Gas Sensor

Abstract

To generate reference voltage with high accuracy without using a DA converter having high resolution.SOLUTION: A gas sensor 10 comprises a control circuit 20 that generates an output signal Vout on the basis of a gas detection signal Vgas output from a sensor section S. The control circuit 20 includes: an amplifier 21 that generates an amplified signal Vamp by amplifying a difference between the gas detection signal Vgas and reference voltage Vref; a calculation section 25 that generates a digital value D2 by amplifying a digital value D1, which indicates a difference between the reference voltage Vref and offset voltage Vf, by m times; a DA converter 24 that generates difference voltage Vdef2 by DA-converting the digital value D2; and a reference voltage generation circuit 26 that generates the reference voltage Vref on the basis of difference voltage Vdef1 obtained by attenuating the difference voltage Vdef2 by 1 / m times and the offset voltage Vf.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas sensor, and more particularly to a gas sensor that uses a DA converter to generate a reference voltage that is compared with a gas detection signal. [Background technology]

[0002] Patent Document 1 discloses a gas sensor including a differential amplifier that compares a gas detection signal output from a semiconductor gas sensor with a reference voltage, and a DA converter that generates the reference voltage. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Unexamined Patent Publication No. 56-40996 Summary of the Invention [Problem to be solved by the invention]

[0004] However, if the resolution of the DA converter is low, it is not possible to generate a reference voltage with high precision. To generate a reference voltage with high precision, a DA converter with high resolution can be used, but using a DA converter with high resolution increases the cost of the gas sensor.

[0005] Therefore, an object of the present invention is to provide a gas sensor that can generate a reference voltage with high precision without using a high-resolution DA converter. [Means for solving the problem]

[0006] A gas sensor according to the present invention includes a sensor section that generates a gas detection signal corresponding to the concentration of a gas to be measured, and a control circuit that generates an output signal indicating the concentration of the gas to be measured based on the gas detection signal. The control circuit includes an amplifier that generates an amplified signal by amplifying the difference between the gas detection signal and a reference voltage that serves as a reference for the gas detection signal; Gas Detection Signal and an offset voltage used to generate the reference voltage, by amplifying a first digital value by a factor of m; a DA converter that generates a first differential voltage by performing DA conversion on the second digital value; and a reference voltage generation circuit that generates a reference voltage based on the second differential voltage, which is obtained by attenuating the first differential voltage by a factor of m, and the offset voltage.

[0007] According to the present invention, the first digital value is amplified by m times to obtain a second digital value, which is input to the DA converter, and the first differential voltage output from the DA converter is attenuated by 1 / m times to obtain a second differential voltage, thereby increasing the apparent resolution of the DA converter by m times. This makes it possible to generate a reference voltage to be supplied to an amplifier with high accuracy without using a high-resolution DA converter.

[0008] In the present invention, the control circuit may further include an AD converter that performs AD conversion on the amplified signal to generate a third digital value, and the calculation unit may generate an output signal based on the third digital value. This makes it possible to generate an output signal indicating the concentration of the measurement target gas through digital calculation. In this case, the DA converter may have a lower resolution than the AD converter. This makes it possible to provide a gas sensor at low cost.

[0009] In the present invention, the calculation unit may change the first digital value in accordance with the environmental temperature. This makes it possible to reduce measurement errors depending on the environmental temperature. In this case, the calculation unit may change the first digital value in accordance with the environmental temperature based on the amplified signal. offset Varies depending on voltage and ambient temperature Second Digital Value It is okay to decide.

[0010] In the present invention, the value of m may be a value expressed as a power of 2. This makes it easy to generate the second digital value based on the first digital value.

[0011] In the present invention, the sensor unit includes first and second detection elements whose resistance values ​​change depending on the concentration of the gas to be measured, and first and second heater resistors that heat the first and second detection elements, respectively, and the gas detection signal is detection element and second detection The first and second detection elements may be heated to first and second temperatures by first and second heater resistors, respectively, during a first period, and the first and second detection elements may be heated to second and first temperatures by first and second heater resistors, respectively, during a second period. This significantly reduces the difference in thermal history between the first and second detection elements, making it possible to suppress measurement errors due to changes over time.

[0012] In the present invention, the sensor unit may further include a temperature sensor that generates a temperature detection signal according to the ambient temperature, and the control circuit may change the heater voltages applied to the first and second heater resistors according to the temperature detection signal, thereby reducing measurement errors according to the ambient temperature.

[0013] In the present invention, the control circuit may further include a first heater voltage generation circuit that generates a first heater voltage in response to the temperature detection signal, and a second heater voltage generation circuit that generates a second heater voltage different from the first heater voltage in response to the temperature detection signal, and the first and second heater voltages may be applied to the first and second heater resistors, respectively, during the first period, and the second and first heater voltages may be applied to the first and second heater resistors, respectively, during the second period. This allows the first and second heater voltage generation circuits to be shared by the first and second heater resistors. [Effects of the Invention]

[0014] As described above, according to the present invention, it is possible to provide a gas sensor that can generate a reference voltage with high precision without using a high-resolution DA converter. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a circuit diagram showing the configuration of a gas sensor 10 according to one embodiment of the present invention. [Figure 2] FIG. 2 is a graph showing the relationship between the environmental temperature and the voltages Va, Vb, and Vd. [Figure 3] FIG. 3 is a graph showing the relationship between the environmental temperature and the voltages Va, Vc, and Ve. [Figure 4] FIG. 4 is a flowchart for explaining operations before and after the reference voltage setting operation. [Figure 5] FIG. 5 is a flowchart illustrating the reference voltage setting operation. [Figure 6] FIG. 6 is a graph showing an example of the relationship between the ambient temperature and the gas detection signal Vgas. [Figure 7] FIG. 7 is a graph showing an example of the relationship between the environmental temperature and the differential voltage Vdef1 (digital value D1). [Figure 8] FIG. 8 is a graph showing an example of the relationship between the offset voltage Vf and the environmental temperature and the differential voltage Vdef2 (digital value D2). [Figure 9] FIG. 9 is a flowchart for explaining operations before and after the gas concentration measurement operation. [Figure 10] FIG. 10 is a timing chart showing the operation of the gas sensor 10. [Figure 11] FIG. 11 is a flowchart for explaining the gas concentration measurement operation (step S32). [Figure 12] FIG. 12 is a graph showing an example of the relationship between the environmental temperature and the reference voltage Vref. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0017] FIG. 1 is a circuit diagram showing the configuration of a gas sensor 10 according to one embodiment of the present invention.

[0018] 1, the gas sensor 10 according to this embodiment includes a sensor section S and a control circuit 20. Although not particularly limited, the gas sensor 10 according to this embodiment detects the concentration of CO2 gas in the atmosphere.

[0019] The sensor unit S is a thermal conduction gas sensor for measuring the concentration of CO2 gas, which is a measurement target gas, and includes first to third sensor units S1 to S3. The first sensor unit S1 includes a first thermistor Rd1 and a first heater resistor MH1 that heats it. Similarly, the second sensor unit S2 includes a second thermistor Rd2 and a second heater resistor MH2 that heats it. The third sensor unit S3 includes a third thermistor Rd3. The first to third thermistors Rd1 to Rd3 are detection elements made of a material with a negative temperature coefficient of resistance, such as a composite metal oxide, amorphous silicon, polysilicon, or germanium. The first and second thermistors Rd1 and Rd2 both detect the concentration of CO2 gas, but have different operating temperatures, as described below. The third thermistor Rd3 also functions as a temperature sensor that detects the ambient temperature.

[0020] As shown in FIG. 1, the first thermistor Rd1 and the second thermistor Rd2 are connected in series between a wiring supplied with a power supply potential Vcc and a wiring supplied with a ground potential GND. The first thermistor Rd1 is heated by a first heater resistor MH1, and the second thermistor Rd2 is heated by a second heater resistor MH2. A gas detection signal Vgas appears at the connection point between the first thermistor Rd1 and the second thermistor Rd2. The gas detection signal Vgas is supplied to the control circuit 20. The third sensor unit S3 is composed of a third thermistor Rd3, and a fixed resistor R1 and the third thermistor Rd3 are connected in series between the wiring supplied with the power supply potential Vcc and the wiring supplied with the ground potential GND. A temperature detection signal Va is output from the connection point between the fixed resistor R1 and the third thermistor Rd3. The temperature detection signal Va is input to the control circuit 20.

[0021] The control circuit 20 includes an amplifier 21, a buffer 22, an AD converter (ADC) 23, a DA converter (DAC) 24, a calculation unit 25, a reference voltage generation circuit 26, and heater voltage generation circuits 27 and 28. The amplifier 21 compares the gas detection signal Vgas with a reference voltage Vref and amplifies the difference between them. The amplified signal Vamp output from the amplifier 21 is input to the AD converter 23. The AD converter 23 converts the amplified signal Vamp into a digital value D3 and supplies this to the calculation unit 25. Meanwhile, the DA converter 24 generates a differential voltage Vdef2 by analog-converting the digital value D2 supplied from the calculation unit 25. The differential voltage Vdef2 is supplied to the reference voltage generation circuit 26.

[0022] The reference voltage generation circuit 26 comprises an offset voltage source 31, an amplifier A1, a variable resistor R7, a resistor R8, and a capacitor C1. The capacitor C1 is a phase compensation element that prevents oscillation of the amplifier A1. The amplifier A1, the variable resistor R7, and the resistor R8 form a subtraction circuit that reduces the level of the differential voltage Vdef2 at an attenuation rate determined by the resistance ratio (R7 / R8) between the variable resistor R7 and the resistor R8. Since an offset voltage Vf from the offset voltage source 31 is input to the positive terminal of the amplifier A1, the reference voltage Vref output from the amplifier A1 is Vf+Vdef(R7 / R8). The offset voltage source 31 is variable, and the offset voltage Vf can be adjusted under the control of the calculation unit 25.

[0023] The temperature detection signal Va is supplied to a buffer 22 and heater voltage generation circuits 27 and 28. The temperature detection signal Va supplied to the buffer 22 is converted into a digital signal by an AD converter 23 and supplied to a calculation unit 25. The heater voltage generation circuits 27 and 28 generate heater voltages Vb and Vc, respectively, based on the temperature detection signal Va. The heater voltages Vb and Vc are supplied to switch circuits SW1 and SW2, respectively. The connection destination of the switch circuits SW1 and SW2 is switched to one of nodes N1 to N3 under the control of the calculation unit 25. When the connection destination of the switch circuits SW1 and SW2 is set to node N1, the heater voltage Vb is used as the heater voltage Vmh1 applied to the heater resistor MH1, and the heater voltage Vc is used as the heater voltage Vmh2 applied to the heater resistor MH2. On the other hand, when the connection destination of the switch circuits SW1 and SW2 is set to node N3, the heater voltage Vb is used as the heater voltage Vmh2 to be applied to the heater resistor MH2, and the heater voltage Vc is used as the heater voltage Vmh1 to be applied to the heater resistor MH1. Also, when the connection destination of the switch circuits SW1 and SW2 is set to node N2, neither of the heater voltages Vb nor Vc is supplied to the heater resistors MH1 and MH2, thereby reducing power consumption.

[0024] The heater voltage generation circuit 27 comprises a reference voltage source 32, an amplifier A2, a variable resistor R3, a resistor R4, and a capacitor C2. The capacitor C2 is a phase compensation element that prevents oscillation of the amplifier A2. The amplifier A2, the variable resistor R3, and the resistor R4 form a subtraction circuit that reduces the level of the temperature detection signal Va at an attenuation rate determined by the resistance ratio (R3 / R4) between the variable resistor R3 and the resistor R4. Since the voltage Vd of the reference voltage source 32 is input to the positive terminal of the amplifier A2, the heater voltage Vb output from the amplifier A2 is Vd+Va(R3 / R4).

[0025] The heater voltage generation circuit 28 comprises a reference voltage source 33, an amplifier A3, a variable resistor R5, a resistor R6, and a capacitor C3. The capacitor C3 is a phase compensation element that prevents oscillation of the amplifier A3. The amplifier A3, the variable resistor R5, and the resistor R6 form a subtraction circuit that reduces the level of the temperature detection signal Va at an attenuation rate determined by the resistance ratio (R5 / R6) between the variable resistor R5 and the resistor R6. Since the voltage Ve of the reference voltage source 33 is input to the positive terminal of the amplifier A3, the heater voltage Vc output from the amplifier A3 is Ve+Va(R5 / R6).

[0026] References voltage source 32 Voltage Vd and reference voltage source 33 The voltages Ve are different from each other. Voltage Vd is set to a level that heats the thermistor Rd1 or Rd2 to 150°C using heater resistor MH1 or MH2, while voltage Ve is set to a level that heats the thermistor Rd1 or Rd2 to 300°C using heater resistor MH1 or MH2. If CO2 gas is present in the measurement atmosphere when the thermistor Rd1 or Rd2 is heated to 150°C, the heat dissipation characteristics of the thermistor Rd1 or Rd2 will change depending on the concentration of CO2 gas. This change will manifest as a change in the resistance value of the thermistor Rd1 or Rd2. On the other hand, if CO2 gas is present in the measurement atmosphere when the thermistor Rd1 or Rd2 is heated to 300°C, the resistance value of the thermistor Rd1 or Rd2 will hardly change.

[0027] Furthermore, the resistance ratio (R3 / R4) between variable resistor R3 and resistor R4 and the resistance ratio (R5 / R6) between variable resistor R5 and resistor R6 may be different from each other. The resistance ratio (R3 / R4) between variable resistor R3 and resistor R4 is designed so that heater voltage Vb remains approximately constant regardless of the ambient temperature, and the resistance ratio (R5 / R6) between variable resistor R5 and resistor R6 is designed so that heater voltage Vc remains approximately constant regardless of the ambient temperature. These resistance ratios can be adjusted by changing the resistance values ​​of variable resistors R3 and R5.

[0028] FIG. 2 is a graph showing the relationship between the environmental temperature and the voltages Va, Vb, and Vd, and FIG. 3 is a graph showing the relationship between the environmental temperature and the voltages Va, Vc, and Ve.

[0029] As shown in FIGS. 2 and 3, as the ambient temperature increases, the resistance of the thermistor Rd3 decreases, causing a decrease in the temperature detection signal Va output from the temperature sensor unit S3. The temperature detection signal Va is supplied to heater voltage generation circuits 27 and 28. The temperature detection signal Va supplied to the heater voltage generation circuit 27 is attenuated at a predetermined attenuation rate by a subtraction circuit consisting of an amplifier A2, a variable resistor R3, and a resistor R4, and a voltage Vd from a reference voltage source 32 is added to the temperature detection signal Va. As a result, the level of the heater voltage Vb decreases as the ambient temperature increases, making it possible to heat the thermistor Rd1 or Rd2 to a constant temperature (e.g., 150°C) regardless of the ambient temperature. Similarly, the temperature detection signal Va supplied to the heater voltage generation circuit 28 is attenuated at a predetermined attenuation rate by a subtraction circuit consisting of an amplifier A3, a variable resistor R5, and a resistor R6, and a voltage Ve from a reference voltage source 33 is added to the temperature detection signal Va. As a result, the level of the heater voltage Vc decreases as the ambient temperature increases, making it possible to heat the thermistor Rd1 or Rd2 to a constant temperature (for example, 300° C.) regardless of the ambient temperature.

[0030] Next, the operation of the gas sensor 10 according to this embodiment will be described.

[0031] The gas sensor 10 according to this embodiment performs a reference voltage setting operation and then a gas concentration measurement operation. As shown in FIG. 4, the reference voltage setting operation (step S12) is executed after the connection destination of the switch circuits SW1 and SW2 is set to node N1 (step S10) and a predetermined delay time has elapsed (step S11). When the connection destination of the switch circuits SW1 and SW2 is set to node N1, the thermistor Rd1 is heated to 150°C, and the thermistor Rd2 is heated to 300°C. After the reference voltage setting operation (step S12) is completed, the connection destination of the switch circuits SW1 and SW2 is temporarily set to node N2 (step S13), and then the connection destination of the switch circuits SW1 and SW2 is set to node N3 (step S14). When the connection destination of the switch circuits SW1 and SW2 is set to node N3, the thermistor Rd1 is heated to 300°C, and the thermistor Rd2 is heated to 150°C. After a predetermined delay time has elapsed (step S15), the connection destination of the switch circuits SW1 and SW2 is set to node N2 again (step S16), which makes it possible to make the thermal histories of thermistors Rd1 and Rd2 approximately the same during the reference voltage setting operation.

[0032] FIG. 5 is a flowchart for explaining the reference voltage setting operation (step S12).

[0033] In the reference voltage setting operation (step S12), a temporary differential voltage Vdef2 is generated by outputting a temporary digital value D2 from the calculation unit 25, and a temporary offset voltage Vf is generated by controlling the offset voltage source 31 (step S20). Next, the resistance value of the variable resistor R7 is made to match the resistance value of the resistor R8 (step S21). As a result, the attenuation ratio of the subtraction circuit included in the reference voltage generation circuit 26 becomes 1, and the level of the reference voltage Vref output from the amplifier A1 becomes Vref=Vf+Vdef2 The temporary reference voltage Vref thus generated is supplied to the amplifier 21 (step S22).

[0034] Next, the ambient temperature is changed continuously or stepwise using a heater or the like (not shown) (step S23). The range of change in ambient temperature is preferably the temperature range expected in actual use, for example, a range of 10°C to 60°C. This operation is performed in an environment where no gas to be measured is present in the atmosphere or where the atmosphere is in a steady state. For example, if the gas to be measured is CO2 gas, it is performed in an environment where the concentration of CO2 gas is a constant concentration in normal air. As a result, a gas detection signal Vgas is output from the sensor unit S in each temperature environment. The gas detection signal Vgas is compared with a temporary reference voltage Vref by the amplifier 21, and an amplified signal Vamp is generated. If the gain of the amplifier 21 is G, the level of the amplified signal Vamp is Vamp=G(Vgas-Vref)+Vref The amplified signal Vamp is converted into a digital value D3 by the AD converter 23 and supplied to the calculation unit 25. The calculation unit 25 reads the digital value D3 at predetermined time intervals (step S24) and stores the digital value D3 in association with the environmental temperature (step S25).

[0035] Next, the calculation unit 25 calculates the actual gas detection signal Vgas from the digital value D3 stored in association with the environmental temperature (step S26). Vgas=(D3+Vref / (G-1)) / G Since the gas detection signal Vgas is calculated by digital calculation, the value of the reference voltage Vref uses the temporary digital value D2 used in step S11 and the set value of the offset voltage Vf. As a result, a first relational expression F1 (or a data table) indicating the relationship between the ambient temperature and the gas detection signal Vgas in a state where the gas to be measured is not present or in a steady state is generated inside the calculation unit 25 (step S26). FIG. 6 is a graph showing an example of the relationship between the ambient temperature and the gas detection signal Vgas.

[0036] Next, the offset voltage Vf is determined (step S27), and the differential voltage Vdef1 (=Vgas-Vf) between the gas detection signal Vgas and the offset voltage Vf is calculated (step S28). The differential voltage Vdef1 is expressed as a digital value D1. This generates a second relational expression F2 (or a data table) that indicates the relationship between the ambient temperature and the digital value D1. The offset voltage Vf is set to the minimum value of the gas detection signal Vgas indicated by the first relational expression F1 or a level slightly smaller than the minimum value of the gas detection signal Vgas. For example, as shown in FIG. 6, if the minimum value of the gas detection signal Vgas is approximately 0.967 V, the offset voltage Vf should be set to 0.96 V. In this case, the relationship between the ambient temperature and the differential voltage Vdef1 is as shown in FIG. 7.

[0037] Here, because the change in the differential voltage Vdef1 according to the environmental temperature is very small, even if the digital value D1 corresponding to the differential voltage Vdef1 is supplied to the DA converter 24 as is, the digital value D1 cannot be accurately converted into the differential voltage Vdef1 depending on the resolution of the DA converter 24. For this reason, in this embodiment, the calculation unit 25 generates a digital value D2 by amplifying the digital value D1 by m times (step S29). The value of m may be determined according to the resolution of the DA converter 24. In particular, the value of m is set to a value expressed as a power of 2 (=2 n (where n is an integer equal to or greater than 1), digital value D2 can be easily generated by simply replacing the lower bits of digital value D1 with the upper bits of digital value D2. This generates a third relational expression F3 (or data table) that indicates the relationship between the environmental temperature and digital value D2. For example, if the temperature value indicating the environmental temperature is Tc, third relational expression F3 can be expressed by the following equation.

[0038]

number

[0039] This completes the reference voltage setting operation (step S12), and a digital value D2 is determined according to the offset voltage Vf and the ambient temperature. The digital value D2 is supplied to the DA converter 24 and converted into a differential voltage Vdef2. FIG. 8 is a graph showing an example of the relationship between the offset voltage Vf, the ambient temperature, and the differential voltage Vdef2. As shown in FIG. 8, the digital value D2 is amplified m times with respect to the digital value D1, and therefore the change in the differential voltage Vdef2 according to the ambient temperature is amplified m times. The offset voltage Vf is constant, and in the example shown in FIG. 8, is 0.96 V.

[0040] FIG. 9 is a flowchart for explaining operations before and after the gas concentration measurement operation.

[0041] As shown in FIG. 9, the gas concentration measurement operation (step S32) is executed after the connection destination of the switch circuits SW1 and SW2 is set to node N1 (step S30) and a predetermined delay time has elapsed (step S31). When the connection destination of the switch circuits SW1 and SW2 is set to node N1, the thermistor Rd1 is heated to 150°C, and the thermistor Rd2 is heated to 300°C. When the gas concentration measurement operation (step S32) is completed, the connection destination of the switch circuits SW1 and SW2 is temporarily set to node N2 (step S33), and then the connection destination of the switch circuits SW1 and SW2 is set to node N3 (step S34). When the connection destination of the switch circuits SW1 and SW2 is set to node N3, the thermistor Rd1 is heated to 300°C, and thermistor Rd2 is heated to 150°C. Then, after a predetermined delay time has elapsed (step S35), the connection destination of the switch circuits SW1 and SW2 is set to node N2 again (step S36). This allows the thermistors Rd1 and Rd2 to have approximately the same thermal history. This operation is repeated as shown in FIG.

[0042] FIG. 11 is a flowchart for explaining the gas concentration measurement operation (step S32).

[0043] As shown in FIG. 11, in the gas concentration measurement operation (step S32), first, offset voltage Vf is generated by controlling offset voltage source 31 (step S40). The level of offset voltage Vf is the level determined in the reference voltage setting operation (step S12) described above. Next, the resistance value of variable resistor R7 is controlled to set the attenuation rate of the subtraction circuit consisting of amplifier A1, variable resistor R7, and resistor R8 to 1 / m (step S41). This can be achieved by setting the resistance value of variable resistor R7 to 1 / m times that of resistor R8.

[0044] Next, the temperature detection signal Va is referenced, and a digital value D2 corresponding to the current ambient temperature is output (step S42). The digital value D2 is determined by reference to a third relational expression F3 (or a data table) that indicates the relationship between the ambient temperature and the digital value D2. As a result, the level of the reference voltage Vref output from the amplifier A1 is Vref=Vf+Vdef2 / m In other words, when the differential voltage Vdef2 amplified by m times by the calculation unit 25 is input to the reference voltage generation circuit 26, it is attenuated by 1 / m times and restored to the same level as the differential voltage Vdef1. FIG. 12 is a graph showing an example of the relationship between the ambient temperature and the reference voltage Vref. As shown in FIG. 12, it can be seen that the relationship between the ambient temperature and the reference voltage Vref is almost the same as the relationship between the ambient temperature and the gas detection signal Vgas shown in FIG. 6.

[0045] In this state, the amplifier 21 compares the gas detection signal Vgas with the reference voltage Vref. Therefore, if no measurement target gas is present in the atmosphere or if the atmosphere is in a steady state, the gas detection signal Vgas and the reference voltage Vref will be approximately equal. In contrast, if the measurement target gas is present in the atmosphere or if there is more measurement target gas present than in the steady state, the difference between the gas detection signal Vgas and the reference voltage Vref is reflected in the digital value D3. Then, the calculation unit 25 generates an output signal Vout indicating the concentration of the measurement target gas based on the digital value D3 (step S43).

[0046] By performing this gas concentration measurement operation (step S32), it is possible to accurately measure the concentration of the measurement target gas in the atmosphere.

[0047] As described above, the gas sensor 10 according to this embodiment can generate the reference voltage Vref with high accuracy, thereby reducing costs, even if the resolution of the DA converter 24 is low. As an example, even if the resolution of the AD converter 23 is 16 bits and the resolution of the DA converter 24 is 12 bits, if the digital value D2 is generated by multiplying the digital value D1 corresponding to the differential voltage Vdef1 by 16, the DA converter 24 can also apparently achieve a resolution equivalent to 16 bits.

[0048] The above describes a preferred embodiment of the present invention, but the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the present invention, and it goes without saying that these modifications are also included within the scope of the present invention.

[0049] For example, in the above embodiment, the measurement target gas is CO2 gas, but the present invention is not limited to this. Also, the sensor unit used in the present invention does not necessarily have to be a thermal conduction sensor, and may be a sensor of another type, such as a catalytic combustion type. [Explanation of symbols]

[0050] 10 Gas Sensor 20 Control circuit 21 Amplifier 22 buffers 23 AD converter 24 DA converter 25 Arithmetic section 26 Reference voltage generation circuit 27,28 Heater voltage generation circuit 31 Offset voltage source 32,33 Reference Voltage Source A1~A3 amplifiers C1~C3 capacitors D1~D3 digital values F1~F3 relational equation MH1,MH2 heater resistors N1 to N3 nodes R1,R4,R6,R8 Resistor R3, R5, R7 variable resistors Rd1~Rd3 thermistor S, S1 to S3 sensor section SW1, SW2 switch circuit

Claims

1. a sensor unit that generates a gas detection signal corresponding to the concentration of a gas to be measured; a control circuit that generates an output signal indicating the concentration of the measurement target gas based on the gas detection signal, The control circuit an amplifier that generates an amplified signal by amplifying the difference between the gas detection signal and a reference voltage that serves as a reference for the gas detection signal; a calculation unit that generates a second digital value by amplifying a first digital value that indicates a difference between the gas detection signal and an offset voltage used to generate the reference voltage by m times; a DA converter that generates a first differential voltage by DA converting the second digital value; a reference voltage generating circuit that generates the reference voltage based on a second differential voltage obtained by attenuating the first differential voltage by 1 / m times and the offset voltage; an AD converter that generates a third digital value by AD converting the amplified signal; The gas sensor is characterized in that the calculation unit generates the output signal based on the third digital value.

2. 2. The gas sensor according to claim 1, wherein the DA converter has a lower resolution than the AD converter.

3. 3. The gas sensor according to claim 1, wherein the calculation unit changes the first digital value in accordance with an ambient temperature.

4. 4. The gas sensor according to claim 3, wherein the calculation unit determines the offset voltage, which is constant regardless of the ambient temperature, and the second digital value, which varies depending on the ambient temperature, based on the amplified signal.

5. 5. The gas sensor according to claim 1, wherein the value of m is a value expressed as a power of two.

6. the sensor unit includes first and second detection elements whose resistance values ​​change depending on the concentration of the measurement target gas, and first and second heater resistors that heat the first and second detection elements, respectively; the gas detection signal appears at a junction of the first detection element and the second detection element; during a first period, the first and second detection elements are heated to first and second temperatures by the first and second heater resistors, respectively; 6. The gas sensor according to claim 1, wherein, in a second period, the first and second detection elements are heated to the second and first temperatures by the first and second heater resistors, respectively.

7. the sensor unit further includes a temperature sensor that generates a temperature detection signal according to an environmental temperature; 7. The gas sensor according to claim 6, wherein the control circuit changes heater voltages applied to the first and second heater resistors in response to the temperature detection signal.

8. the control circuit further includes a first heater voltage generation circuit that generates a first heater voltage in response to the temperature detection signal, and a second heater voltage generation circuit that generates a second heater voltage different from the first heater voltage in response to the temperature detection signal; During the first period, the first and second heater voltages are applied to the first and second heater resistors, respectively; 8. The gas sensor according to claim 7, wherein the second and first heater voltages are applied to the first and second heater resistors, respectively, during the second period.

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