependence due to the temperature dependence of the capacitive element, and the oscillation frequency also changes depending on the temperature. On the other hand, it is known that the temperature coefficient of the drain current during conduction of a C-MOSFET depends on the gate voltage-threshold voltage, and in particular, if the gate voltage-threshold voltage is changed, can also be negative. Therefore, the pressure detection capacitor has temperature characteristics, but by adjusting the drive voltage (gate voltage, drive voltage that can change the gate voltage) including the gate voltage of the C-MOSFET, the gate voltage - threshold value of the C-MOSFET can be adjusted. By adjusting the voltage, the temperature coefficient of the drain current during conduction can be adjusted, and the CR time constant can be made constant regardless of temperature. The present invention is characterized in that the drive voltage of the C-MOSFET is adjusted so as to eliminate the temperature dependence of the CR time constant. Therefore, the temperature dependence of the oscillation frequency can be eliminated simply by adjusting the drive voltage.

【Example】

The present invention will be described below based on specific examples. This embodiment relates to a capacitive pressure detection circuit. FIG. 1 is a circuit diagram showing a capacitance detection circuit according to a first embodiment of the present invention. Transistor Tri and transistor Tr2. Transistor Tr3 and transistor Tr4. Transistor Tr
5 and transistor Tr8. Transistor Tr6 and transistor Tr9. The transistor Tr7 and the transistor Tr10 are each a C40SFET, and when one transistor is in a conductive or cutoff state, the other transistor is in a cutoff or conduction state, respectively. Also, C. is a pressure detection capacitance whose capacitance changes according to externally detected pressure. The C-MOSF [IT, which is composed of the transistor Tr3 and the transistor Tr4, is a circuit for charging or discharging the pressure detection capacitor C. When the transistor Tr4 is conductive, current flows from the power source v0° to the pressure sensing capacitor ff1c, and the pressure sensing capacitor C is charged from the power source V,n.
The potential vX at one end of the pressure detection capacitor C is the transistor Tr.
4 and the pressure detection capacitor CX. On the other hand, when the transistor Tr3 is in a conductive state, the charge charged in the pressure detection capacitor C8 is discharged via the transistor Tr3, and the potential VX is an index of a time constant determined by the on-resistance of the transistor Tr3 and the pressure detection capacitor C8. Increase with function. Further, the transistors Tr5 to TrlO are Schmitt trigger circuits, and when the input potential Vx exceeds the upper limit value Vu, the transistors Try and Tr9 are turned on, and the transistors Tr5. Tr6 is turned off and the input potential V
When X falls below the lower limit vL, transistors Tr8. T
r9 is turned off, and transistors Tr5. Tr6 is turned on. Then, when the output potential Vs is at the L level, the transistor Tr7 is turned on and the transistor TrlO is turned off, and when the output potential Vs is at the H level, the transistor Tr7 is turned off and the transistor T10 is turned on, so that the Schmitt trigger circuit is activated. The transistors Trll and Tr12 are output transistors.
The potential Vs is inverted and output. Further, the potential Vs is fed back to the transistors Tri and Tr2, and the transistor Tri to the transistor TrlO form an oscillation circuit. In the circuit with the above configuration, the potential vs, the potential vX, the potential V
The waveform of ouL is shown in FIG. Potential vX in Fig. 2
is the pressure detection capacitor flcx and the transistor Tr3. Tr
It increases or decreases in relation to the time constant determined by the on-resistance of 4. Therefore, the output potential V. Regarding the detected pressure, the oscillation frequency of the transistor Tr3. Since the on-resistance of Tr4 remains unchanged, it is inversely proportional to the size of the pressure detection capacitance Cx, and the size of the capacitance, that is, the pressure can be detected from the size of this frequency. On the other hand, the temperature coefficient of the drain current of MOSPET is the 6th
It has the characteristics shown in the figure, and can be arbitrarily changed to be positive or negative depending on the magnitude of the gate voltage-threshold voltage. In the circuit with the above configuration, the gate voltage vG is approximately equal to the drive voltage vno, so the temperature coefficient of the drain current is compensated for the temperature coefficient of the pressure detection capacitance Cx so that the CR time constant does not have temperature dependence. is the driving voltage V. . It can be set by adjusting. When the temperature coefficient of drain current was measured, it was found that n-MOS
PET telt, gate voltage = threshold voltage + 1.5Vf
7), in the p-10s FET, the temperature coefficient became zero when the gate voltage = threshold voltage - 1°Ov. Therefore, when the relationship between the driving voltage Vlli1 and the oscillation frequency f was measured while changing the temperature from 20° C. to 50° C., the characteristics shown in FIG. 3 were obtained. As a result, it was found that the oscillation frequency had no temperature dependence when the drive voltage VDD was about 2.5V. In this way, the drive voltage V may be experimentally set to a value that has no temperature dependence of the oscillation frequency. In addition to using the Schmitt trigger circuit described above, the oscillation circuit may also be configured using an unstable multivibrator shown in FIG. That is, the transistor Tr2 constituting the C-MOSFBT
0°Tr21 and amplifier A1. A2 and pressure detection capacity C
x constitutes an unstable multivibrator. That is, the pressure detection capacitor C1I is charged by the conduction of the transistor Tr20, and discharged by the conduction of the transistor Tr21. Similarly, when the relationship between the driving voltage VD++ and the oscillation frequency f was measured while changing the temperature at 10° C., 21° C., and 41° C., the characteristics shown in FIG. 5 were obtained. As a result, the drive voltage V. is about 3. It was found that the oscillation frequency had no temperature dependence when Ov. In this way, the drive voltage is experimentally set to a value that has no temperature dependence of the oscillation frequency. As described above, the present invention is a circuit aimed at temperature compensation of capacitance, so it can be used in a pressure detection circuit of a capacitance type pressure sensor, as well as a detection circuit of a sensor that detects a physical quantity as a capacitance change, and a capacitance. It can be used in oscillation circuits with good temperature stability in which the oscillation frequency is determined by the element.

【Effect of the invention】

The present invention provides a charging/discharging circuit for a capacitive element using a complementary MO
It is constructed using S-type field effect transistors and has an oscillation circuit that oscillates at a period corresponding to the time constant of the charge/discharge circuit, and compensates for the temperature variation of the capacitive element using the temperature characteristics of the transistor's operating characteristics. As a result, the drive voltage of the complementary MOS transistor is adjusted so that there is no temperature dependence of the oscillation frequency, making it possible to compensate for the temperature of the capacitive element with a simple circuit and facilitating integration. Particularly when used in a capacitive pressure detection circuit, highly accurate pressure measurement independent of temperature becomes possible. Furthermore, since it is composed of complementary MOS transistors, etc., integration becomes easy.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a capacitance detection circuit according to a specific embodiment of the present invention, FIG. 2 is a timing chart showing its operating characteristics, and FIG. 3 is a diagram showing the oscillation frequency and driving voltage of the circuit of the embodiment. Measurement diagram showing the relationship between using temperature as a parameter, No. 4
The figure is a circuit diagram showing a capacitance detection circuit according to another embodiment, FIG. 5 is a measurement diagram showing the relationship between the oscillation frequency and drive voltage by the capacitance detection circuit using temperature as a parameter, and FIG. 6 is a 1103F [! FIG. 3 is a characteristic diagram showing the voltage dependence of the temperature coefficient of the drain current of T. ri ~Tr12゜Tr20゜

Claims (1)

[Scope of Claims] A capacitive element, a complementary MOS field effect transistor in which when one is in a conducting or blocking state, the other is in a blocking or conducting state, the capacitive element and the complementary MOS field effect transistor. The capacitor is charged through one transistor of the complementary MOS field effect transistor and discharged through the other transistor, thereby increasing the capacitance. an oscillation circuit that oscillates at a frequency according to the capacitance of the element, and compensates the temperature characteristics of the capacitive element with the temperature characteristics of the operating characteristics of the transistor so that the oscillation frequency does not have temperature dependence; A capacitive temperature compensation circuit characterized by adjusting the driving voltage of complementary MOS transistors.