JP2021089155A - Gas sensor - Google Patents

Abstract

To automatically change current flowing in a heater resistor according to an environmental temperature.SOLUTION: A gas sensor 10A includes a gas sensor part S1 including a thermistor Rd1 and a heater resistor MH1, a temperature sensor part S3, and a voltage application part V1 for applying a control voltage Vmh1 to the heater resistor MH1. The voltage application part V1 includes an analog buffer A1 that linearly changes the control voltage Vmh1 based on the output voltage Va of the temperature sensor part S3. The current flowing through the heater resistor MH1 can thus be automatically changed according to the environmental temperature without performing digital processing or the like.SELECTED DRAWING: Figure 1

Description

The present invention relates to a gas sensor that detects gas contained in an atmosphere, and more particularly to a gas sensor having a heater resistance that heats a temperature measuring body such as a thermistor.

The gas sensor detects the concentration of the gas to be measured contained in the atmosphere. Among them, the gas sensor of the type that heats a temperature measuring body such as a thermistor by a heater resistance is excellent in miniaturization. For example, the gas sensor described in Patent Document 1 connects a thermistor heated by a heater resistor and a reference resistor in series, and detects the concentration of the gas to be measured based on the potential of the connection point.

The gas sensor described in Patent Document 1 further includes an environmental temperature measuring element. Then, the thermistor is heated at a constant temperature regardless of the environmental temperature by finely adjusting the amount of current flowing through the heater resistor according to the environmental temperature obtained by the environmental temperature measuring element. In the gas sensor described in Patent Document 1, the voltage value output from the environmental temperature measuring element is A / D converted, and the indicated value indicating the amount of current flowing through the heater resistor is calculated based on the obtained digital value. Further, the indicated value is D / A converted to generate a current flowing through the heater resistor.

JP-A-2017-9472

However, in the method described in Patent Document 1, it is necessary to increase the number of bits of the A / D converter and the D / A converter in order to control the current flowing through the heater resistor with high accuracy, and the circuit scale becomes large. There was a problem. In addition, it is not always easy to control the current flowing through the heater resistor with high accuracy because an error occurs in the calculation of the indicated value. Moreover, since a predetermined time is required for A / D conversion, calculation, and D / A conversion, it is not easy to increase the response speed to a change in the environmental temperature.

Therefore, an object of the present invention is to automatically change the amount of current flowing through the heater resistor according to the environmental temperature in a gas sensor of a type that heats a resistance temperature detector such as a thermistor by a heater resistor without performing digital processing. And.

The gas sensor according to the present invention includes a first gas sensor unit including a first thermometer whose resistance value changes according to the concentration of the gas to be measured and a first heater resistance for heating the first thermometer, and an environment. A temperature sensor unit including a second thermometer whose resistance value changes according to temperature, and a first voltage application unit that applies a first control voltage to the first heater resistance are provided, and a first voltage is provided. The application unit includes a first buffer circuit that changes the first control voltage based on the output voltage of the temperature sensor unit.

According to the present invention, since the first control voltage is linearly changed by using the first buffer circuit based on the output voltage of the temperature sensor unit, the first heater is not subjected to digital processing or the like. The amount of current flowing through the resistor can be automatically changed according to the ambient temperature. This makes it possible to heat the first resistance temperature detector to a constant temperature regardless of the environmental temperature.

In the present invention, the first voltage application unit includes a first voltage dividing circuit that generates a first adjustment voltage based on the output voltage of the temperature sensor unit, and a first reference voltage that generates a first reference voltage. The first buffer circuit, further including a source, may generate a first control voltage by adding or subtracting a first adjustment voltage with respect to a first reference voltage. According to this, it is possible to adjust the amount of change of the first adjustment voltage according to the environmental temperature by the voltage division ratio of the first voltage divider circuit.

The gas sensor according to the present invention includes a first switch circuit connected between the first buffer circuit and the temperature sensor unit, and a second switch connected between the first buffer circuit and the first reference voltage source. It may be further provided with a circuit. According to this, since the first control voltage can be intermittently applied to the first heater resistor, it is possible to reduce the power consumption.

In the gas sensor according to the present invention, a second control voltage is applied to the second gas sensor unit including the third thermometer and the second heater resistor for heating the third thermometer, and the second heater resistor. A second voltage application unit is further provided, the first temperature measuring body and the third temperature measuring body are connected in series, and the second voltage application unit is a second based on the output voltage of the temperature sensor unit. A second buffer circuit that linearly changes the control voltage is included, and the first control voltage and the second control voltage have different values from each other, whereby the first temperature measuring body and the third temperature measuring body are mutually different. It may be heated to a different temperature. According to this, it is possible to obtain a detection signal according to the concentration of the gas to be measured from the connection point between the first temperature measuring body and the third temperature measuring body.

In the present invention, the second voltage application unit includes a second voltage dividing circuit that generates a second adjustment voltage based on the output voltage of the temperature sensor unit, and a second reference voltage that generates a second reference voltage. The second buffer circuit, further including the source, may generate a second control voltage by adding or subtracting a second adjustment voltage with respect to the second reference voltage. According to this, it is possible to adjust the amount of change of the second adjustment voltage according to the environmental temperature by the voltage division ratio of the second voltage divider circuit.

In the present invention, the voltage dividing ratios of the first voltage dividing circuit and the second voltage dividing circuit may be different from each other. According to this, it is possible to individually design the amount of change of the first and second adjustment voltages according to the environmental temperature.

The gas sensor according to the present invention further includes a third switch circuit connected between the second buffer circuit and the second reference voltage source, and the first switch circuit includes the first and second buffer circuits and the temperature. It may be connected between the sensor units. According to this, since the first and second control voltages can be intermittently applied to the first and second heater resistors, respectively, it is possible to reduce the power consumption.

The gas sensor according to the present invention may further include a sample hold circuit connected between the first buffer circuit and the first heater resistor to hold the first control voltage. According to this, it is possible to fix the first control voltage during heating using the first heater resistor.

In the present invention, the temperature sensor unit further includes a first resistor connected in series to the second resistance temperature detector and a second resistor connected in parallel to the second resistance temperature detector. The output voltage may be the voltage at the connection point between the second resistance temperature detector and the first resistor. According to this, the amount of change in the output voltage according to the environmental temperature can be adjusted by the resistance values of the first and second resistors.

As described above, according to the gas sensor according to the present invention, it is possible to automatically change the amount of current flowing through the heater resistor according to the ambient temperature without performing digital processing. Therefore, it is possible to control the current flowing through the heater resistor with high accuracy and high speed in response to a change in the environmental temperature while reducing the circuit scale.

FIG. 1 is a circuit diagram showing a configuration of a gas sensor 10A according to the first embodiment of the present invention. FIG. 2 is a top view for explaining the configuration of the sensor unit S. FIG. 3 is a cross-sectional view taken along the line AA shown in FIG. FIG. 4 is a graph showing the relationship between the environmental temperature and the voltages Va, Vb, Vd, and Vmh1. FIG. 5 is a graph showing the relationship between the environmental temperature and the voltages Va, Vc, Ve, and Vmh2. Figure 6 is a graph showing the relationship between the environmental temperature and voltage _Vmh1, Vmh2, _{V CO2.} FIG. 7 is a timing diagram showing an example of the control timing of the switch circuits SW1 to SW3 in the first embodiment. FIG. 8 is a circuit diagram showing the configuration of the gas sensor 10B according to the second embodiment of the present invention. FIG. 9 is a timing diagram showing an example of the control timing of the switch circuits SW1 to SW3 in the second embodiment.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing a configuration of a gas sensor 10A according to the first embodiment of the present invention.

As shown in FIG. 1, the gas sensor 10A according to the first embodiment includes a sensor unit S and a signal processing circuit 20. Although not particularly limited, the gas sensor 10A according to the present embodiment detects the concentration _{of CO 2 gas in the atmosphere.}

The sensor unit S is _{a heat conduction type gas sensor for detecting the concentration of CO 2} gas, which is a gas to be detected, and has a first gas sensor unit S1, a second gas sensor unit S2, and a temperature sensor unit S3. There is. The first gas sensor unit S1 includes a thermistor Rd1 which is a first temperature measuring body and a heater resistor MH1 for heating the thermistor Rd1. Similarly, the second gas sensor unit S2 includes a thermistor Rd2 which is a third temperature measuring body and a heater resistor MH2 which heats the thermistor Rd2. On the other hand, the temperature sensor unit S3 includes a thermistor Rd3 which is a second temperature measuring body.

As shown in FIG. 1, the thermistors Rd1 and Rd2 are connected in series between the wiring to which the power supply potential Vcc is supplied and the wiring to which the ground potential GND is supplied. On the other hand, the thermistor Rd3 and the resistor R1 are connected in series between the wiring to which the power supply potential Vcc is supplied and the wiring to which the ground potential GND is supplied, and the resistor R2 is connected in parallel with the thermistor Rd3. There is. Thermistors Rd1 to Rd3 are made of materials having a negative temperature coefficient of resistance, such as composite metal oxides, amorphous silicon, polysilicon, and germanium. Of these, the thermistors Rd1 and Rd2 both _{detect the concentration of CO 2} gas, but their operating temperatures are different from each other as described later.

The thermistor Rd1 is heated by the heater resistor MH1. The heating temperature of the thermistor Rd1 by the heater resistor MH1 is, for example, 150 ° C. _{If CO 2} gas is present in the measurement atmosphere while the thermistor Rd1 is heated, the heat dissipation characteristics of the thermistor Rd1 change according to its concentration. Such a change appears as a change in the resistance value of the thermistor Rd1. When the heating temperature of the thermistor Rd1 is 150 ° C., the resistance value of the thermistor Rd1 changes with the first sensitivity according to the concentration of _{CO 2 gas.} The first sensitivity has a sensitivity capable of sufficiently changing the _{detection voltage V CO2} appearing at the connection point between the thermistor Rd1 and the thermistor Rd2. Further, when water vapor is present in the measurement atmosphere, the heat dissipation characteristics of the thermistor Rd1 change according to the concentration thereof.

The thermistor Rd2 is heated by the heater resistor MH2. The heating temperature of the thermistor Rd2 by the heater resistor MH2 is, for example, 300 ° C. _{Even if CO 2} gas is present in the measurement atmosphere while the thermistor Rd2 is heated, the resistance value of the thermistor Rd2 hardly changes. This is because when the heating temperature of the thermistor Rd2 is 300 ° C., the resistance value of the thermistor Rd2 _{changes with the second sensitivity according to the concentration of CO 2} gas, but the second sensitivity is higher than the first sensitivity. This is because it is significantly lower, preferably 1/10 or less of the first sensitivity, and more preferably almost zero. Therefore, _{even if the concentration of CO 2} gas changes, the resistance value of the thermistor Rd2 hardly changes. On the other hand, when water vapor is present in the measurement atmosphere, the heat dissipation characteristics of the thermistor Rd2 change according to its concentration.

As described above, the thermistor Rd1 and the thermistor Rd2 are connected in series, and the detection voltage V _CO2 is output from the connection point. On the other hand, the output voltage Va of the temperature sensor unit S3 is output from the connection point between the thermistor Rd3 and the resistor R1. Although it is not essential to provide the resistor R2 in the temperature sensor unit S3, by connecting the resistor R2 in parallel to the thermistor Rd3, it is possible to improve the linearity of the output voltage Va with respect to the temperature change. The detection voltage V _CO2 and the output voltage Va are input to the signal processing circuit 20.

The signal processing circuit 20 includes voltage application units V1 and V2, a buffer 25, a differential amplifier 26, an AD converter (ADC) 27, a DA converter (DAC) 28, a control unit 29, and switch circuits SW1 to SW3. The buffer 25 generates a temperature signal Vtemp by buffering the output voltage Va. Further, the differential amplifier 26 _{compares the detected voltage V CO2} with the reference voltage Vref and amplifies the difference. The temperature signal Vtemp output from the buffer 25 and the gas detection signal Vamp output from the differential amplifier 26 are input to the AD converter 27.

The AD converter 27 digitally converts the temperature signal Vtemp and the gas detection signal Vamp, and supplies the values to the control unit 29. On the other hand, the DA converter 28 generates a reference voltage Vref by analog-converting the reference signal supplied from the control unit 29. The control unit 29 generates an output signal Vout indicating the _{current concentration of CO 2} gas based on the digitally converted gas detection signal Vamp. The control unit 29 also controls the switch circuits SW1 to SW3.

The voltage application unit V1 is a circuit for generating a control voltage Vmh1 applied to the heater resistor MH1, and includes a voltage dividing circuit 21, a reference voltage source 23, and an analog buffer A1. Similarly, the voltage application unit V2 is a circuit for generating a control voltage Vmh2 applied to the heater resistor MH2, and includes a voltage dividing circuit 22, a reference voltage source 24, and an analog buffer A2.

The voltage dividing circuit 21 included in the voltage applying unit V1 generates the adjusting voltage Vb when the switch circuit SW3 is in the ON state. The voltage dividing circuit 21 has a configuration in which resistors R3 and R4 are connected in series, and an adjusted voltage Vb is obtained from the connection points of the resistors R3 and R4. The adjustment voltage Vb is supplied to the analog buffer A1. Further, the reference voltage source 23 generates a reference voltage Vd. The reference voltage Vd is supplied to the analog buffer A1 when the switch circuit SW1 is in the ON state. The analog buffer A1 generates a control voltage Vmh1 by subtracting the adjustment voltage Vb from the reference voltage Vd (Vmh1 = Vd−Vb). The control voltage Vmh1 is applied to the heater resistor MH1.

The voltage dividing circuit 22 included in the voltage applying unit V2 generates the adjusting voltage Vc when the switch circuit SW3 is in the ON state. The voltage dividing circuit 22 has a configuration in which resistors R5 and R6 are connected in series, and an adjusted voltage Vc is obtained from the connection points of the resistors R5 and R6. The adjustment voltage Vc is supplied to the analog buffer A2. Further, the reference voltage source 24 generates a reference voltage Ve. The reference voltage Ve is supplied to the analog buffer A2 when the switch circuit SW2 is in the ON state. The analog buffer A2 generates the control voltage Vmh2 by subtracting the adjustment voltage Vc from the reference voltage Ve (Vmh2 = Ve−Vc). The control voltage Vmh2 is applied to the heater resistor MH2.

FIG. 2 is a top view for explaining the configuration of the sensor unit S. Further, FIG. 3 is a cross-sectional view taken along the line AA shown in FIG. The drawings are schematic, and for convenience of explanation, the relationship between the thickness and the plane dimension, the ratio of the thickness between the devices, and the like are different from the actual structure within the range where the effect of the present embodiment can be obtained. It doesn't matter if you do.

The sensor unit S is a heat conduction type gas sensor that detects the gas concentration based on the change in heat dissipation characteristics according to the concentration of _{CO 2 gas, and as shown in FIGS. 2 and 3, the two gas sensor units S1 and S2} A temperature sensor unit S3 arranged between the gas sensor unit S1 and the gas sensor unit S2, and a ceramic package 51 accommodating these sensor units S1 to S3 are provided.

The ceramic package 51 is a box-shaped case having an open upper portion, and a lid 52 is provided on the upper portion. The lid 52 has a plurality of vents 53, which allows CO ₂ gas in the atmosphere to flow into the ceramic package 51. The lid 52 is omitted in FIG. 2 in consideration of the legibility of the drawing.

Although not particularly limited, in the present embodiment, three sensor units S1 to S3 are integrated on a single substrate 61. Three cavities 61a to 61c corresponding to the three sensor units S1 to S3 are formed on the substrate 61.

The substrate 61 protects the insulating films 62 and 63, the heater resistors MH1 and MH2 provided on the insulating films 63, the heater protective films 64 covering the heater resistors MH1 and MH2, and the heater protection films 64 at positions overlapping the cavities 61a to 61c, respectively. The thermistors Rd1 to Rd3 and thermistor electrodes 35, 45, 65 provided on the film 64, and a thermistor protective film 66 covering the thermistors Rd1 to Rd3 and the thermistor electrodes 35, 45, 65 are provided.

The substrate 61 is not particularly limited as long as it has an appropriate mechanical strength and is a material suitable for fine processing such as etching, and is not particularly limited, and is a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, or a quartz substrate. , Glass substrate and the like can be used. The insulating films 62 and 63 are made of an insulating material such as silicon oxide or silicon nitride. The heater resistors MH1 and MH2 are metal materials made of materials having a relatively high melting point, for example, molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), and the like. Iridium (Ir) or an alloy containing any two or more of these is suitable. Thermistors Rd1 to Rd3 are made of materials having a negative temperature coefficient of resistance, such as composite metal oxides, amorphous silicon, polysilicon, and germanium. Here, the thermistor is used as the resistance temperature detector because the resistance temperature coefficient is larger than that of the platinum resistance temperature detector and the like, so that a large detection sensitivity can be obtained. As the material of the heater protective film 64, the same material as that of the insulating film 63 can be used.

Thermistor electrodes 35, 45, 65 are a pair of electrodes having a predetermined interval, the thermistor Rd1 is provided between the pair of thermistor electrodes 35, the thermistor Rd2 is provided between the pair of thermistor electrodes 45, and the pair of thermistors. A thermistor Rd3 is provided between the electrodes 65. As a result, the resistance value between the pair of thermistor electrodes 35, 45, 65 is determined by the resistance value of the thermistors Rd1 to Rd3, respectively. The materials of the thermista electrodes 35, 45, 65 include molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or any of these. An alloy containing two or more kinds is suitable.

As shown in FIG. 2, both ends of the heater resistor MH1 are connected to the electrode pads 37a and 37b, respectively, and both ends of the heater resistor MH2 are connected to the electrode pads 47a and 47b, respectively. Further, both ends of the thermistor electrode 35 are connected to electrode pads 37c and 37d, respectively, both ends of the thermistor electrode 45 are connected to electrode pads 47c and 47d, respectively, and both ends of the thermistor electrode 65 are connected to electrode pads 67a and 67b, respectively. .. These electrode pads are connected to the package electrode 54 provided on the ceramic package 51 via the bonding wire 55. The package electrode 54 is connected to the signal processing circuit 20 shown in FIG. 1 via an external terminal 56 provided on the back surface of the ceramic package 51.

The above is the configuration of the gas sensor 10A according to the present embodiment. Next, the operation of the gas sensor 10A according to the present embodiment will be described.

The gas sensor 10A according to the present embodiment _{utilizes the fact that the thermal conductivity of CO 2} gas is significantly different from the thermal conductivity of air, and the gas detection voltage changes the heat dissipation characteristics of the thermistas Rd1 and Rd2 depending on the concentration of _{CO 2 gas.} Take out as _{V CO2.} However, since the thermal conductivity of the measurement atmosphere _{changes not only with the concentration of CO 2} gas but also with the humidity, that is, the concentration of water vapor, the influence of humidity causes a measurement error. Therefore, in the gas sensor 10A according to the present embodiment, the thermistor Rd1 is heated to, for example, 150 ° C. by using the heater resistor MH1, and the thermistor Rd2 is heated to, for example, 300 ° C. by using the heater resistor MH2. When the heating temperature is 150 ° C, _{the heat dissipation characteristics of the thermistor Rd1 change greatly depending on the concentration of CO 2} gas, whereas when the heating temperature is 300 ° C, the heat dissipation characteristics of the thermistor Rd2 _{change greatly according to the concentration of CO 2 gas.} The heat dissipation characteristics hardly change. On the other hand, when the heating temperature is 150 ° C. or 300 ° C., the heat dissipation characteristics of the thermistors Rd1 and Rd2 change depending on the humidity. Therefore, by connecting the thermistors Rd1 and Rd2 in series, the influence of humidity is canceled. can do. Here, when there is a difference in the sensitivities of the thermistors Rd1 and Rd2 with respect to humidity, it is possible to reduce the difference in sensitivity by connecting a correction resistor in parallel with the thermistor Rd1 or thermistor Rd2.

Further, in the gas sensor 10A according to the present embodiment, the levels of the control voltages Vmh1 and Vmh2 applied to the heater resistors MH1 and MH2 are automatically adjusted by the voltage application units V1 and V2, respectively, even if the environmental temperature changes.

FIG. 4 is a graph showing the relationship between the environmental temperature and the voltages Va, Vb, Vd, and Vmh1, and FIG. 5 is a graph showing the relationship between the environmental temperature and the voltages Va, Vc, Ve, and Vmh2.

As shown in FIG. 4, when the environmental temperature rises, the resistance value of the thermistor Rd3 decreases, so that the output voltage Va of the temperature sensor unit S3 rises. The output voltage Va is divided by the voltage dividing circuit 21 at a predetermined voltage dividing ratio, and the adjusted voltage Vb is generated. The adjustment voltage Vb also increases according to the environmental temperature, but since the voltage is divided by the voltage dividing circuit 21, the change according to the level of the adjustment voltage Vb and the environmental temperature becomes smaller than the output voltage Va. Then, the adjustment voltage Vb is subtracted from the reference voltage Vd by the analog buffer A1 (Vd−Vb), and the control voltage Vmh1 is generated. As a result, the level of the control voltage Vmh1 decreases as the environmental temperature rises, so that the thermistor Rd1 can be heated at a constant temperature (for example, 150 ° C.) regardless of the environmental temperature.

Similarly, the output voltage Va is divided by the voltage dividing circuit 22 at a predetermined voltage dividing ratio, and the adjusted voltage Vc is generated. As shown in FIG. 5, the adjustment voltage Vc also increases according to the environmental temperature, but since the voltage is divided by the voltage dividing circuit 22, the level of the adjustment voltage Vc and the change according to the environmental temperature are different from the output voltage Va. Also becomes smaller. Then, the adjustment voltage Vc is subtracted from the reference voltage Ve by the analog buffer A2 (Ve-Vc), and the control voltage Vmh2 is generated. As a result, the level of the control voltage Vmh2 decreases as the environmental temperature rises, so that the thermistor Rd2 can be heated at a constant temperature (for example, 300 ° C.) regardless of the environmental temperature.

Figure 6 is a graph showing the relationship between the environmental temperature and voltage _Vmh1, Vmh2, _{V CO2.}

The changes in the control voltages Vmh1 and Vmh2 shown in FIG. 6 are the same as those in FIGS. 4 and 5, and become lower as the environmental temperature increases. As a result, the thermistors Rd1 and Rd2 are heated at a desired constant temperature _{regardless of the environmental temperature. Therefore, if the concentration of CO 2} gas contained in the measurement atmosphere is constant, gas detection is performed regardless of the environmental temperature. The level of voltage V _{CO2 is constant.}

As described above, according to the present embodiment, the thermistors Rd1 and Rd2 can be heated at a desired constant temperature regardless of the environmental temperature. Moreover, since the control voltages Vmh1 and Vmh2 according to the temperature change are not adjusted by digital processing but linearly by using a voltage dividing circuit and an analog buffer, it is caused by the resolution of the DA converter or AD converter. Not only does the error of the control voltages Vmh1 and Vmh2 not occur, but also the control voltages Vmh1 and Vmh2 can be adjusted at an extremely high speed.

FIG. 7 is a timing diagram showing an example of control timing of the switch circuits SW1 to SW3. In the example shown in FIG. 7, by turning on the switch circuits SW1 to SW3 simultaneously and intermittently, the adjustment voltages Vb and Vc are intermittently generated, and the reference voltages Vd and Ve are intermittently generated in the analog buffers A1 and A2. You are typing in. As a result, the control voltages Vmh1 and Vmh2 are generated only during the period when the adjustment voltages Vb and Vc and the reference voltages Vd and Ve are generated, so that the power consumption can be reduced.

FIG. 8 is a circuit diagram showing the configuration of the gas sensor 10B according to the second embodiment of the present invention.

As shown in FIG. 8, the gas sensor 10B according to the second embodiment is different from the gas sensor 10A according to the first embodiment shown in FIG. 1 in that sample hold circuits SH1 and SH2 are added. Since the other basic configurations are the same as those of the gas sensor 10A according to the first embodiment, the same elements are designated by the same reference numerals, and duplicate description will be omitted.

The sample hold circuit SH1 is connected between the analog buffer A1 and the heater resistor MH1, and performs a sampling operation and a hold operation of the control voltage Vmh1 under the control of the control unit 29. Similarly, the sample hold circuit SH2 is connected between the analog buffer A2 and the heater resistor MH2, and performs a sampling operation and a hold operation of the control voltage Vmh2 under the control of the control unit 29.

FIG. 9 is a timing diagram showing an example of the control timing of the switch circuits SW1 to SW3 in the second embodiment. In the example shown in FIG. 9, the on-time of the switch circuits SW1 to SW3 is shortened as compared with the example shown in FIG. 7, and the sample hold circuits SH1 and SH2 are used during the period when the switch circuits SW1 to SW3 are on. Sampling operation is being performed. Then, after performing the sampling operation, the sampled control voltages Vmh1 and Vmh2 are output by performing the hold operation for a certain period of time. According to this, even if the output voltage Va of the temperature sensor unit S3 changes (rises) due to the heating operation by the heater resistors MH1 and MH2, the control voltages Vmh1 and Vmh2 can be kept constant, respectively. Further, even if the environmental temperature changes during the hold period, the control voltages Vmh1 and Vmh2 are kept constant, so that stable control can be performed.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention, and these are also the present invention. Needless to say, it is included in the range.

For example, in the above embodiment _{, the case where the measurement target gas is CO 2} gas has been described as an example, but the present invention is not limited thereto. Further, it is not essential that the sensor unit used in the present invention is a heat conduction type sensor, and another type of sensor such as a contact combustion type sensor may be used.

Further, in the above embodiment, the adjustment voltage is subtracted from the reference voltage by the analog buffer, but the reference voltage and the adjustment voltage may be added. For example, when a material having a positive temperature coefficient of resistance such as a platinum temperature detector is used for the temperature sensor unit S3, or when the positions of the thermistor Rd3 and the resistor R1 are exchanged, the output voltage Va increases as the environmental temperature increases. When designed to be low, the heater resistance can be heated at a constant temperature by adding the reference voltage and the adjustment voltage with the analog buffer.

10A, 10B Gas sensor 20 Signal processing circuit 21 and 22 Voltage division circuit 23,24 Reference voltage source 25 Buffer 26 Differential amplifier 27 AD converter 28 DA converter 29 Control unit 35, 45, 65 Thermista electrodes 37a to 37d, 47a to 47d, 67a, 67b Electrode pad 51 Ceramic package 52 Lid 53 Vent 54 Package electrode 55 Bonding wire 56 External terminal 61 Substrate 61a to 61c Cavity 62, 63 Insulation film 64 Heater protection film 65 Thermista electrode 66 Thermista protection film A1, A2 Analog buffer ( Buffer circuit)
MH1, MH2 Heater resistance R1 to R6 Resistance Rd1 to Rd3 Thermistor S Sensor unit S1 First gas sensor unit S2 Second gas sensor unit S3 Temperature sensor unit SH1, SH2 Sample hold circuit SW1 to SW3 Switch circuit V1, V2 Voltage application unit

Claims (9)

A first resistance temperature detector whose resistance value changes according to the concentration of the gas to be measured, a first gas sensor unit including a first heater resistance for heating the first resistance temperature detector, and a first gas sensor unit.
A temperature sensor unit that includes a second resistance temperature detector whose resistance value changes according to the environmental temperature,
A first voltage application unit for applying a first control voltage to the first heater resistor is provided.
The first voltage application unit is a gas sensor including a first buffer circuit that changes the first control voltage based on the output voltage of the temperature sensor unit. The first voltage application unit includes a first voltage dividing circuit that generates a first adjustment voltage based on the output voltage of the temperature sensor unit, and a first reference voltage source that generates a first reference voltage. Including
The first buffer circuit according to claim 1, wherein the first buffer circuit generates the first control voltage by adding or subtracting the first adjustment voltage with respect to the first reference voltage. Gas sensor. A first switch circuit connected between the first buffer circuit and the temperature sensor unit,
The gas sensor according to claim 2, further comprising a second switch circuit connected between the first buffer circuit and the first reference voltage source. A third temperature measuring body, a second gas sensor unit including a second heater resistor for heating the third temperature measuring body, and
A second voltage application unit for applying a second control voltage to the second heater resistor is further provided.
The first resistance thermometer and the third resistance temperature detector are connected in series and
The second voltage application unit includes a second buffer circuit that linearly changes the second control voltage based on the output voltage of the temperature sensor unit.
The first control voltage and the second control voltage have different values, whereby the first temperature measuring body and the third temperature measuring body are heated to different temperatures. The gas sensor according to claim 3. The second voltage application unit includes a second voltage dividing circuit that generates a second adjusted voltage based on the output voltage of the temperature sensor unit, and a second reference voltage source that generates a second reference voltage. Including
The second buffer circuit according to claim 4, wherein the second buffer circuit generates the second control voltage by adding or subtracting the second adjustment voltage with respect to the second reference voltage. Gas sensor. The gas sensor according to claim 5, wherein the voltage dividing ratios of the first voltage dividing circuit and the second voltage dividing circuit are different from each other. Further comprising a third switch circuit connected between the second buffer circuit and the second reference voltage source.
The gas sensor according to claim 5 or 6, wherein the first switch circuit is connected between the first and second buffer circuits and the temperature sensor unit. The invention according to any one of claims 1 to 7, further comprising a sample hold circuit connected between the first buffer circuit and the first heater resistor and holding the first control voltage. The described gas sensor. The temperature sensor unit further includes a first resistor connected in series to the second resistance temperature detector and a second resistor connected in parallel to the second resistance temperature detector.
The gas sensor according to any one of claims 1 to 8, wherein the output voltage is a voltage at a connection point between the second temperature measuring body and the first resistor.

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