JP6358133B2 - Gas concentration sensor signal processing device - Google Patents

Description

  The present invention relates to a signal processing device for a gas concentration sensor.

  The gas concentration sensor is, for example, a sensor that detects exhaust gas of an internal combustion engine, and outputs a detection signal that changes according to the gas concentration. Normally, when stoichiometric (theoretical air-fuel ratio), the sensor output current is 0 A, and the directions of current flow in the rich state and the lean state are opposite to each other. For example, a detection resistor is used for detecting the sensor current, and the fuel injection can be appropriately controlled by detecting the voltage across the detection resistor through which the sensor current flows.

  An example of a signal processing device of this gas concentration sensor is disclosed in Patent Document 1. In the technology described in Patent Document 1, the A / D converter uses a single-ended input device, and the analog front-end circuit provided in front of the A / D converter also includes a single-ended output differential amplifier. Used.

JP 2004-205488 A

  Since a single-ended input A / D converter measures a signal voltage according to a potential difference from the ground, it is more susceptible to noise than a differential input type A / D converter. For this reason, it is desirable to use a differential input type A / D converter for detection of sensor signals that are likely to be affected by noise.

  In general, an analog processing circuit can be configured using, for example, two differential amplifiers in order to process a differential signal. If the power supply voltage is VDD at this time, the differential input type A / D converter has a differential input voltage range of −VDD / 2 to + VDD / 2 and an in-phase input voltage range of VDD / 2. There are many. When detecting the sensor current using this circuit format, if the current detection range in the rich state and the current detection range in the lean state are equal in absolute value (for example, current detection range: −25 mA to +25 mA), the A / D converter The differential input voltage range can be fully used.

  However, when the absolute value of the current detection range is not equal between the rich state and the lean state (for example, current detection range: −10 mA to +40 mA), the full range of the current detection range is constant, but the differential input voltage range is sufficient. It cannot be used for. In addition, the resistance for current-voltage conversion must be reduced to, for example, 5/8 (25 mA / 40 mA). In this case, the detection accuracy is lowered.

  An object of the present invention is to provide a signal processing device for a gas concentration sensor that can fully utilize the differential input voltage range even when the absolute values of the detection ranges are not equal between the rich state and the lean state. It is in.

  According to the first aspect of the present invention, the gas concentration sensor detects the gas concentration of the non-detection gas and outputs a sensor signal. The analog processing unit inputs the sensor signal as a differential signal, and the differential signal is analog. To process. At this time, the offset applying unit applies an offset during analog processing of the differential signal by the analog processing unit. Since the differential input type A / D converter performs A / D conversion on the differential signal processed by the analog processing unit, the differential input voltage range can be fully utilized. Since the digital processing unit excludes the value corresponding to the offset applied from the A / D conversion result of the A / D conversion unit by digital processing, it is possible to exclude the influence of the offset given in advance. Thereby, even when the absolute values of the detection ranges are not equal between the rich state and the lean state, the differential input voltage range of the differential input type A / D converter can be fully utilized.

1 is a block diagram schematically showing an example of an electrical configuration of a signal processing device of a gas concentration sensor according to a first embodiment. Example showing the range of use of the A / D converter Example showing range of use of A / D converter showing comparative example Example showing range of use of A / D converter showing comparative example Configuration example of offset application unit of comparative example Block diagram schematically showing an example of an electrical configuration of a signal processing apparatus of a gas concentration sensor according to a second embodiment (one form when adjusting resolution: part 1) Block diagram schematically showing an example of the electrical configuration of the signal processing device of the gas concentration sensor (one form when adjusting the resolution (when adjusting the stoichiometry): Part 2) Example (1) showing range of use of A / D converter Example (2) showing range of use of A / D converter

  Hereinafter, several embodiments of a signal processing device of a gas concentration sensor will be described with reference to the drawings. In the following description, components having the same or similar functions as those described in each embodiment are denoted by the same reference numerals or similar symbols, and description thereof will be omitted as necessary in the second and subsequent embodiments.

(First embodiment)
1 to 5 are explanatory diagrams of the first embodiment. FIG. 1 is a schematic block diagram showing an electrical configuration of a signal processing device 1 of a gas concentration sensor. A signal processing apparatus 1 shown in FIG. 1 is an air-fuel ratio sensor for detecting an oxygen concentration in exhaust gas and specifying an air-fuel ratio by using an exhaust combustion gas discharged from a vehicle engine (not shown) as a detected gas. Various control processes are performed. The signal processing apparatus 1 is configured by, for example, a so-called ASIC (specific application IC), an analog processing unit 2, an offset adding unit 3, an A / D converter (corresponding to an A / D conversion unit) 4, and a DSP 5 as a digital processing unit. , A storage unit 6, a drive unit 7, a bias application unit 8, and a current detection resistor 10 flowing through the air-fuel ratio sensor 9. The A / D converter 4 is of a differential input type, operates by being supplied with a power supply voltage VDD, has a differential input voltage range of −VDD / 2 to + VDD / 2, and has an in-phase input voltage range. Is assumed to be VDD / 2.

  When the signal processing apparatus 1 receives the sensor signal of the air-fuel ratio sensor 9, the signal processing apparatus 1 controls energization of the air-fuel ratio sensor 9 based on the sensor signal. The resistor 10 is connected between the first node N1 of the output of the bias applying unit 8 and the second node N2 serving as the current conduction node of the air-fuel ratio sensor 9, and the bias applying unit 8 is defined as the output node. The air-fuel ratio sensor 9 is configured to energize the resistor 10 with a bias voltage Vcom (for example, 2.5 V = VDD / 2) applied. The increase / decrease in the element current of the air / fuel ratio sensor 9 corresponds to the increase / decrease (lean / rich) of the air / fuel ratio. For example, when the air / fuel ratio becomes lean, the element current becomes positive (refer to FIG. 1 for the current direction) and the air / fuel ratio becomes rich. Then, the device current becomes negative (refer to FIG. 1 for the current direction).

  The DSP 5 operates based on a program stored in the storage unit 6. The storage unit 6 includes a volatile memory or a non-volatile memory such as a flash memory, and various parameters (for example, an offset voltage described later) are stored in advance in the flash memory. The analog processing unit 2 differentially amplifies the voltage between the first node N1 and the second node N2 of the resistor 10 using the offset voltage provided by the offset applying unit 3. The analog processing unit 2 includes a first buffer 11 connected to one first node N1 of the resistor 10, a second buffer 12 connected to the other second node N2 of the resistor 10, a first differential amplifier 13, and , A second differential amplifier 14.

  The offset applying unit 3 includes a resistance voltage dividing circuit 15, switches 16 and 17, a third buffer 18, and a fourth buffer 19. When the DSP 5 controls the switches 16 and 17, the offset applying unit 3 applies the offset voltage.

  The resistance voltage dividing circuit 15 is configured by connecting a plurality of resistors R1 to R4 in series between a power supply terminal 20 (corresponding to a first power supply line) and a ground terminal 21 (corresponding to a second power supply line), and divides the power supply voltage VDD. Output to the switches 16 and 17. The switches 16 and 17 are configured to be switchable from a control logic such as a DSP 5, for example, and are configured to be capable of switching the output of the divided voltage by the resistance voltage dividing circuit 15.

  The third buffer 18 outputs the first offset voltage (corresponding to the first offset) V1 to the analog processing unit 2, and the fourth buffer 19 outputs the second offset voltage (corresponding to the second offset) V2 to the analog processing unit 2. . The first offset voltage V1 is set to 0.65 times the power supply voltage VDD, for example, and the second offset voltage V2 is set to 0.35 times the power supply voltage VDD, for example. In this case, the offset applying unit 3 applies a voltage obtained by adding or minus 0.15 · VDD from the median value 0.5 · VDD of the power supply voltage VDD as the first offset voltage V1 and the second offset voltage V2. In other words, the first offset voltage V1 and the second offset voltage V2 are in opposite directions with respect to the median value (0.5 · VDD) of the input possible range (VDD to 0) of the A / D converter 4 and The voltage having the same absolute value (0.15 · VDD) is obtained by adding or subtracting the median value.

  On the other hand, the first buffer 11 of the analog processing unit 2 is configured using an operational amplifier OP1 in which the first node N1 of the resistor 10 is connected to the non-inverting input terminal and the output terminal is feedback-connected to the inverting input terminal. The second buffer 12 of the analog processing unit 2 is configured using an operational amplifier OP2 in which the second node N2 of the resistor 10 is connected to the non-inverting input terminal and the output terminal is feedback-connected to the inverting input terminal.

  The first differential amplifier 13 includes, for example, an operational amplifier OP3 and resistors R5 to R8. The first differential amplifier 13 inverts the output signal of the first buffer 11 and inputs the output signal of the second buffer 12 in the normal direction to perform differential amplification. Thereby, the first differential amplifier 13 inverts the detection voltage of the differential signal of the sensor signal of the air-fuel ratio sensor 9 to differentially amplify it. For example, in the circuit configuration of FIG. 1, a resistor R5 is connected between the output node of the first buffer 11 and the inverting input terminal of the operational amplifier OP3, and a resistor is connected between the output terminal and the inverting input terminal of the third operational amplifier OP3. R6 is connected. A resistor R7 is connected between the output node of the second buffer 12 and the non-inverting input terminal of the operational amplifier OP3, and a resistor R8 is connected between the output node of the third buffer 18 and the non-inverting input terminal of the operational amplifier OP3. Is connected.

  The second differential amplifier 14 includes, for example, an operational amplifier OP4 and resistors R9 to R12. The second differential amplifier 14 inverts the output signal of the second buffer 12 and inputs the output signal of the first buffer 11 in the normal direction to perform differential amplification. As a result, the second differential amplifier 14 differentially amplifies the detection voltage of the differential signal of the sensor signal of the air-fuel ratio sensor 9. For example, in the circuit configuration of FIG. 1, a resistor R11 is connected between the output node of the second buffer 12 and the inverting input terminal of the operational amplifier OP4, and a resistor R12 is connected between the output terminal and the inverting input terminal of the operational amplifier OP4. It is connected. A resistor R9 is connected between the output node of the first buffer 11 and the non-inverting input terminal of the operational amplifier OP4, and a resistor R10 is connected between the output node of the fourth buffer 19 and the non-inverting input terminal. ing. In FIG. 1, elements having the same characteristics are employed for the operational amplifiers OP3 and OP4, and the resistance values of the resistors R5 to R12 are all set to the same value. Note that the operational amplifiers OP3 and OP4 do not have to employ elements having the same characteristics, and the resistance values of the resistors R5 to R12 may not be set to the same value.

  The output voltage Vadinm of the first differential amplifier 13 is given to the negative input terminal of the differential input type A / D converter 4, and the output voltage Vadinp of the second differential amplifier 14 is supplied to the A / D converter. 4 positive input terminals. In other words, the A / D converter 4 inputs the differential signal processed by the analog processing unit 2. The A / D converter 4 calculates and converts the differential voltage Vad (= Vadinp−Vadinm) of these analog output voltages Vadinp and Vadinm into a digital output to the DSP 5. Here, the A / D converter 4 operates by receiving the power supply voltage VDD and the ground voltage, and the input differential voltage Vad (= Vadinp−Vadinm) is expressed by the following equation (1). Configured to work with a range.

−VDD / 2 ≦ Vad ≦ VDD / 2 (1)
Although not usable outside this operating range, the A / D converter 4 outputs a digital value corresponding to the output current of the air-fuel ratio sensor 9 to the DSP 5 within this operating range. The DSP 5 is configured to subtract an offset, which will be described later, and to feedback control the energization current of the air-fuel ratio sensor 9 by the drive unit 9 based on the sensor signal of the air-fuel ratio sensor 9.

  The operation of the above configuration will be described. As described above, the first offset voltage V1 is set to 0.65 times the power supply voltage VDD, for example, and the second offset voltage V2 is set to 0.35 times the power supply voltage VDD, for example. Accordingly, when the voltage at the first node N1 of the resistor 10 is Vinp and the voltage at the second node N2 of the resistor 10 is Vinm, the output voltages Vadinm and Vadinp are expressed by the following equations (2) and (3).

Vadinm = 0.65 · VDD + (Vinm-Vinp) ÷ 2 ... (2)
Vadinp = 0.35 · VDD + (Vinp-Vinm) ÷ 2 ... (3)
Therefore, when the differential input type A / D converter 4 calculates these difference values,
Vad = Vadinp-Vadinm
= Vinp-Vinm-0.3 · VDD (4)
Is required. The DSP 5 can calculate the voltage between the terminals of the resistor 10 by subtracting the shift voltage 0.3 · VDD. Thereby, the sensor signal of the air-fuel ratio sensor 9 can be obtained accurately.

  In the present embodiment, the first differential amplifier 13 and the second differential amplifier 14 are intentionally opposite to each other on the basis of the median value of the input possible range (VDD to 0) of the A / D converter 4. In addition, the first offset voltage V1 and the second offset voltage V2 (offset) having the same absolute value are applied. The significance of this will be described.

  Assuming that the stoichiometric air-fuel ratio (stoichiometric) is assumed, the output current of the air-fuel ratio sensor 9 is 0 [A], and the voltage across the terminals of the detection resistor 10 is 0 [V]. In this case, since Vinp = Vinm in the equations (2) and (3), Vadinm = 0.65 · VDD and Vadinp = 0.35 · VDD. At this time, the positions where the input differential voltage Vad on the vertical axis in FIG. The operating points P1 and P2 are operating points located at a ratio of 0.2 if the input possible range X1 in the equation (1) is “1”.

  For example, when the air-fuel ratio transitions from the stoichiometric operating points P1 and P2 to the rich state, the current flowing through the air-fuel ratio sensor 9 transitions from 0A in the negative direction, and Vinp <Vinm. The change at this time is shown in FIG. 2, but the voltage Vadinp decreases and the voltage Vadinm increases (see arrow Y1 in FIG. 2). The differential voltage Vad input to the A / D converter 4 has a minimum value of −0.5 · VDD. Therefore, the operating points P3 and P4 at this time are the voltage Vadinp = 0.25 · VDD and the voltage Vadinm = 0.75 · VDD, and the effective operating range of the output current d of the air-fuel ratio sensor 9 is a negative first. The predetermined current value is about I1 (for example, −10 mA).

  Conversely, for example, when the air-fuel ratio transitions to the lean state from the operating points P1 and P2 that become stoichiometric, the current flowing through the air-fuel ratio sensor 9 transitions from 0A in the positive direction and Vinp> Vinm. The change at this time is also shown in FIG. 2, but the voltage Vadinp increases and the voltage Vadinm decreases (see arrow Y2 in FIG. 2). The differential voltage Vad input to the A / D converter 4 has a maximum value of + 0.5 · VDD. Therefore, the operating points P5 and P6 at this time are the voltage Vadinp = 0.75 · VDD and the voltage Vadinm = 0.25 · VDD, and the effective operating range of the output current of the air-fuel ratio sensor 9 is a positive second predetermined value. The current value is about I2 (for example, +40 mA).

Here, when the absolute value of the first predetermined current value I1 and the absolute value of the second predetermined current value I2 are compared, it can be expressed as the following equation (5).
| I1 | <| I2 | (5)
Therefore, by setting the offset voltages V1 and V2 to other than 0V, the effective detection range of the output current of the air-fuel ratio sensor 9 can be shifted.

  For example, in recent years, there is a demand for lean burn, and considering that there are many cases of operation in a lean state than the stoichiometric air-fuel ratio, the effective operating range in the rich state is reduced while the effective operating range in the lean state is expanded. This can meet this demand.

  Further, as shown in FIG. 1, the offset voltages V1 and V2 can be switched by providing switches 16 and 17. At this time, the resistance (R1 + R2) and the resistance (R3 + R4) of the resistance voltage dividing circuit 15 are common so that the DSP 5 controls the switches 16 and 17 so that the offset voltages V1 and V2 are both 0.5 · VDD. Consider switching to connect to connection node N3.

  In this case, since the resistance voltage dividing circuit 15 outputs 0.5 · VDD, the input offset voltages V1 and V2 of the first differential amplifier 13 and the second differential amplifier 14 are both equal to 0.5 · VDD. Become. The operating point P7 in this case is shown in FIG. For example, the absolute value of the first predetermined current value I1 and the absolute value of the second predetermined current value I2 can be set to be the same as the following equation (6).

| I1 | = | I2 | (6)
In this case, the DSP 5 may change the shift voltage from 0.3 · VDD to 0. Since the A / D converter 4 operates at the power supply voltage VDD to 0 V, the offset applying unit 3 sets 0.5 · VDD to the first offset voltage that can be canceled by the A / D converter 4 acquiring the difference. V1 and the second offset voltage V2 are applied. For this reason, the DSP 5 does not need to remove the value corresponding to the offset from the output digital value of the A / D converter 4, and can use the digital value input from the A / D converter 4 as it is.

  Thereby, the DSP 5 can switch between the case where the expression (5) is satisfied and the case where the expression (6) is satisfied by switching the switches 16 and 17. For example, the current detection range of the sensor signal of the air-fuel ratio sensor 9 can be -10 mA to +40 mA, or the current detection range can be -25 mA to +25 mA.

  FIG. 4 shows a comparative example corresponding to FIG. 2, and FIG. 5 shows a comparative example of a portion corresponding to the offset applying unit 3 of FIG. For example, even if an operation is performed so as to generate a bias (−10 mA to +40 mA) in the current detection range of the air-fuel ratio sensor 9, as shown in FIG. Is given as 0.5 · VDD as it is, as shown in FIG. 4, the operation lower limit value of the A / D converter 4 is limited (see operation points P103 and P104). The use range of the D converter 4 is limited. For example, as described above, even if it is desired to cause the current detection range of the air-fuel ratio sensor 9 to be biased, for example, from -10 mA to +40 mA, it can be obtained only from -10 mA to +25 mA.

  On the other hand, according to the present embodiment, the offset applying unit 3 applies an offset during analog processing of the differential signal by the analog processing unit 2, and the DSP 5 provides from the A / D conversion result of the A / D converter 4. The value corresponding to the offset is digitally processed to remove the offset. For this reason, the influence of the offset given previously can be excluded. For this reason, even if the absolute values of the detection ranges are not equal between the rich state and the lean state, the differential input voltage range of the A / D converter 4 can be fully utilized.

  Since the offset applying unit 3 applies the first and second offset voltages V1 and V2 to the first and second differential amplifiers 3 and 4, the current detection range of the sensor signal of the air-fuel ratio sensor 9 can be shifted. For this reason, the current detection range of the sensor signal of the air-fuel ratio sensor 9 can be changed variously. As a result, various control methods can be supported.

  The offset applying unit 3 may be configured so that the first and second offset voltages V1 and V2 can be set to different values. Further, the offset applying unit 3 applies offsets that are in the opposite directions and have the same absolute value as the first and second offset voltages V1 and V2, with reference to the median value of the input range of the A / D converter 4. It is good to configure.

(Second Embodiment)
6 to 9 show additional explanatory views of the second embodiment. About the structure which has the same or similar function as the said embodiment, the same or similar code | symbol is attached | subjected and description is abbreviate | omitted as needed. As shown in FIG. 6, the signal processing device 201 includes an analog processing unit 202 instead of the analog processing unit 2. The analog processing unit 202 includes a first differential amplifier 213 that replaces the first differential amplifier 13 and a second differential amplifier 214 that replaces the second differential amplifier 14. In the present embodiment, the DSP 5 has functions as a resolution adjustment unit and a stoichiometric amplification degree adjustment unit.

  The first differential amplifier 213 includes an operational amplifier OP3, resistors R5, R205, and R6 connected in series between the output node of the first buffer 11 and the output node of the operational amplifier OP3, and the output node of the second buffer 12. A switch 218 that enables connection switching between one of the terminals of the resistor R205 and the inverting input terminal of the operational amplifier OP3, and a resistor R7, R207, R8 connected in series with the output node of the three buffer 18; A switch 219 that enables connection switching between either one of the terminals of R207 and the non-inverting input terminal of the operational amplifier OP3.

  The second differential amplifier 214 includes an operational amplifier OP4, resistors R9, R209, and R10 connected in series between the output node of the first buffer 11 and the output node of the fourth buffer 19, and the output node of the second buffer 12. And a switch 220 that enables connection switching between any one of the terminals of the resistor R209 and the non-inverting input terminal of the operational amplifier OP4, and the resistors R11, R211, and R12 connected in series between the output node of the operational amplifier OP4 and the operational amplifier OP4. A switch 221 that enables connection switching between either one of the terminals of the resistor R211 and the inverting input terminal of the operational amplifier OP4. The switches 218 to 221 can be switched and controlled by the DSP 5 (control unit). Other configurations are the same as those in the above-described embodiment, and thus description thereof is omitted.

  The operation of the above configuration will be described. A switch 218 is connected between both terminals of the operational amplifier OP3 and the resistor R205, and a switch 219 is connected between both terminals of the operational amplifier OP3 and the resistor R207. It is provided to adjust the degree of amplification.

  When switching the switch 218 to the common connection point side of the resistors R205 and R6, the DSP 5 switches the switch 219 to the common connection point side of the resistors R207 and R8. Further, when switching the switch 220 to the common connection point side of the resistors R209 and R10, the DSP 5 switches the switch 221 to the common connection point side of the resistors R211 and R12. For easy understanding of the explanation, if the resistance values of the resistors R5 to R12, R205, R207, R209, and R211 are set to the same value, the gains of the first differential amplifier 213 and the second differential amplifier 214 are set to the same value. Can be set.

  The values of the resistors R5, R7, R9, and R11 are the resistance value Ra, the values of the resistors R205, R207, R209, and R211 are the resistance value Rb, and the values of the resistors R6, R8, R10, and R12 are the resistance value Rc. As shown, when the switches 218 to 221 are switched, the amplification degree can be set to Rc / (Ra + Rb). A characteristic diagram corresponding to FIG. 2 at this time is shown in FIG. As shown in FIG. 8, the current detection range of the sensor signal of the air-fuel ratio sensor 9 can be shifted (see operating points P1 to P6). This description is omitted.

  As shown in FIG. 7, when the DSP 5 switches the switch 218 to the common connection point side of the resistors R205 and R5, the switch 219 switches to the common connection point side of the resistors R207 and R7. Further, when switching the switch 220 to the common connection point side of the resistors R209 and R9, the DSP 5 switches the switch 221 to the common connection point side of the resistors R211 and R11. As shown in FIG. 7, when the switches 218 to 221 are switched, the amplification degrees of the differential amplifiers 213 and 214 can be set to (Rb + Rc) / Ra. As a result, the DSP 5 can switch the switches 218 to 221 to change the amplification degrees of the first and second differential amplifiers 213 and 214. Since the sensitivity of the input voltage of the A / D converter 4 is changed by changing the amplification degrees of the first and second differential amplifiers 213 and 214 to be the same, the resolution by the A / D converter 4 is adjusted. it can.

  In the configuration of FIG. 7, the DSP 5 switches and controls the switches 16 and 17, so that the resistance voltage dividing circuit 15 sets its output voltage to 0.5 · VDD, which are set to the same value. Therefore, the offset applying unit 3 applies 0.5 · VDD that can be canceled by the A / D converter 4 to the first and second differential amplifiers 13 and 14, respectively. The DSP 5 does not need to remove the value corresponding to the offset from the output digital value of the A / D converter 4 and can use the digital value output from the A / D converter 4 as it is. As a result, digital processing can be eliminated and processing can be speeded up.

  Therefore, the DSP 5 switches and controls the switches 16, 17, 218 to 221, whereby the median value of the current detection range of the air-fuel ratio sensor 9 can be set to 0 A on the circuit configuration and the amplification degree can be increased. The sensor signal of the air-fuel ratio sensor 9 needs to be adjusted to 0 A (stoichiometric) because, for example, external factors fluctuate. At the time of this stoichiometry adjustment, in order to detect “0A” with high sensitivity, the amplification degree of the sensor signal of the air-fuel ratio sensor 9 is preferably set higher than the normal amplification degree. This is because the resolution of the A / D converter 4 can be increased by making the amplification degree higher than usual, so that the sensitivity of the voltage near 0 A can be increased and the stoichiometry can be easily adjusted.

  The DSP 5 performs switching control of the switches 16, 17, 218 to 221 as shown in FIGS. 6 and 7, so that the current detection range of the air-fuel ratio sensor 9 can be made lean burn (FIG. 6), stoichiometric. It is possible to switch to a configuration (FIG. 7) in which the sensitivity is high when adjusting the metrics.

  As described above, according to the present embodiment, the DSP 5 adjusts the amplification levels of the first differential amplifier 213 and the second differential amplifier 214 while keeping the same amplification levels, so that the A / D converter 4 Resolution can be adjusted. In addition, at the time of stoichiometry adjustment, the offset applying unit 3 uses the same value as the first offset voltage V1 and the second offset voltage V2 that can be canceled when the A / D converter 4 A / D converts the differential voltage. Since the DSP 5 adjusts the amplification degree of the first differential amplifier 213 and the second differential amplifier 214 to an amplification degree higher than the normal amplification degree, the sensitivity at the time of stoichiometry adjustment can be increased. .

(Other embodiments)
The present invention is not limited to the above-described embodiment, and for example, the following modifications or expansions are possible.
Although the DSP 5 is shown as a digital processing unit, the digital processing unit is not limited to the DSP 5 as long as the offset added by the offset applying unit 3 can be removed by digital processing.

The A / D converter 4 has shown a form in which a single power supply voltage between the voltage VDD and the ground is used as the power supply voltage, but both power supplies (for example, + VDD to −VDD) may be used.
In addition, the error components of the differential amplifiers 13 and 14 are added to the shift voltage, and the error components are stored in the storage unit 6 and set as constants used by the DSP 5, whereby the solid error components of the differential amplifiers 13 and 14 are stored. Can be corrected. In addition, an error correction process associated with the individual difference of the air-fuel ratio sensor 9 may be considered.

  The offset voltages V1 and V2 generated by the resistance voltage dividing circuit 15 may be further subdivided so as to be discretely changeable in three or more stages. Further, although a mode in which a plurality of offset voltages V1, V2 can be switched discretely has been shown, it is needless to say that it can be configured to be continuously switchable.

  In the drawing, 1, 201 are signal processing devices (signal processing devices for gas concentration sensors), 2, 202 are analog processing units, 3 is an offset applying unit, 4 is an A / D conversion unit, and 5 is a DSP (digital processing unit, Resolution adjusting unit, stoichiometric amplification degree adjusting unit), 13, 213, a first differential amplifier, and 14, 214, a second differential amplifier.

Claims (7)

An analog processing unit (2,202) that inputs a sensor signal of a gas concentration sensor that detects a gas concentration of a gas to be detected as a differential signal and performs analog processing;
An offset applying unit (3) for applying an offset during analog processing of the differential signal by the analog processing unit;
A differential input type A / D converter (4) for A / D converting a differential signal after processing by the analog processing unit;
A signal processing apparatus for a gas concentration sensor, comprising: a digital processing unit (5) that removes an offset by digitally processing a value corresponding to the given offset from an A / D conversion result of the A / D conversion unit. The analog processing unit (2,202)
A first differential amplifier (13, 213) to which a detection voltage for detecting a differential signal of the sensor signal is inverted and differentially amplified and a first offset (V1) is applied by the offset applying unit;
A second differential amplifier (14, 214) that differentially amplifies a detection voltage for detecting a differential signal of the sensor signal and is provided with a second offset (V2) by the offset applying unit;
The A / D converter performs A / D conversion on a differential voltage between an output voltage of the first differential amplifier and an output voltage of the second differential amplifier as a differential signal after processing by the analog processing unit. The signal processing apparatus for a gas concentration sensor according to claim 1.   The signal processing apparatus of the gas concentration sensor according to claim 2, wherein the offset applying unit (3) can set the first offset and the second offset to different values.   The offset assigning unit (3) assigns, as the first offset and the second offset, offsets that are in opposite directions and have the same absolute value with reference to the median value of the input range of the A / D converter. The signal processing apparatus for a gas concentration sensor according to claim 2 or 3, wherein The offset applying unit (3) can set the first offset and the second offset to the same value that can be canceled when the A / D converter (4) A / D converts the differential voltage. Yes,
When the offset applying unit (3) sets the first offset and the second offset to the same value, the digital processing unit (5) outputs the A / D conversion result of the A / D conversion unit (4). 3. The signal processing apparatus for a gas concentration sensor according to claim 2, wherein digital processing for removing the offset of the gas concentration sensor is unnecessary.   The resolution of the A / D converter (4) is adjusted by adjusting the value of the amplification level while making the amplification levels of the first differential amplifier (213) and the second differential amplifier (214) the same. The signal processing apparatus for a gas concentration sensor according to any one of claims 2 to 5, further comprising a resolution adjusting unit (5) for performing the processing. When adjusting stoichiometry,
The offset applying unit (3) sets the first offset and the second offset to the same value that can be canceled when the A / D converter (4) A / D converts the differential voltage,
A stoichiometric amplification degree adjustment unit (5) for adjusting the amplification degree of the first differential amplifier (213) and the second differential amplifier (214) to an amplification degree higher than a normal amplification degree; 6. A signal processing device for a gas concentration sensor according to claim 2, wherein the signal processing device is a gas concentration sensor.

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