Abstract:
A signal processing device includes: a signal processing circuit that processes an input signal, and outputs a signal corresponding to the input signal; an offset input device that inputs a diagnosis offset signal as an internal signal in a passage between an input side and an output side of the signal processing circuit; a self-diagnosis device that performs a self-diagnosis of the signal processing circuit based on a variation in the signal output from the signal processing circuit when the diagnosis offset signal input by the offset input device is varied by a predetermined amount; and an extraction device that removes a component of the diagnosis offset signal from the signal output from the signal processing circuit, and extracts only a signal corresponding to the input signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on Japanese Patent Application No. 2014-232609 filed on Nov. 17, 2014, the disclosure of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a signal processing device having a signal processing circuit that processes an input signal from a sensor unit for detecting, for example, a physical quantity, and outputs a signal corresponding to the input signal. 
       BACKGROUND 
       [0003]    For example, a capacitive acceleration sensor device mounted in an automobile airbag system includes a semiconductor acceleration sensor chip (sensor element), and a signal processing device mainly having a C/V conversion circuit that processes a detection signal from the sensor chip (for example, refer to JP-2009-75097 A (Patent Literature 1). 
         [0004]    The acceleration sensor device is provided with a self-diagnosis function for diagnosing whether the acceleration sensor device per se operates normally, or not (a predetermined sensitivity is obtained, or abnormality such as foreign matter is present in the sensor chip). The self-diagnosis function forcedly supplies a self-diagnosis signal different from a carrier at the time of a normal acceleration detection to the acceleration sensor chip to perform a diagnosis according to whether a signal commensurate with the self-diagnosis signal is obtained, or not. 
         [0005]    In the above Patent Literature 1, in order to realize the self-diagnosis function, there is a need to provide a process (phase) of the self-diagnosis separately from the normal acceleration detection time. For that reason, the self-diagnosis function needs to be executed at the time of starting the use of the sensor device (at the time of starting an engine), or to be performed with a changeover of the phase from the normal operation phase to the self-diagnosis process as occasion demands. In other words, up to now, the self-diagnosis can be performed only when the sensor unit is not used, and it is desirable that the self-diagnosis function can be always executed even during the use of the sensor unit (during acceleration detection). 
       SUMMARY 
       [0006]    It is an object of the present disclosure to provide a signal processing device having a signal processing circuit that processes an input signal from, for example, a sensor unit, which always executes a self-diagnosis function. 
         [0007]    According to an example aspect of the present disclosure, a signal processing device includes: a signal processing circuit that processes an input signal, and outputs a signal corresponding to the input signal; an offset input device that inputs a diagnosis offset signal as an internal signal in a passage between an input side and an output side of the signal processing circuit; a self-diagnosis device that performs a self-diagnosis of the signal processing circuit based on a variation in the signal output from the signal processing circuit when the diagnosis offset signal input by the offset input device is varied by a predetermined amount; and an extraction device that removes a component of the diagnosis offset signal from the signal output from the signal processing circuit, and extracts only a signal corresponding to the input signal. 
         [0008]    In the above signal processing device, when the offset input device forcibly inputs the diagnosis offset signal into the signal processing circuit, the signal in the signal processing circuit is varied with a variation amount corresponding to the diagnosis offset signal, according to a predetermined variation of the diagnosis offset signal. Thus, the self-diagnosis device monitors a variation of the signal with respect to the diagnosis offset signal, and the device can determine whether the signal processing circuit functions normally. 
         [0009]    At the same time as the self-diagnosis, the extraction device extracts only the signal corresponding to the input signal from the signal output from the signal processing circuit by cancelling a variation of the diagnosis offset signal. Thus, the device always detects the physical quantity detected by the sensor unit. Thus, the signal processing device includes the signal processing circuit, the device always executes the self-diagnosis function without setting a phase for performing the self-diagnosis at a period other than a normal operation period. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
           [0011]      FIG. 1  is a diagram schematically illustrating an electric configuration of a main portion of a semiconductor acceleration sensor device according to a first embodiment of the disclosure; 
           [0012]      FIG. 2  is a timing chart illustrating an example of a waveform of a carrier, an offset input, and outputs of respective units; 
           [0013]      FIG. 3A  is a schematic top view of a sensor chip, and  FIG. 3B  is a vertically cross-sectional front view of the sensor chip; 
           [0014]      FIG. 4  is a diagram illustrating a modification of a pattern of an offset input; 
           [0015]      FIG. 5  is a diagram corresponding to  FIG. 1  according to a second embodiment of the disclosure; 
           [0016]      FIGS. 6A to 6C  are diagrams illustrating a signal in each section; 
           [0017]      FIG. 7  is a diagram corresponding to  FIG. 1  according to a third embodiment of the disclosure; and 
           [0018]      FIG. 8  is a timing chart illustrating an example of a waveform of a carrier, an offset input, and so on. 
       
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       [0019]    Hereinafter, a description will be given of a capacitive semiconductor acceleration sensor device according to a first embodiment of the disclosure with reference to  FIGS. 1 to 3 .  FIG. 1  is a diagram schematically illustrating an electric configuration of a capacitive semiconductor acceleration sensor device  11 , and  FIGS. 3A and 3B  are schematic views of a sensor chip  12  in the capacitive semiconductor acceleration sensor device  11 . As illustrated in  FIG. 1 , the semiconductor acceleration sensor device  11  includes the sensor chip  12  as a sensor unit (sensor element), and a signal processing device  13  according to this embodiment. 
         [0020]    First, a schematic configuration of the sensor chip  12  will be described. As illustrated in  FIG. 3B , the sensor chip  12  has an acceleration detection unit  14  as a physical quantity detection unit which is located in a rectangular region of a center portion of the sensor chip  12 . The acceleration detection unit  14  is formed, for example, in such a manner that a rectangular (square) SOI substrate where a single crystal silicon layer  12   c  is formed over a support substrate  12   a  made of silicon through an oxide film  12   b  is provided as a base, and grooves are produced in the single crystal silicon layer  12   c  of a surface of the SOI substrate through a micromachining technique. 
         [0021]    In that case, the acceleration detection unit  14  has a detection axis (X-axis) in one direction, and detects an acceleration in an anteroposterior direction (X-axial direction) in  FIG. 3A . The acceleration detection unit  14  includes a movable electrode part  15  that is displaced in the X-axial direction according to an action of acceleration, and a pair of first and second fixed electrode parts  16 ,  17  on left and right sides. In the acceleration detection unit  14 , the movable electrode part  15  includes a weight part  15   a , spring parts  15   b , and an anchor part  15   c . The weight part  15   a  extends in the center of the acceleration detection unit  14  in the anteroposterior direction. The spring parts  15   b  are provided on both ends of the weight part  15   a  in the anteroposterior direction, and each shaped into a slender rectangular frame in a lateral direction. The anchor part  15   c  is disposed in front of the front side spring part  15   b  in  FIG. 3A . The movable electrode part  15  also includes multiple thin movable electrodes  15   d  extending from the weight part  15   a  toward the lateral direction in a so-called pectinate shape. 
         [0022]    As illustrated in  FIG. 3B , the movable electrode part  15  floats in a so-called cantilevered state where the oxide film  12   b  on a lower surface side of the sensor chip  12  is removed except for the anchor part  15   c , and only the anchor part  15   c  is supported by the support substrate  12   a . As illustrated in  FIG. 1 , an upper surface part of the anchor part  15   c  is equipped with an input terminal  18  formed of an electrode pad. As will be described later, a carrier D 1  is input to the input terminal  18 . 
         [0023]    On the contrary, the first fixed electrode part  16  on the left side includes a rectangular base  16   a , multiple fixed electrodes  16   b  extending from the rectangular base  16   a  to the right in a pectinate shape, and a fixed electrode wire part  16   c  extending forward from the base  16   a . The respective fixed electrodes  16   b  are disposed to be adjacent to each other in parallel through a small gap immediately on a rear side of the respective movable electrodes  15   d . As illustrated in  FIG. 1 , an upper surface of a front end of the fixed electrode wire part  16   c  is equipped with a first output terminal  19  formed of an electrode pad. 
         [0024]    The second fixed electrode part  17  on the right side includes a rectangular base  17   a , multiple fixed electrodes  17   b  extending from the rectangular base  17   a  to the left in a pectinate shape, and a fixed electrode wire part  17   c  extending forward from the base  17   a . The respective fixed electrodes  17   b  are disposed to be adjacent to each other in parallel through a small gap immediately on a front side of the respective movable electrodes  15   d . As illustrated in  FIG. 1 , an upper surface of a front end of the fixed electrode wire part  17   c  is equipped with a second output terminal  20  formed of an electrode pad. 
         [0025]    As a result, capacitors C 1  and C 2  (refer to  FIG. 1 ) having the movable electrode part  15  as a common electrode are formed between the movable electrode part  15  (movable electrodes  15   d ) and the first fixed electrode part  16  (fixed electrodes  16   b ) and between the movable electrode part  15  (movable electrodes  15   d ) and the second fixed electrode part  17  (fixed electrodes  17   b ), respectively. Capacitances of those capacitors C 1  and C 2  differentially change according to a displacement of the movable electrode part  15  caused by the action of acceleration in the X-axial direction, and therefore the acceleration can be extracted as a change in capacitance values. 
         [0026]    Although not shown in detail, the sensor chip  12  has a so-called stack structure implemented on a circuit chip where the respective circuits of the signal processing device  13  are formed. The sensor chip  12  is housed in, for example, a package made of ceramic. The first and second output terminals (electrode pads  19  and  20 ) of the sensor chip  12  are connected to first and second input terminals (not illustrated) disposed in the signal processing device  13 , respectively. The electric connections are performed by bonding wire connections or bump connections. 
         [0027]    Then, the signal processing device  13  according to this embodiment will be described. As illustrated in  FIG. 1 , the signal processing device  13  has a signal processing circuit  21  for processing the signal from the sensor chip  12 . In addition, the signal processing device  13  includes a carrier signal input circuit  22 , a control logic circuit  23 , a determination logic circuit  24 , a diagnosis offset input circuit  25 , and a moving average filter circuit (MAF)  26 . The control logic circuit  23  and the determination logic circuit  24  each mainly include a computer, and perform controls and determinations to be described later with a software configuration of the computer. 
         [0028]    The signal processing circuit  21  includes a fully differential C/V conversion circuit  27  that converts a capacitance change into a voltage change, a sample and hold (S/H) circuit  28  that samples and holds a voltage signal output from the C/V conversion circuit  27  at a predetermined timing, and an A/D conversion circuit  29  that converts a signal output from the sample and hold circuit  28  into a digital signal. The output signal processed in the signal processing circuit  21  is output from the A/D conversion circuit  29 . 
         [0029]    The C/V conversion circuit  27  includes a fully differential amplifier  30  having two non-inverting and inverting input terminals and two first and second output terminals, a capacitor  31  and a first switch  32  which are connected in parallel to each other between the non-inverting input terminal of the fully differential amplifier  30  and the first output terminal on a negative side, and a capacitor  33  and a second switch  34  which are connected in parallel to each other between the inverting input terminal of the fully differential amplifier  30  and the second output terminal on a positive side. The first output terminal  19  of the sensor chip  12  is connected to the non-inverting input terminal of the fully differential amplifier  30 , and the second output terminal  20  of the sensor chip  12  is connected to the inverting input terminal of the fully differential amplifier  30 . 
         [0030]    The carrier signal input circuit  22  generates the carrier D 1 , and inputs the carrier D 1  to the movable electrode part  15  (input terminal  18 ) of the sensor chip  12  on the basis of a command from the control logic circuit  23 . As illustrated in  FIG. 2 , the carrier D 1  amplitudes between a predetermined voltage (for example, 5V equal to a power source voltage) and 0V, and is formed into a pulse shape (rectangular waveform) having a frequency of, for example, 120 kHz. In this situation, the carrier D 1  is always supplied to the movable electrode part  15  during the operation of the acceleration sensor device  11 . 
         [0031]    The diagnosis offset input circuit  25  inputs a diagnosis offset to any internal signal of the signal processing circuit  21  on the basis of the command from the control logic circuit  23 . Therefore, the diagnosis offset input circuit  25  functions as offset input device. In this embodiment, the output signal is input to an input side of the C/V conversion circuit  27  (fully differential amplifier  30 ). In detail, as will be described in the description of the operation later, the diagnosis offset input circuit  25  inputs offset signals S 1  and S 2  to the non-inverting input terminal and the inverting input terminal of the fully differential amplifier  30 , respectively. Those offset signals S 1  and S 2  have magnitudes corresponding to +0.5 G and −0.5 G, for example, in acceleration conversion, respectively. 
         [0032]    In this situation, as illustrated in  FIG. 2 , the diagnosis offset input circuit  25  alternately inputs the positive offset signal S 1  and the negative offset signal S 2  to the positive side and the negative side with a substantially equal amplitude in synchronization with the timing of sampling of the signal from the signal processing circuit  21  (carrier D 1  at timing of Hi). In other words, the positive and negative offsets are input with a deflection width corresponding to 1 G (predetermined amount) (varied with an equal amplitude). As illustrated in  FIG. 1 , an output signal from the signal processing circuit  21  (A/D conversion circuit  29 ) is input to the determination logic circuit  24 , and a self-diagnosis (determination of whether abnormality is present, or not) is performed on the basis of a variation in the output signal. 
         [0033]    In addition, an output signal from the signal processing circuit  21  (A/D conversion circuit  29 ) is input to the moving average filter circuit  26 . The moving average filter circuit  26  calculates an average value [{X(n)+X(n−1)}/2] between a present signal X(n) and a last signal X(n−1) from the A/D conversion circuit  29 . Through the calculation in the moving average filter circuit  26 , the offset signals S 1  and S 2  (two offset inputs) are canceled, and only a signal (acceleration detection signal) corresponding to the input signal to the signal processing circuit  21 , that is, corresponding to the detection signal of the sensor chip  12  is extracted. 
         [0034]    Therefore, the determination logic circuit  24  functions as self-diagnosis device, and the moving average filter circuit  26  functions as extraction device. The first and second switches  32  and  34  of the C/V conversion circuit  27  are intended for reset of the capacitors  31  and  33 , and as illustrated in  FIG. 2 , are turned on at an appropriate timing (rising timing of the pulse of the carrier D 1 ) by the control logic circuit  23 . 
         [0035]    Then, the operation of the above configuration will be described also with reference to  FIG. 2 .  FIG. 2  illustrates a relationship of a waveform of the carrier D 1  input to the movable electrode part  15  of the sensor chip  12 , and the offset signals S 1  and S 2  input to the input side of the C/V conversion circuit  27  (fully differential amplifier  30 ) in the signal processing circuit  21  by the diagnosis offset input circuit  25 , in the operation of the semiconductor acceleration sensor device  11 .  FIG. 2  illustrates an example of an output signal from the C/V conversion circuit  27 , an output signal from the sample and hold circuit  28 , an output signal from the A/D conversion circuit  29 , and an output signal from the moving average filter circuit  26  together.  FIG. 2  illustrates a state in which no abnormality is present in the sensor chip  12  and the signal processing device  13 , and the acceleration of, for example, 1 G acts on the sensor chip  12  and the signal processing device  13 . 
         [0036]    As described above, in the operation of the semiconductor acceleration sensor device  11 , the offset signal S 1  (+0.5 G equivalent) and the offset signal S 2  (−0.5 G equivalent) are always alternately input in synchronization with the carrier D 1 . When it is assumed that the output signal from the A/D conversion circuit  29  when receiving the offset signal S 1  is X 1  (number 1 in each white circle in  FIG. 2 ), and the output signal from the A/D conversion circuit  29  when receiving the offset signal S 2  is X 2  (number 2 in each white circle in  FIG. 2 ), the signal X 1  and the signal X 2  are alternately output from the A/D conversion circuit  29 . 
         [0037]    Those output signals X 1  and X 2  are input to the determination logic circuit  24  to perform the abnormality diagnosis. In the case of normal (no abnormality), the magnitude of the signal X 1  corresponds to +0.5 G, the magnitude of the signal X 2  corresponds to +1.5 G, and those signals are alternately output. On the contrary, when the abnormality is present in the signal processing circuit  21  or the sensor chip  12 , since the magnitude of the amplitude between the signal X 1  and the signal X 2 , or an average value between the signal X 1  and the signal X 2  is changed, it can be determined that the abnormality occurs in the signal processing circuit  21  or the sensor chip  12 . 
         [0038]    For example, when abnormality that the sensitivity is too high is present, a value (X 2 −X 1 ) of the amplitude between the signal X 1  and the signal X 2  is larger than the 1 G equivalent. When abnormality that the sensitivity is too low is present, the value (X 2 −X 1 ) of the amplitude between the signal X 1  and the signal X 2  is smaller than the 1 G equivalent. When the abnormality of polarity inversion is present, the value of the amplitude between the signal X 1  and the signal X 2  is smaller than the 1 G equivalent. If the offset abnormality is present, the average value {(X 1 +X 2 )}/2} between the signal X 1  and the signal X 2  is deviated from the 1 G equivalent. In this way, the abnormality is determined by the determination logic circuit  24  according to the output signals X 1  and X 2 . 
         [0039]    The output signals X 1  and X 2  from the A/D conversion circuit  29  are input to the moving average filter circuit  26 , and an average of the output signals X 1  and X 2  and the last output signal is taken twice. In other words, when the signal X 2  is input to the moving average filter circuit  26 , an average {(X 1 +X 2 )/2} between the input signal X 2  and the last signal X 1  is obtained. When the signal X 1  is input to the moving average filter circuit  26 , an average {(X 2 +X 1 )/2} between the input signal X 1  and the last signal X 2  is obtained. As a result, through the moving average filter circuit  26 , the offset signals S 1  and S 2  (two offset inputs) are canceled, and only a signal (for example, 1.0 G equivalent) corresponding to the input signal to the signal processing circuit  21 , that is, corresponding to the detection signal of the sensor chip  12  is extracted. 
         [0040]    As described above, according to the signal processing device  13  of this embodiment, the diagnosis offset signals S 1  and S 2  can be forcedly input to the C/V conversion circuit  27  in the signal processing circuit  21  by the diagnosis offset input circuit  25 . The output signal from the signal processing circuit  21  (A/D conversion circuit  29 ) is varied with the variation commensurate with the offset according to a predetermined amount of variation of the offset input. As a result, the determination logic circuit  24  monitors the output variation to the offset input, thereby being capable of diagnosing whether the signal processing circuit  21  operates normally, or not. 
         [0041]    At the same time as the above self-diagnosis, the variation in the offset input is canceled by the moving average filter circuit  26  to enable only a portion corresponding to the input signal (acceleration detection signal) to be extracted from the output signal from the signal processing circuit  21  (A/D conversion circuit  29 ), and the acceleration detected by the sensor chip  12  can be always detected. Therefore, this embodiment is provided with the signal processing circuit  21 , and obtains such excellent advantages that the self-diagnosis function can be always executed unlike the conventional art that provides the self-diagnosis phase at a time other than the normal operation. 
         [0042]    In the above first embodiment, the offset signal S 1  on the positive side and the offset signal S 2  on the negative side are alternately input by the diagnosis offset input circuit  25  in synchronization with the carrier D 1 . Alternatively, the disclosure can employ another pattern of the input (variation) of the offset signals. In other words, as a modification of the pattern of the offset signal input, the input and input stop (offset is 0) of the offset signal S 1  on the positive side, and the input and input stop (offset is 0) of the offset signal S 2  on the negative side can be repeated in order in synchronization with the carrier D 1  (at timing when the carrier D 1  is Hi). 
         [0043]    In this event, as illustrated in  FIG. 4 , in the normal case, the output signal from the signal processing circuit  21  (A/D conversion circuit  29 ) repeats 1.5 G equivalent, 1 G equivalent, 0.5 G equivalent, and 1 G equivalent in correspondence with the input pattern of the offset signal. Similarly, in this case, when the abnormality is present in the signal processing circuit  21  or the sensor chip  12 , since the magnitude of the amplitude of the output signal from the A/D conversion circuit  29 , or an average value of the magnitude is changed, it can be determined in the determination logic circuit  24  that the abnormality occurs in the signal processing circuit  21  or the sensor chip  12 . The offset abnormality can be determined according to the output signal from the A/D conversion circuit  29  at the time of stopping the offset input regardless of whether a failure is present in the signal processing device  13 , or not. 
         [0044]    In the moving average filter circuit  26 , an average value [{X(n)+2*X(n−1)+X(n−2) }/4] is calculated according to the present signal X(n), the last signal X(n−1), and a second last signal X(n−2) from the A/D conversion circuit  29  so that the signals at the time of inputting the positive and negative offset signal are input one by one. Alternatively, an average value [{X(n)+X(n−1)+X(n−2)+X(n−3)}/4] is calculated. As a result, the acceleration detected by the sensor chip  12  can be always detected. 
       Second Embodiment 
       [0045]      FIGS. 5 and 6  illustrate a second embodiment of the disclosure. The second embodiment is different from the above first embodiment in the configuration of a signal processing circuit  41 . In other words, in the signal processing circuit  41  according to this embodiment, a chopping circuit  42  is disposed on an input side (subsequent stage to an input portion of the offset signals S 1  and S 2  by the diagnosis offset input circuit  25 ) of the totally differential C/V conversion circuit  27 . 
         [0046]    The chopping circuit  42  includes a third switch  43 , a fourth switch  44 , a fifth switch  45 , and a sixth switch  46 . The third switch  43  is inserted between a first output terminal  19  and a non-inverting input terminal of a fully differential amplifier  30 . The fourth switch  44  is inserted between a second output terminal  20  and an inverting input terminal of the fully differential amplifier  30 . The fifth switch  45  is inserted between the first output terminal  19  and the inverting input terminal of the fully differential amplifier  30 . The sixth switch  46  is inserted between the second output terminal  20  and the non-inverting input terminal of the fully differential amplifier  30 . 
         [0047]    The chopping circuit  42 , that is, the third to sixth switches  43  to  46  are controlled in on/off operation by the control logic circuit  23 . In this situation, a state in which the third switch  43  and the fourth switch  44  are on, and the fifth switch  45  and the sixth switch  46  are off in the chopping circuit  42  is called “forward state”. In the forward state, an offset signal S 1  is input to the non-inverting input terminal of the fully differential amplifier  30 , and an offset signal S 2  is input to the inverting input terminal of the fully differential amplifier  30 . 
         [0048]    On the contrary, a state in which the third switch  43  and the fourth switch  44  are off, and the fifth switch  45  and the sixth switch  46  are on in the chopping circuit  42  is called “inversion state”. In the inversion state, the offset signal S 1  is input to the inverting input terminal of the fully differential amplifier  30 , and the offset signal S 2  is input to the non-inverting input terminal of the fully differential amplifier  30 . 
         [0049]    In this case, the offset signal S 1  on the positive side, the offset signal S 1  on the positive side, the offset signal S 2  on the negative side, and the offset signal S 2  on the negative side are repetitively input to the positive side and the negative side with a substantially equal amplitude in the stated order from the diagnosis offset input circuit  25  in synchronization with a carrier D 1  (at a timing when the carrier D 1  is Hi). The forward state, the inversion state, the forward state, and the inversion state are alternately switched by the chopping circuit  42  at a timing synchronous with the above input. 
         [0050]      FIGS. 6A to 6C  illustrate a signal (Vcv+) of an acceleration (G) from a sensor chip  12 , an offset signal (positive offset input is Voff+, negative offset input is Voff−) input from the diagnosis offset input circuit  25 , and an output signal (VADO+ in a case including the positive offset input, and VADO− in a case including the negative offset input) from an A/D conversion circuit  29 , in eight sections (eight cycles of the carrier D 1 ) of AD 1  to AD 8 .  FIG. 6A  illustrates data that remains chopped, and  FIG. 6B  illustrates data when chopping is demodulated (ADCh 1  to ADCh 8 ).  FIG. 6C  illustrates an extracted signal by a moving average filter circuit  26 . 
         [0051]    The section AD 1  shows an appearance in which the offset signal S 1  on the positive side is input to the chopping circuit  42 , and the chopping circuit  42  is in the forward state, and the section AD 2  shows an appearance in which the offset signal S 1  on the positive side is input to the chopping circuit  42 , and the chopping circuit  42  is in the inversion state. The section AD 3  shows an appearance in which the offset signal S 2  on the negative side is input to the chopping circuit  42 , and the chopping circuit  42  is in the forward state, and the section AD 4  shows an appearance in which the offset signal S 2  on the negative side is input to the chopping circuit  42 , and the chopping circuit  42  is in the inversion state. A pattern of those sections AD 1  to AD 4  is also repeated in the sections AD 5  to AD 8 . 
         [0052]    As is apparent from  FIG. 6 , similarly, in a configuration where the chopping circuit  42  described above is provided, even if the offset input from the diagnosis offset input circuit  25  is performed in the order of positive, positive, negative, and negative to implement the signal inversion by the chopping circuit  42 , the output signal from the A/D conversion circuit  29  which has been subjected to the demodulation of the chopping is alternately deflected to the positive and negative. As a result, the abnormality determination (self-diagnosis) can be performed by the determination logic circuit  24 . In the moving average filter circuit  26 , with the calculation of an average value of four output signals, a variation in the offset input is canceled, only a portion corresponding to an acceleration detection signal of the sensor chip  12  can be extracted to always detect the acceleration. 
         [0053]    Therefore, similarly, the second embodiment is provided with the signal processing circuit  41 , and obtains such excellent advantages that the self-diagnosis function can be always executed unlike the conventional art that provides the self-diagnosis phase at a time other than the normal operation. In the second embodiment, the chopping circuit  42  is disposed in the subsequent stage to the input portion of the offset signals S 1  and S 2  by the diagnosis offset input circuit  25 . Alternatively, the chopping circuit  42  may be disposed on an output side of the C/V conversion circuit  27  or on an output side of the sample and hold circuit  28 , and can be implemented under the same control. 
       Third Embodiment, and Other Embodiments 
       [0054]    Subsequently, a third embodiment of the disclosure will be described with reference to  FIGS. 7 and 8 .  FIG. 7  schematically illustrates an electric configuration of a main portion of a semiconductor acceleration sensor device  51  according to this embodiment. The semiconductor acceleration sensor device  51  includes a sensor chip  52  as a sensor unit, and a signal processing device  53 . In the semiconductor acceleration sensor device  51 , the sensor chip  52  includes a movable electrode part  15 , and a pair of fixed electrode parts  16  and  17 , and capacitors C 1  and C 2  are configured by those components. 
         [0055]    The sensor chip  52  is equipped with first and second input terminals  54  and  55  connected to the fixed electrode parts  16  and  17 , respectively, and an output terminal  56  connected to the movable electrode part  15 . The input terminals  54  and  55  are connected with a carrier input circuit  57 , and pulsed carriers whose potential has an amplitude between Vp (for example, 5V) and Vm (for example, 0V), and are opposite in phase to each other are supplied to the input terminals  54  and  55 . The output terminal  56  is connected to a signal processing circuit  58  of the signal processing device  53 . 
         [0056]    The signal processing circuit  58  includes a single end C/V conversion circuit  59 , a sample and hold (S/H) circuit  60 , and an A/D conversion circuit  61 . The C/V conversion circuit  59  includes an arithmetic amplifier  62 , and a feedback capacitor  63  and a switch  64  which are connected in parallel to each other between a non-inverting input terminal and an output terminal of the arithmetic amplifier  62 . The output terminal  56  is connected to the non-inverting input terminal of the arithmetic amplifier  62 . A predetermined (DC) voltage signal, for example, an intermediate voltage Vref of a carrier is input to an inverting input terminal of the arithmetic amplifier  62 . 
         [0057]    In addition, the signal processing device  53  includes a control logic circuit  23 , a determination logic circuit  24 , and a moving average filter circuit (MAF)  26 . The signal processing device  53  also includes a diagnosis offset input circuit  65 . The diagnosis offset input circuit  65  inputs an offset signal S 1  (for example, a signal corresponding to +0.5 G, for example, in acceleration conversion) to an input side of the C/V conversion circuit  59  (arithmetic amplifier  62 ) on the basis of a command from the control logic circuit  23 . In this case, as illustrated in  FIG. 8 , the diagnosis offset input circuit  65  alternately performs an input and an input stop (offset is 0) of the offset signal S 1  for each one cycle of Hi and Lo of the carrier D 1  in synchronization with a timing of sampling of a signal from the signal processing circuit  58 . 
         [0058]    As in the above first embodiment ( FIG. 2 ),  FIG. 8  illustrates a signal of each component when no abnormality is present in the sensor chip  52  and the signal processing device  53 , and the acceleration of, for example, 1 G acts on the sensor chip  52  and the signal processing device  53 . The waveform of the offset signal S 1  is different from that in the first embodiment, but the output signal from the C/V conversion circuit  59  is equal to that in the first embodiment. As a result, although not shown, an output signal from the sample and hold circuit  60 , an output signal from the A/D conversion circuit  61 , and an output signal from the moving average filter circuit  26  are equal to those shown in  FIG. 2 . 
         [0059]    As a result, similarly, in this embodiment, the determination logic circuit  24  monitors the output variation to the offset input, thereby being capable of diagnosing whether the signal processing circuit  58  operates normally, or not. At the same time as the above self-diagnosis, the variation in the offset input is canceled by the moving average filter circuit  26  to enable only a portion corresponding to the input signal (acceleration detection signal) to be extracted from the output signal from the signal processing circuit  58  (A/D conversion circuit  61 ), and the acceleration detected by the sensor chip  52  can be always detected. Therefore, similarly, the third embodiment is provided with the signal processing circuit  58 , and can obtain such an advantageous effect that the self-diagnosis function can be always executed. 
         [0060]    Although not described in the above respective embodiments, the signal processing circuit may provide a zero point adjustment mechanism that adjusts an output (zero point) of the sensor unit in a state where a physical quantity does not act on the sensor unit. In the case of providing the zero point adjustment mechanism as described above, the offset input device (diagnosis offset input circuit) can also function as the zero point adjustment mechanism, and the configuration can be more simplified. In the above respective embodiments, the moving average filter is employed as the extraction device. Alternatively, the extraction device can be configured by the combination of a low pass filter or a band pass filter with the moving average filter. 
         [0061]    In the above respective embodiments, the offset signal is input to the input side of the C/V conversion circuit by the diagnosis offset input circuit. Alternatively, the output signal may be input to the input side of the sample and hold circuit, or the input side of the A/D conversion circuit. In addition, in the above respective embodiments, the disclosure is applied to the semiconductor acceleration sensor device. Alternatively, the disclosure can be applied to another capacitive semiconductor sensor device such as a yaw rate sensor. Further, the disclosure can be applied to the general signal processing devices. The signal processing circuit may include no C/V conversion circuit, and the signal waveforms in the respective components merely show an example, and the disclosure can be implemented with an appropriate change without departing from the spirit of the disclosure. 
         [0062]    While the present disclosure has been described with reference to embodiments thereof, it is to be understood that the disclosure is not limited to the embodiments and constructions. The present disclosure is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the present disclosure.