Abstract:
In an existing disconnection detection circuit for a bridge circuit, consideration is not taken into the fact that an offset voltage or temperature characteristic of a bridge output is degraded. Provided is a disconnection detection circuit for a bridge circuit capable of suppressing a change in a characteristic of a sensor to a minimal extent. A disconnection detection circuit  8   a  for a bridge circuit in accordance with the present invention comprises conducting means  9  and  10  each of which causes a current to flow from an output terminal of the bridge circuit to a predetermined potential, potential difference detecting means  12  and  13  each of which detects a potential difference between the potential at the output terminal of the bridge circuit and the predetermined potential, and a disconnection detecting means  14  that detects a disconnection on the basis of the outputs of the potential difference detecting means.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a disconnection detection circuit for a bridge circuit, or more particularly, to a disconnection detection circuit for a bridge circuit that little affects an output voltage of a bridge circuit. 
       BACKGROUND ART 
       [0002]    Existing examples of a disconnection detection circuit for a bridge circuit include a sensor bridge circuit described in Japanese Unexamined Patent Application Publication No. Hei6-249730. 
       CITATION LIST 
     Patent Literature 
       [0003]    Patent literature 1: Japanese Unexamined Patent Application Publication No. Hei6-249730 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    An existing technology described in Japanese Unexamined Patent Application Publication No. Hei6-249730 is such that: as shown in  FIG. 2 , resistors  19  and  20  are connected to outputs of a bridge circuit composed of sensor element resistors  15 ,  16 ,  17 , and  18 , and to a power supply and a ground respectively; when the output of the bridge circuit is disconnected, an output voltage of the bridge circuit is largely varied; and a sensor output obtained by amplifying the output voltage of the bridge circuit using an amplifier  21  is largely varied so that the fact that the bridge circuit has been disconnected can be detected. 
         [0005]    However, in the foregoing disconnection detection circuit, consideration is not taken into the fact that since the resistors  19  and  20  are asymmetrically connected in parallel with the sensor bridge, an offset voltage or temperature characteristic of the sensor is degraded. 
         [0006]    The present invention addresses the foregoing situation. An object of the present invention is to provide a disconnection detection circuit for a bridge circuit which suppresses a change in a characteristic of a sensor to a minimalextent. 
       Solution to Problem 
       [0007]    In order to solve the aforesaid problem, a current is caused to flow from an output terminal of a bridge circuit to a predetermined potential, a potential difference between the potential at the output terminal of the bridge circuit and the predetermined potential is detected, and a disconnection is detected based on the potential difference. 
       Advantageous Effects of Invention 
       [0008]    According to the present invention, an adverse effect which a disconnection detection circuit imposes on an output voltage of a sensor bridge circuit can be reduced. Therefore, an offset voltage or temperature characteristic of a sensor bridge output can be improved, and the disconnection of the sensor bridge can be highly precisely detected. Eventually, a highly precise and highly reliable sensor can be provided. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a circuit diagram of a disconnection detection circuit for a bridge circuit of a first embodiment; 
           [0010]      FIG. 2  is a diagram showing a disconnection detection circuit for a bridge circuit of an existing technology; 
           [0011]      FIG. 3  is a circuit diagram of a disconnection detection circuit for a bridge circuit of a second embodiment; 
           [0012]      FIG. 4  is a diagram showing a drain current characteristic of transistors  24  and  25 ; 
           [0013]      FIG. 5  is a circuit diagram of a disconnection detection circuit for a bridge circuit of a third embodiment; 
           [0014]      FIG. 6  is a timing chart of control signals for switches  26 ,  28 ,  29 ,  30 ,  31 , and  32 ; 
           [0015]      FIG. 7  is a circuit diagram of a disconnection detection circuit for a bridge circuit of a fourth embodiment; 
           [0016]      FIG. 8  is a circuit diagram of a disconnection detection circuit for a bridge circuit of a fifth embodiment; 
           [0017]      FIG. 9  is a circuit diagram of a disconnection detection circuit for a bridge circuit of a sixth embodiment; 
           [0018]      FIG. 10  is a circuit diagram of a disconnection detection circuit for a bridge circuit of a seventh embodiment; 
           [0019]      FIG. 11  is a configuration diagram of a system of an eighth embodiment including a disconnection detecting means; 
           [0020]      FIG. 12  is a configuration diagram of a system of a ninth embodiment including a disconnection detecting means; and 
           [0021]      FIG. 13  is a configuration diagram of a system of a tenth embodiment including a disconnection detecting means. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0022]    Now, referring to  FIG. 1  to  FIG. 13 , embodiments of the present invention will be described below. 
         [0023]    To begin with, a disconnection detection circuit for a bridge circuit that is a first embodiment of the present invention will be described in conjunction with  FIG. 1 .  FIG. 1  is a circuit diagram of the disconnection detection circuit for a bridge circuit of the first embodiment. 
         [0024]    A detection element  1  is formed with a bridge circuit having a series circuit composed of sensor element resistors  2  and  4  and series resistors, which include sensor element resistors  3  and  5 , connected in parallel with each other. When the resistance values of the sensor element resistors  2 ,  3 ,  4 , and  5  vary depending on a measurement physical quantity, an intermediate voltage between the sensor element resistors  2  and  4  and an intermediate voltage between the sensor element resistors  3  and  5  vary. Incidentally, the intermediate voltage between the sensor element resistors  3  and  5  is inputted to an output terminal A, and fetched into outside of the detection element  1  through the output terminal A. The intermediate voltage between the sensor element resistors  2  and  4  is inputted to an output terminal B, and fetched into outside of the detection element  1  through the output terminal B. An output voltage (a voltage between the output terminals A and B) of the bridge circuit fetched through the output terminal A and output terminal B is amplified by an amplifier  6 , and outputted to outside as a sensor output via a switching circuit  7 . For the sensor element resistors, for example, platinum (Pt), tantalum (Ta), molybdenum (Mo), or silicon (Si) is selected. 
         [0025]    A disconnection detection circuit  8   a  includes a resistor  10  that causes a current to flow into the output terminal A, a resistor  9  that causes a current to flow into the output terminal B, a reference voltage source  11  that regenerates a reference voltage, a comparator  12  that compares the voltage at the output terminal A with the voltage at the reference voltage source  11  so as to detect the disconnection of the output terminal A, a comparator  13  that compares the voltage at the output terminal B with the voltage at the reference voltage source  11  so as to detect the disconnection of the output terminal B, and an OR circuit  14  that obtains an OR of the comparator  12  and comparator  13 . If the disconnection detection circuit  8   a  detects a disconnection, the switching circuit  7  fixes the sensor output to a ground voltage or a supply voltage. 
         [0026]    Next, actions of the disconnection detection circuit  8   a  will be described. The disconnection detection circuit  8   a  detects the disconnections of the output terminal A and output terminal B. 
         [0027]    If the output terminal A is disconnected, the potential at the output terminal A is brought to a ground potential by the resistor  10 . The potential at the output terminal A is compared with the voltage at the reference voltage source  11  by the comparator  12 . When the output terminal A is disconnected, the disconnection is reflected on the output of the comparator  12 . 
         [0028]    If the output terminal B is disconnected, the potential at the output terminal B is brought to the ground potential by the resistor  9 . The potential at the output terminal B is compared with the voltage at the reference voltage source  11  by the comparator  13 . When the output terminal B is disconnected, the disconnection is reflected on the output of the comparator  13 . 
         [0029]    Therefore, if the output terminal A or output terminal B is disconnected, the disconnection is reflected on the output of the OR circuit that obtains the OR of the outputs of the comparators  12  and  13 . Owing to the constitution, the disconnection detection circuit  8  can detect the disconnections of the output terminal A and output terminal B. 
         [0030]    Next, the features of the disconnection detection circuit  8   a  of the present embodiment will be described below. The disconnection detection circuit  8   a  is a circuit that is symmetrical with respect to the output terminals A and B of the bridge circuit of the detection element  1 , whereby an adverse effect on an output voltage of the bridge circuit of the detection element  1  can be minimized. Specifically, a circuit to be connected to the output terminal A includes the resistor  10  and comparator  12 , and a circuit to be connected to the output terminal B includes the resistor  9  and comparator  13 . Thus, since the identical circuits are connected to the respective output terminals, adverse effects which the disconnection detection circuit  8   a  imposes on the output terminal A and output terminal B respectively are identical to each other. Accordingly, an adverse effect on the output voltage of the bridge circuit of the detection element  1  (a difference voltage between the output terminal A and output terminal B) can be reduced. 
         [0031]    Next, a disconnection detection circuit for a bridge circuit that is a second embodiment of the present invention will be described in conjunction with  FIG. 3  and  FIG. 4 . Incidentally,  FIG. 3  is a circuit diagram of the disconnection detection circuit for a bridge circuit of the second embodiment, and  FIG. 4  is a diagram showing a drain current characteristic of transistors  24  and  25 . The disconnection detection circuit for a bridge circuit of the second embodiment is such that the resistors  9  and  10  of the disconnection detection circuit for a bridge circuit of the first embodiment are changed into a current mirror circuit composed of transistors  23 ,  24 , and  25 . 
         [0032]    In the disconnection detection circuit  8   b  for a bridge circuit of the present embodiment, the resistors  9  and  10  are changed into a current mirror circuit composed of the transistors  23 ,  24 , and  25 . A constant current source  22  is connected to the transistor  23 , so that the drain currents of the transistors  24  and  25  exhibit a characteristic shown in  FIG. 4 . Accordingly, when the output terminals A and B are not disconnected, the impedances of the transistors  24  and  25  with respect to the output terminals A and B can be raised. Therefore, an adverse effect on the output voltage of the bridge circuit can be further reduced. 
         [0033]    When the disconnection of the output terminal A or output terminal B is detected, the impedance of the transistor  24  or  25  with respect to the output terminal A or B can be diminished. Therefore, since the voltage at the disconnected output terminal A or B can be dropped, a margin of a threshold for the comparator  12  or  13  can be increased. Eventually, precision in disconnection detection can be improved. 
         [0034]    Next, a disconnection detection circuit for a bridge circuit of a third embodiment of the present invention will be described in conjunction with  FIG. 5  and  FIG. 6 .  FIG. 5  is a circuit diagram of the disconnection detection circuit for abridge circuit of the third embodiment, and  FIG. 6  is a timing chart of control signals for switches  26 ,  28 ,  29 ,  30 ,  31 , and  32 . The third embodiment is such that the switches  26 ,  28 ,  29 ,  30 ,  31 , and  32  and capacitors  27 ,  33 , and  34  are added to the first embodiment. 
         [0035]    In the present embodiment, the switches  26 ,  28 ,  29 ,  30 ,  31 , and  32  and capacitors  27 ,  33 , and  34  are added so that detection of a bridge voltage and disconnection detection can be executed in time-sharing manner. Specifically, at timing P 1 , the switches  26  and  30  are made in order to connect the output terminals A and B to the amplifier  6 , whereby the output voltage of the bridge circuit is detected. At this time, the switches  28  and  29  enter a broken state. Therefore, the disconnection detection circuit  8   c  does not affect the output voltage of the bridge circuit because the bridge circuit is completely disconnected from the disconnection detection circuit  8   c . The capacitor  27  is included to hold the voltage at the timing P 1  in preparation for the timing P 2  when the switches  26  and  30  are broken. 
         [0036]    Thereafter, the switches  28 ,  29 ,  31 , and  32  are made at the timing P 2  in order to connect the output terminals A and B to the disconnection detection circuit  8   c , whereby the disconnection of the output terminal A or B of the bridge circuit is detected. At this time, the switches  26  and  30  are left broken and completely disconnected from the amplifier  6 . Therefore, an input resistance of the amplifier  6  does not affect the disconnection detection circuit  8 . The capacitors  33  and  34  are included to hold the voltages attained at the timing P 2  in preparation for the timing P 1  when the switches  28 ,  29 ,  31 , and  32  are broken. 
         [0037]    Next, a disconnection detection circuit for a bridge circuit that is a fourth embodiment of the present invention will be described in conjunction with  FIG. 7 .  FIG. 7  is a circuit diagram of the disconnection detection circuit for a bridge circuit of the fourth embodiment. 
         [0038]    To begin with, a detection element  35  is a half bridge circuit composed of sensor element resistors  36  and  37 . When the sensor element resistors  36  and  37  vary depending on a measurement physical quantity, a voltage at an output terminal of the bridge circuit is varied. An output voltage of the half bridge circuit is amplified by an amplifier  39  and outputted to outside as a sensor output. A supply voltage Vcc is connected to a power terminal of the bridge circuit, and a predetermined voltage is fed to a ground terminal via a reference voltage source  38 . A disconnection detection circuit  41  includes a constant current source  42  that feeds a current to the output terminal of the half bridge circuit, a reference voltage source  43  that generates a reference voltage, and a comparator  44  that compares a voltage at the output terminal of the half bridge circuit with the value of the reference voltage source  43  so as to detect the disconnection of the output terminal. If the disconnection detection circuit  41  detects a disconnection, a switching circuit  40  fixes the sensor output to a ground voltage or supply voltage. 
         [0039]    Next, actions of the disconnection detection circuit will be described below. The disconnection detection circuit detects the disconnection of the output terminal of the half bridge circuit. To begin with, if the output terminal is disconnected, the potential at the output terminal is brought to a ground potential by the constant current source  42 . The potential at the output terminal is compared with the voltage of the reference voltage source  43  by the comparator  44 . Therefore, if the output terminal is disconnected, the disconnection is reflected on the output of the comparator  44 . Thus, the disconnection detection circuit  41  detects the disconnection of the output terminal. 
         [0040]    Next, the features of the disconnection detection circuit will be described below. Assuming that the sensor element resistor  36  is a component whose resistance varies, like a thermistor, by several digits depending on temperature, the voltage at the output terminal changes from near the voltage at the power terminal of the detection element  35  to near the voltage at the ground terminal. 
         [0041]    Assuming that the voltage at the ground voltage is 0 V, when the disconnection of the output terminal has to be reliably detected, it is necessary to increase the current of the constant current source  42  and to set the voltage of the reference voltage source  43  to almost 0 V. This is because, since the output of the detection element  35  at a normal time changes from near the voltage at the power terminal of the detection element  35  to near the voltage at the ground terminal, it is necessary to bring the voltage at a disconnection time to a voltage that falls outside the output at the normal time, and to bring the voltage of the reference voltage source  43 , which is a voltage to be compared by the comparator  44 , to the voltage that falls outside the output at the normal time. Therefore, in order to reliably bring the voltage at the disconnection time to the voltage that falls outside the output at the normal time, that is, in order to bring the voltage to almost 0 V, it is necessary to increase the current of the constant current source  42  and to bring a comparison voltage of the comparator  44  to near almost 0 V. However, when the current of the constant current source  42  is increased, an adverse effect on a sensor output is intensified. When the voltage of the reference voltage source is brought to almost 0 V, a margin of a threshold for the comparator  44  nearly runs out. Eventually, precision in disconnection detection is degraded. 
         [0042]    In the present embodiment, as a voltage at a ground terminal of the detection element  35 , a voltage of several volts is applied using the reference voltage source  38 . In this case, even if the sensor element resistor  36  is a component whose resistance varies, like a thermistor, by several digits depending on temperature, the voltage at the output terminal changes merely from a voltage at a power terminal of the detection element  35  to the voltage of the reference voltage source  38  that is the voltage at the ground terminal. Therefore, the current of the constant current source  42  can be diminished because it should merely be equal to or lower than the voltage of the reference voltage source  38  at the disconnection time of the output terminal of the detection element  35 . In addition, since the voltage of the reference voltage source  43  that is a reference value for disconnection detection can be set to the voltage of the reference voltage source  38 , the margin of the threshold for the comparator  44  can be increased. Eventually, precision in disconnection detection can be improved. 
         [0043]    Next, a disconnection detection circuit for a bridge circuit which is a fifth embodiment of the present invention will be described in conjunction with  FIG. 8 .  FIG. 8  is a circuit diagram of the disconnection detection circuit for a bridge circuit of the fifth embodiment. Incidentally, the disconnection detection circuit for a bridge circuit of the fifth embodiment is, contrary to the disconnection detection circuit for a bridge circuit of the fourth embodiment, such that a voltage at a power terminal of a detection element  35  is dropped by several volts using a reference voltage source  45 . 
         [0044]    To begin with, the detection element  35  is a half bridge circuit composed of sensor element resistors  36  and  37 . The sensor element resistors  36  and  37  vary depending on a measurement physical quantity, whereby a voltage at an output terminal of the bridge circuit is varied. The output voltage of the bridge circuit is amplified by an amplifier  39  and outputted to outside as a sensor output. A power terminal of the bridge circuit is connected to a supply voltage Vcc via a reference voltage source  45 . A voltage that is lower than the supply voltage Vcc by the voltage of the reference voltage source  45  is fed to the power terminal. A ground terminal is provided with a ground potential. A disconnection detection circuit  46  includes a constant current source  47  that feeds a current into the output terminal, a reference voltage source  48  that generates a reference voltage, and a comparator  49  that compares the voltage at the output terminal with the value of the reference voltage source  48  so as to detect the disconnection of the output terminal. If the disconnection detection circuit  46  detects a disconnection, a switching circuit  40  fixes the sensor output to a ground voltage or supply voltage. 
         [0045]    Next, actions of the disconnection detection circuit will be described below. The disconnection detection circuit detects the disconnection of the output terminal. If the output terminal is disconnected, the potential at the output terminal is brought to the supply voltage by the constant current source  47 . The potential at the output terminal is compared with the voltage of the reference voltage source  48  by the comparator  49 . Therefore, if the output terminal is disconnected, the disconnection is reflected on the output of the comparator  49 . Accordingly, the disconnection detection circuit  46  detects the disconnection of the output terminal. 
         [0046]    Next, the features of the disconnection detection circuit will be described below. When the sensor element resistor  36  is, like a thermistor, a component whose resistance value varies by several digits depending on temperature, the voltage at the output terminal changes from the voltage at the power terminal of the detection element  35  to the voltage at the ground terminal. Assume that the voltage at the power terminal is equal to the supply voltage Vcc. In this case, for reliably detecting the disconnection of the output terminal, it is necessary to increase the current of the constant current source  47  and to set the voltage of the reference voltage source  48  to almost the supply voltage Vcc. However, when the current of the constant current source  47  is increased, an adverse effect on the sensor output is intensified. When the voltage of the reference voltage source  48  is set to almost the supply voltage Vcc, a margin of a threshold for the comparator  49  nearly runs out. Therefore, precision in disconnection detection is degraded. 
         [0047]    In the present embodiment, the reference voltage source  45  is used to drop the voltage at the power terminal of the detection element  35  so that the voltage becomes lower than the supply voltage Vcc by several volts. In this case, even if the sensor element resistor  36  is, like a thermistor, a component whose resistance value varies by several digits depending on temperature, the voltage at the output terminal merely changes from a voltage, which is lower by several volts than the supply voltage that is equal to the voltage at the power terminal of the detection element  35 , to the ground voltage. Therefore, the current of the constant current source  47  can be diminished because when the output terminal of the detection element  35  is disconnected, the voltage at the power supply should merely approach the supply voltage with a margin equivalent to the voltage of the reference voltage source  45 . In addition, the voltage of the reference voltage source  48  that is a reference value for disconnection detection can be set with a margin equivalent to the voltage of the reference voltage source  45 . Therefore, the margin of the threshold for the comparator  49  can be increased. Eventually, precision in disconnection detection can be improved. 
         [0048]    Next, a disconnection detection circuit for a bridge circuit that is a sixth embodiment of the present invention will be described below in conjunction with  FIG. 9 .  FIG. 9  is a circuit diagram of the disconnection detection circuit for a bridge circuit of the sixth embodiment. 
         [0049]    To begin with, a detection element  50  is a bridge circuit composed of sensor element resistors  51 ,  52 ,  53 , and  54 . The sensor element resistors  51 ,  52 ,  53 , and  54  vary depending on a measurement physical quantity, whereby voltages at output terminals A and B of the bridge circuit are varied. An output voltage of the bridge circuit (a voltage between the output terminals A and B) is analog-to-digital converted by a delta-sigma modulator  55  (hereinafter, a ΔΣ modulator), and outputted to outside as a sensor output. Incidentally, the ΔΣ modulator  55  includes an SC integrator that is composed of switches  56 ,  58 ,  63 , and  65  which act at the timing P 1  shown in  FIG. 6 , switches  59 ,  60 ,  61 , and  62  which act at the timing P 2  shown in  FIG. 6 , capacitors  57 ,  64 ,  66 , and  68 , and an amplifier  67 , a comparator  69  that compares the output of the SC integrator, a local digital-to-analog (D/A) converter  70  that outputs a voltage according to the output of the comparator  69 . A disconnection detection circuit  72  includes a switch  74  that acts at the timing P 2  shown in  FIG. 6  so as to link the output terminal A and a constant current source  76 , a switch  73  that acts at the timing P 2  shown in  FIG. 6  so as to link the output terminal B and a constant current source  75 , the constant current source  76  that feeds a current into the output terminal A, the constant current source  75  that feeds a current into the output terminal B, a sample-and-hold circuit that samples voltages across the constant current sources  75  and  76  at the timing P 2  shown in  FIG. 6  and is composed of switches  77  and  78  and capacitors  79  and  80 , a reference voltage source  81  that generates a reference voltage, a comparator  82  that compares a voltage across the capacitor  79  with the value of the reference voltage source  82  so as to detect the disconnection of the output terminal A, a comparator  83  that compares a voltage across the capacitor  80  with the value of the reference voltage source  81  so as to detect the disconnection of the output terminal B, and an OR circuit  84  that obtains an OR of the comparator  82  and comparator  83 . In addition, there is an AND circuit  71  that, if the disconnection detection circuit  72  detects a disconnection, fixes the output of the ΔΣ modulator  55  to a ground. 
         [0050]    Next, actions of the present embodiment will be described below. The ΔΣ modulator  55  discharges the capacitors  57  and  64  at the timing P 2 . At the timing P 1 , the ΔΣ modulator  55  samples the output voltage of the bridge circuit and charges the capacitors using the SC integrator. The disconnection detection circuit  72  acts at the timing P 2 , which is a non-sampling period of the ΔΣ modulator  55 , so as not to affect the action of the ΔΣ modulator  55 . The disconnection detection circuit  72  turns on the switches  73 ,  74 ,  77 , and  78  at the timing P 2  so that a constant current flows into the output terminals A and B of the bridge circuit. At this time, if the output terminal A or output terminal B is disconnected, the voltage across the associated constant current source  75  or  76  is dropped to almost a ground potential. The voltage is held by the sample-and-hold circuit including the switches  77  and  78  and capacitors  79  and  80 , and compared by the comparator  82  or  83 , whereby the disconnection of the output terminal A or B is detected. 
         [0051]    Next, the features of the present embodiment will be described below. Since the disconnection detection circuit  72  is a circuit symmetrical with respect to the output terminals A and B of the bridge circuit of the detection element  50 , an adverse effect on the output voltage of the bridge circuit of the detection element  50  can be minimized. 
         [0052]    Since the disconnection detection circuit  72  acts during the non-sampling period of the ΔΣ modulator  55 , the action of the disconnection detection circuit  72  does not affect the ΔΣ modulator  55 . In reverse, the action of the ΔΣ modulator  55  does not affect the disconnection detection circuit  72 . 
         [0053]    When the ΔΣ modulator  55  is employed in a detection circuit for an output voltage of a bridge circuit, it is very easy to fix an output signal to a value that cannot be outputted as an ordinary sensor output because an AND circuit alone is needed. 
         [0054]    Next, a disconnection detection circuit for a bridge circuit that is a seventh embodiment of the present invention will be described in conjunction with  FIG. 10 .  FIG. 10  is a circuit diagram of the disconnection detection circuit for a bridge circuit of the seventh embodiment. The present embodiment is such that the constant current sources  75  and  76  of the sixth embodiment are replaced with a switched capacitor circuit composed of switches  85  and  87  and capacitors  86  and  88 . 
         [0055]    In the present embodiment, faster detection than that achieved using the constant current sources  75  and  76  is enabled by replacing the constant current sources  75  and  76  with a switched capacitor circuit. This is because the switched capacitor circuit provides less impedance than the constant current sources do. This makes it possible to speed up an operating clock for the ΔΣ modulator  55 . Eventually, precision of the ΔΣ modulator  55  and responsiveness thereof can be improved. 
         [0056]    Next, a system that is an eighth embodiment of the present invention and includes a disconnection detecting means will be described in conjunction with  FIG. 11 .  FIG. 11  is a configuration diagram of the system of the eighth embodiment including the disconnection detecting means. 
         [0057]    The present embodiment includes an airflow sensor  89  that detects an air flow rate Q, an intake air temperature sensor  90  that detects intake air temperature Ta, a disconnection detector  91  that detects the disconnection of the intake air temperature sensor  90 , a correction circuit  92  that corrects the air flow rate Q, which is an output signal of the airflow sensor  89 , with the intake air temperature Ta that is an output signal of the intake air temperature sensor  90 , and a switching circuit  93  that, if the disconnection detector  91  detects a disconnection, fixes the signal of the intake air temperature Ta, which is handed to the correction circuit  92 , to 25° C. 
         [0058]    The present embodiment is the system in which if the intake air temperature sensor  90  is disconnected and outputs the signal having an extremely large error, the correction circuit  92  is prevented from performing excess correction processing and outputting a signal, which has an extremely large error, as a sensor output (air flow rate signal). In the present embodiment, if the intake air temperature sensor  90  is disconnected, the switching circuit  93  fixes the signal of the intake air temperature Ta, which is fed to the correction circuit  92 , to 25° C. Thus, excessive correction is prevented. Accordingly, even if the intake air temperature sensor  90  is disconnected, an error in the sensor output can be suppressed. In particular, as far as an airflow sensor that measures an intake air flow rate of an automobile is concerned, if an error caused by the airflow sensor is large, a fatal phenomenon that an engine is not started takes place. In particular, such an event must be avoided that although the airflow sensor  89  does not fail, the sensor output becomes extremely abnormal because of the failure of the intake air temperature sensor  90 . The present system can avoid the event. 
         [0059]    Next, a system that is a ninth embodiment of the present invention and includes a disconnection detecting means will be described in conjunction with  FIG. 12 .  FIG. 12  is a configuration diagram of the system of the ninth embodiment including the disconnection detecting means. The present embodiment is a system that includes the disconnection detecting means and has the switching circuit  93 , which is included in the system of the eighth embodiment including the disconnection detecting means, changed into a switching circuit  94 . 
         [0060]    In the present embodiment, the switching circuit  93  is changed into the switching circuit  94 . If the intake air temperature sensor is disconnected, the correction circuit  92  is bypassed in order to prevent excessive correction. Thus, even if the intake air temperature sensor  90  is disconnected, an error in the sensor output can be suppressed. 
         [0061]    Next, a system that is a tenth embodiment of the present invention and includes a disconnection detecting means will be described in conjunction with  FIG. 13 .  FIG. 13  is a configuration diagram of the system of the tenth embodiment including the disconnection detecting means. The present embodiment is a system that includes the disconnection detecting means, has a circuit temperature sensor  95  added to the system of the eighth embodiment including the disconnection detecting means, and has a switching destination at a disconnection time by the switching circuit  93  changed to another. 
         [0062]    In the present embodiment, the circuit temperature sensor  95  is added, and the switching destination at a disconnection time by the switching circuit  93  is changed to another. If the intake air temperature sensor is disconnected, the switching destination of the correction circuit  92  is set to the circuit temperature sensor  95  in order to prevent excessive correction. This is attributable to the fact that in a steady state, there is no large difference between the intake air temperature Ta and circuit temperature Tlsi. Accordingly, even if the intake air temperature sensor  90  is disconnected, an error in the sensor output can be suppressed. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           1 ,  35 ,  50 : detection element 
           2 ,  3 ,  4 ,  5 ,  15 ,  16 ,  17 ,  18 ,  36 ,  37 ,  51 ,  52 ,  53 ,  54 : sensor element resistor 
           6 ,  21 ,  39 ,  67 : amplifier 
           7 ,  40 ,  93 ,  94 : switching circuit 
           8 ,  8   a ,  8   b ,  8   c ,  41 ,  46 : disconnection detection circuit 
           9 ,  10 ,  19 ,  20 : resistor 
           11 ,  38 ,  43 ,  45 ,  58 ,  81 : reference voltage source 
           12 ,  13 ,  44 ,  49 ,  69 ,  82 ,  83 : comparator 
           14 ,  84 : OR circuit 
           22 ,  42 ,  47 ,  75 ,  76 : constant current source 
           23 ,  24 ,  25 : transistor 
           26 ,  28 ,  29 ,  30 ,  31 ,  32 ,  56 ,  58 ,  59 ,  60 ,  61 ,  62 ,  63 ,  65 ,  73 ,  74 ,  77 ,  78 ,  85 ,  87 : switch 
           27 ,  33 ,  34 ,  57 ,  64 ,  66 ,  68 ,  79 ,  80 ,  86 ,  88 : capacitor 
           55 : ΔΣ modulator 
           70 : local D/A converter 
           71 : AND circuit 
           72 : disconnection detection circuit 
           89 : airflow sensor 
           90 : intake air temperature sensor 
           91 : disconnection detector 
           92 : correction circuit