Patent Publication Number: US-10768020-B2

Title: Capacitive sensor

Description:
TECHNICAL FIELD 
     The present invention relates to an A/D converter and a capacitive sensor using the same. 
     BACKGROUND ART 
     In MEMS capacitive acceleration sensors, angular velocity sensors, angle sensors, and the like, a CV conversion amplifier that converts a change ΔC in a capacitance value generated in an MEMS capacitive element into a voltage signal ΔV is used. It is necessary for the CV conversion amplifier to make a capacitance-voltage conversion gain ΔV/ΔC as large as possible in to relieve a noise specification of a circuit block at a subsequent stage. 
     However, in the past, if the capacitance-voltage conversion gain is increased, there is a problem in that an amplitude range of an output voltage of the CV conversion amplifier is remarkably decreased. The reason is that in the case of a pseudo differential CV conversion amplifier in which two single-ended operational amplifiers are used in parallel, if the capacitance-voltage conversion gain is increased, a center voltage level of an output of each single-ended operational amplifier considerably deviates from a desired value (usually, about ½ of a power voltage). 
     Further, in the case of a fully differential CV conversion amplifier in which one fully differential operational amplifier is used, if the capacitance-voltage conversion gain is increased, a common mode voltage of an input of the fully differential operational amplifier considerably deviates from a desired value. If the amplitude range of the output voltage of the CV conversion amplifier is decreased, an input signal allowable range of a sensor is narrowed. For example, in the case of an acceleration sensor, a range of an input acceleration signal which is normally detectable is narrowed. 
     In this regard, in the past, the output amplitude range of the CV conversion amplifier is secured such that a signal of a high voltage level among differential output signals of the CV conversion amplifier is determined by an OR circuit, held in a peak hold circuit, and a voltage for adjusting the center voltage level of the output is continuously generated. Further, when a setting of an adjustment voltage is stored as a digital value in advance, it is possible to select a suitable parameter in accordance with characteristics of a capacitive sensor. A configuration to which such a common mode voltage adjustment analog circuit is added is described in, for example, Patent Document 1. 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: JP 2007-3300 A 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the configuration to which the common mode voltage adjustment analog circuit described above is added, the capacitance-voltage conversion gain of the CV conversion amplifier and the amplitude range of the output voltage can be secured, but since the voltage for adjusting the center voltage level of the output is continuously generated through an analog circuit, there is a problem in that power consumption is increased. Further, since the signal of the high voltage level among the differential output signals of the CV conversion amplifier is detected by the OR circuit and used for the generation of the adjustment voltage, when there is a large differential signal, the amplitude range of the output voltage is likely to be narrowed without detecting the common mode signal with a high degree of accuracy. Further, in a case in which the setting of the adjustment voltage is stored as the digital value in advance, an A/D converter for converting an analog adjustment voltage to a digital signal is necessary separately from an analog sensor, and a capacitance change or an adjustment voltage of an MEMS of an adjustment target fluctuates due to temperature fluctuation or the like, an adjustment accuracy deteriorates, and thus the amplitude range of the output voltage is likely to be narrowed. 
     In this regard, an object of the present invention to provide a CV conversion amplifier which is small in an increase in current consumption and capable of adjusting the common mode voltage with a high degree of accuracy and securing the capacitance-voltage conversion gain and the amplitude range of the output voltage and a capacitive sensor using the same which is low in power consumption and wide in an input signal allowable range. 
     Solutions to Problems 
     According to one aspect of the present invention to solve the above problem, provided is a capacitive sensor including first and second detection capacitors, a CV conversion circuit that includes first and second feedback capacitors and obtains a voltage based on capacitance values of the first and second feedback capacitors, an AD converter that performs analog digital conversion on an input voltage and obtains a digital signal, a digital control unit that receives the digital signal as an input, and first and second digitally controlled variable capacitors having capacitance values which are controlled by the digital control unit. In the capacitive sensor, in a case in which a physical quantity serving as a measurement target is not substantially zero, capacitance values of the first and second detection capacitors are changed from capacitance values in a case in which the physical quantity is substantially zero in accordance with the physical quantity as the measurement target, and change amounts of the capacitance values of the first and second detection capacitors are opposite in sign to each other and substantially equal in an absolute value to each other. The first and second feedback capacitors accumulate charges in which the capacitance values of the first detection capacitor, the second detection capacitor, the first digitally controlled variable capacitor, and the second digitally controlled variable capacitor are reflected. A common mode voltage level of an input voltage of the CV conversion circuit or a common mode voltage level of an output voltage is controlled in accordance with control of the first and second digitally controlled variable capacitors. 
     According to another aspect of the present invention, provided is a capacitive sensor including a first MEMS capacitive element having a capacitance value changing in accordance with reflection of a change in a physical quantity, a second MEMS capacitive element having a capacitance value changing in accordance with reflection of a change in a physical quantity, a first variable capacitor connected with the first MEMS capacitive element, a second variable capacitor connected with the second MEMS capacitive element, and a first input terminal, a second input terminal, a first output terminal, and a second output terminal. The sensor includes a CV conversion circuit that generates a voltage in which a change in the capacitance value of the first MEMS capacitive element and a change in the capacitance value of the second MEMS capacitive element are reflected, a detection circuit that detects an average voltage of input voltages of the first input terminal and the second input terminal of the CV conversion circuit or an average voltage of output voltages of the first output terminal and the second output terminal of the CV conversion circuit detection circuit, an analog digital converter that converts the average voltage into a digital average voltage signal, and a control circuit that changes the capacitance values of the first variable capacitor and the second variable capacitor on the basis of the digital average voltage signal, wherein a connection point of the first MEMS capacitive element and the first variable capacitor is connected to the first input terminal, and a connection point of the second MEMS capacitive element and the second variable capacitor is connected to the second input terminal. 
     According to another aspect of the present invention, provided is a capacitive sensor including a sensor unit that includes a pair of capacitors configured with a fixed electrode and a movable electrode, applies a voltage to one of the fixed electrode and the movable electrode, extracts charges accumulated in the pair of capacitors from the other electrode, and obtains two output signals, a CV converting unit that receives the two output signals as an input, reflects the charges extracted from the sensor unit in a capacitance value of a feedback capacitor, converts a voltage signal, and obtains a differential output, an AD converter that converts the differential output of the CV converting unit into a digital signal, and a variable capacitor having a capacitance value which is controlled on the basis of the digital signal. In the capacitive sensor, one electrode of the variable capacitor is connected to the other electrode from which the accumulated charges are extracted, and a common mode output potential of the differential output is controlled by controlling a capacitance value of the variable capacitor. 
     Effects of the Invention 
     According to the present invention, unlike the adjustment by the analog circuit, it is unnecessary to constantly perform an operation, and it is possible to adjust the common mode voltage level with low power consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram of a problem to be solved by an embodiment. 
         FIG. 2  is a circuit diagram illustrating a first embodiment of the present invention. 
         FIG. 3  is an operation timing diagram of the first embodiment of the present invention. 
         FIG. 4  is a circuit diagram for supplementarily describing the first embodiment of the present invention. 
         FIG. 5  is a circuit diagram illustrating a second embodiment of the present invention. 
         FIG. 6  is an operation timing diagram of the second embodiment of the present invention. 
         FIG. 7  is a circuit diagram illustrating a third embodiment of the present invention. 
         FIG. 8  is an operation timing diagram of the third embodiment of the present invention. 
         FIG. 9  is a circuit diagram illustrating a fourth embodiment of the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, exemplary embodiments will be described in detail with reference to the appended drawings. However, the present invention is not interpreted to be limited to the description of the embodiments set forth below. It would be easily understood by those skilled in the art that a specific configuration of the present invention can be modified within the scope not departing from the spirit of the present invention. 
     In a configuration of the invention to be described below, parts having the same or similar functions are denoted by the same reference numerals in different drawings, and redundant descriptions may be omitted. 
     In this specification, notations such as “first,” “second,” and “third” are attached to identify constituent elements and need not necessarily limit numbers or an order. Further, numbers identifying constituent elements are used for each context, and the numbers used in one context does not necessarily indicate the same configuration in other contexts. A constituent element identified by a certain number is not precluded from doubling as a function of a constituent element identified by another number. 
     A position, a size, a shape, a range, or the like of each component illustrated in the drawings or the like may not indicate an actual position, an actual size, an actual shape, an actual range, or the like in order to facilitate understanding of the invention. Therefore, the present invention is not necessarily limited to a position, a size, a shape, a range, or the like illustrated on the drawings or the like. 
     In order to understand a configuration and effects of the present embodiment, first, a problem to be solved by the present embodiment will be described. 
       FIG. 1  illustrates a configuration of a pseudo differential CV conversion amplifier according to a related art and a problem of a reduction in the amplitude range of the output voltage of the output voltage caused because the center voltage level of the output deviates from a desired value (usually about ½ of the power voltage). 
     Referring to  FIG. 1 , since two single-ended operational amplifiers  3   a  and  3   b  are constantly in a closed loop state, potentials of nodes connected to inverting input terminals of the operational amplifiers constantly become V DD /2 (V DD  is a power voltage). 
     During a period in which a carrier clock (Carrier CLK) ϕ COM  is a high voltage (a voltage value V CAR ), and a clock signal ϕ 1  is a high voltage, operational amplifier side electrodes of a pair of two detection MEMS capacitive elements  1   a  and  1   b  are charged with charges of (C+ΔC)*(V CAR −V DD /2) and charges of −(C−ΔC)*(V CAR −V DD /2). Capacitance values of the two detection MEMS capacitive elements  1   a  and  1   b  are indicated by C+ΔC and C−ΔC, respectively. C indicates capacitance values of the two detection MEMS capacitive elements when a signal such as acceleration is not applied to the sensor. ΔC indicates capacitance value changes occurring in the two detection MEMS capacitive elements when a signal such as acceleration is applied to the sensor. 
     Further, since the clock signal ϕ 1  is a high voltage, switches  13   a  and  13   b  connected in parallel with feedback capacitive elements (a capacitance value C F )  4   a  and  4   b  of the operational amplifiers  3   a  and  3   b  are in an ON state, both electrodes of the feedback capacitive elements  4   a  and  4   b  are short-circuited, and the charges on the electrodes of the feedback capacitive element are discharged to zero. 
     Then, the carrier clock ϕ COM  and the clock signal ϕ 1  transition from the high voltage to a low voltage. Since the carrier clock ϕ COM  is the low voltage (0 potential), potentials of the carrier clock side electrodes of the two detection MEMS capacitive elements  1   a  and  1   b  are zero. Therefore, charges of (C+ΔC)*V DD /2 and charges of (C−ΔC)*V DD /2 are induced to the operational amplifier side electrodes of the two detection MEMS capacitive elements, respectively. As a result, difference charges with −(C+ΔC)*(V CAR −V DD /2) and −(C−ΔC)*(V CAR −V DD /2) accumulated on the operational amplifier side electrodes of the two detection MEMS capacitive elements  1   a  and  1   b  until then are transferred from the operational amplifier input side electrodes of the feedback capacitive elements  4   a  and  4   b  to the detection MEMS capacitive elements  1   a  and  1   b.    
     Since clock signal ϕ 1  is the low voltage, the switches  13   a  and  13   b  connected in parallel with the feedback capacitive elements  4   a  and  4   b  are in an OFF state, and thus the difference charges are supplied only from feedback capacitive elements. Since the charges of the operational amplifier input side electrodes of the feedback capacitive elements  4   a  and  4   b  have been zero until then, charges Q FP  of the operational amplifier input side electrode of the feedback capacitive element  4   a  on the upper side of  FIG. 1  eventually become Q FP =0-[(C+ΔC)*V DD /2−{−(C+ΔC)*(V CAR −V DD /2)}]=−ΔC*V CAR −C*V CAR . Further, charges Q FN  of the operational amplifier input side electrode of the feedback capacitive element  4   b  on the lower side of  FIG. 1  become Q FN =0−[(C−ΔC)*V DD /2−{−(C−ΔC)*(V CAR −V DD /2)}]=ΔC*V CAR −C*V CAR . 
     Therefore, an output V OUTP  of the operational amplifier  3   a  on the upper side of  FIG. 1  and an output V OUTN  of the operational amplifier  3   b  on the lower side become as follows since V OUTP =V DD /2−Q FP /C F  and V OUTN =V DD /2−Q FN /C F . 
     
       
         
           
             
               
                 
                   
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     A differential output V OUT (=V OUTP −V OUTN ) and an output common mode voltage V CMO  (=(V OUTP +V OUTN )/2) of the CV conversion amplifier become as follows. 
     
       
         
           
             
               
                 
                   
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     Therefore, due to the capacitance values C of the detection MEMS capacitive elements  1   a  and  1   b , the output common mode voltage level V CMO  deviates from a desired value V DD /2 by V CAR *C/C F . Since the feedback capacitance value C F  is unable to be set to a large value in order to increase the capacitance-voltage conversion gain, the deviation V CAR *C/C F  becomes a large voltage. Accordingly, the amplitude range of the output voltage of the CV conversion amplifier is remarkably decreased. 
     A representative capacitive sensor of the present invention to be described below includes a common mode detection circuit that performs resistance voltage division on a differential output signal of the CV amplifier, a digital signal processing unit that includes an output terminal which is connected to one input terminal of a differential A/D converter via a switch and a fixed potential connected to the other input terminal via a switch, detects a common mode voltage through an A/D converter, and assigns a deviation from a desired center voltage level as a digital value, and a D/A converter which is controlled in accordance with the digital value, and compensates an output center voltage level and sets it to an appropriate center voltage level. 
     First Embodiment 
       FIG. 2  illustrates a first embodiment of the present invention. This is an example of a pseudo differential CV conversion amplifier. In this embodiment, a circuit that detects an output center voltage of the pseudo differential CV conversion amplifier is installed, the output center voltage is converted into a digital signal, and a capacitance value of a digitally controlled variable capacitor is controlled in accordance with the digital signal. First, a configuration will be described. 
     A capacitive MEMS  1  includes two detection MEMS capacitive elements  1   a  (a capacitance value C) and  1   b  (a capacitance value C), and one of electrodes thereof is a movable electrode which can be mechanically moved. The movable electrodes of the detection MEMS capacitive elements  1   a  and  1   b  are connected to a carrier clock ϕ CHOP , and the other electrodes (fixed electrodes) are connected to inverting input terminals of operational amplifiers  3   a  and  3   b , respectively. Further, feedback capacitive elements  4   a  (a capacitance value C F ) and  4   b  (a capacitance value C F ) and resistive elements  5   a  (a resistance value R F ) and  5   b  (a resistance value R F ) are installed between the inverting input terminals and the output terminals of the operational amplifiers  3   a  and  3   b . A switch may be installed between the feedback capacitive elements  4   a  and  4   b  and the operational amplifiers  3   a  and  3   b . Further, the non-inverting input terminals of the operational amplifiers  3   a  and  3   b  are connected to a voltage V B . In this case, V B =V DD /2. 
     In the configuration of  FIG. 2 , unlike the configuration of  FIG. 1 , the resistive elements  5   a  and  5   b  are installed between the inverting input terminals and the output terminals of the operational amplifiers  3   a  and  3   b  instead of the switches  13   a  and  13   b . For this reason, a signal for driving the switch is unnecessary. A difference lies in that in the configuration of  FIG. 1 , the capacitance change is sampled and held as a voltage change, whereas in the configuration of  FIG. 2 , the capacitance change is continuously detected as a voltage change. 
     In the present configuration, digitally controlled variable capacitors  2   a  (a capacitance value C DIG ) and  2   b  (a capacitance value C DIG ) are connected in parallel with the detection MEMS capacitive elements  1   a  and  1   b. ϕ′   CHOP_B  obtained by inverting the carrier clock ϕ CHOP  and changing a level appropriately is connected to the digitally controlled variable capacitors  2   a  and  2   b . The digitally controlled variable capacitor may be called a capacitive D/A converter and has the capacitance value which can be controlled in accordance with a digital signal. 
     Outputs of the operational amplifiers  3   a  and  3   b  of the CV conversion amplifier are connected to differential input terminals of an A/D converter  9  via differential voltage detection switches  7   a  and  7   b  and filters  18   a  and  18   b . The A/D converter  9  removes a common mode noise and interference using differential inputs. 
     Common mode voltage detection resistors  6   a  and  6   b  are inserted in series between the outputs of the operational amplifiers  3   a  and  3   b  via common mode voltage generation switches  17   a  and  17   b , and a joint node  200  of the common mode voltage detection resistors  6   a  and  6   b  is connected to a normal phase input terminal of the A/D converter  9  via a common mode voltage detection switch  8   a . Further, a reversed phase input terminal of the A/D converter  9  is connected to the voltage V B  via a common mode voltage detection switch  8   b . Further, the output of the A/D converter  9  is input to a switch  10 , and an output of the switch  10  serves as a sensor output. The output of the A/D converter  9  is input to a digital control unit  12  via a switch  11 . An output D CAL  of the digital control unit  12  is input to the digitally controlled variable capacitors  2   a  and  2   b.    
     Next, an operation of the circuit of  FIG. 1  will be described. The movable electrode of the detection MEMS capacitive element  1   a  and the movable electrode of the detection MEMS capacitive element  1   b  are mechanically coupled to move as one body and mechanically function as one weight (a mass body). When a signal such as acceleration is not applied to the sensor, since no force such as inertial force acts on the weight, the weight, that is, the movable electrode of the detection MEMS capacitive element  1   a  and the movable electrode of the detection MEMS capacitive element  1   b  are positioned at initial positions. At this time, since an electrode structure is designed so that a distance between the movable electrode and the fixed electrode of the detection MEMS capacitive element  1   a  is equal to a distance between the movable electrode and the fixed electrode of the detection MEMS capacitive element  1   b , the capacitance values of the detection MEMS capacitive element  1   a  and the detection MEMS capacitive element  1   b  are equal to each other, and the values thereof are indicated by C. 
     If a signal such as acceleration is applied to the sensor, the weight receives force such as an inertial force proportional to the signal such as the acceleration, and thus the weight, that is, the positions of the movable electrode of the detection MEMS capacitive element  1   a  and the movable electrode of the detection MEMS capacitive element  1   b  are displaced as one body in proportion to the signal such as the acceleration. Accordingly, if the movable electrode of the detection MEMS capacitive element  1   a  is displaced to approach the fixed electrode of the detection MEMS capacitive element  1   a , the movable electrode of the detection MEMS capacitive element  1   b  conversely gets away from the fixed electrode of the detection MEMS capacitive element  1   b  by the same displacement amount. Further, if the movable electrode of the detection MEMS capacitive element  1   a  is displaced to get away from the fixed electrode of the detection MEMS capacitive element  1   a , the movable electrode of the detection MEMS capacitive element  1   b  conversely approaches the fixed electrode of the detection MEMS capacitive element  1   b  by the same displacement amount. If the displacement amount, that is, the capacitance value change according to the change amount of the plate interval is indicated by ΔC, the capacitance value of the detection MEMS capacitive element  1   a  becomes C+ΔC, and the capacitance value of the detection MEMS capacitive element  1   b  becomes C−ΔC. 
       FIG. 3  illustrates waveforms of a carrier clock  40  and a signal  41  applied to the digitally controlled variable capacitor (common mode voltage adjustment capacitors)  2   a  and  2   b  used in the present embodiment. A phase of ϕ CHOP_B  of a signal  42  applied to the digitally controlled variable capacitors  2   a  and  2   b  is inverted from ϕ CHOP  of the carrier clock  40 , and the voltage level is decided in accordance with the common mode voltage compensation range. The signal  41  applied to the carrier clock  40  and the digitally controlled variable capacitors  2   a  and  2   b  is assumed to be input between an adjustment mode and a normal mode to be described below. 
     Next, an operation in a period (the adjustment mode) in which the capacitance value C DIG  is decided will be described. In the adjustment mode, the common mode voltage generation switches  17   a  and  17   b  are turned on. Accordingly, an average voltage of a normal phase side output voltage (an output voltage of the operational amplifier  3   a ) and a reversed phase side output voltage (an output voltage of the operational amplifier  3   b ) of the CV conversion amplifier, that is, the output common mode voltage level V CMO  of the CV conversion amplifier (=the center voltage level of the outputs of the respective operational amplifiers  3   a  and  3   b ) is generated at the joint node  200  of the common mode voltage detection resistors  6   a  and  6   b . Further, in the adjustment mode, the common mode voltage detection switches  8   a  and  8   b  are turned on, and the output common mode voltage level V CMO  of the CV conversion amplifier generated at the joint node  200  is input to the normal phase input terminal of the A/D converter  9  via a filter  18   a , and the voltage V B  is input to the reversed phase input terminal of the A/D converter  9  via a filter  18   b . The filters  18   a  and  18   b  are, for example, low-pass filters for extracting a frequency band necessary for detecting a physical quantity and have a function of passing, for example, a band of 1 MHz or less. 
     In the adjustment mode, since the differential voltage detection switches  7   a  and  7   b  are turned off, the outputs of the operational amplifiers  3   a  and  3   b  of the CV conversion amplifier are not input to the A/D converter  9  via the filters  18   a  and  18   b . The A/D converter  9  converts a difference voltage between the voltage of the normal phase input terminal and the voltage of the reversed phase input terminal, that is, a difference between the output common mode voltage levels V CMO  and V B  of the CV conversion amplifier, that is, V CMO −V DD /2 into a digital value. Here, V B  is V DD /2, but other voltage values may be used as well. 
     The digital value is supplied to the digital control unit  12 . In a case in which the digital value is positive, this means that the current common mode voltage level V CMO  of the CV conversion amplifier is higher than V DD /2. In this case, the digital control unit  12  updates the currently output digital compensation value D CAL  to a larger value and outputs it. On the other hand, in a case in which the digital value is negative, this means that the output common mode voltage level V CMO  of the current CV conversion amplifier is lower than V DD /2. In this case, the digital control unit  12  updates the currently output digital compensation value D CAL  to a smaller value and outputs it. 
     Further, the digitally controlled variable capacitors  2   a  and  2   b  convert the digital compensation value D CAL  supplied from the digital control unit  12  into the capacitance value C DIG . In a case in which the current output common mode voltage level V CMO  of the CV conversion amplifier is higher than V DD /2, D CAL  increases, and thus the capacitance value C DIG  increases as well. As a result, the output common mode voltage level V CMO  of the CV conversion amplifier moves in a direction in which it gets lower than now. On the other hand, in a case in which the output common mode voltage level V CMO  of the current CV conversion amplifier is lower than V DD /2, D CAL  decreases, and thus the capacitance value C DIG  decreases as well. As a result, the output common mode voltage level V CMO  of the CV conversion amplifier moves in a direction in which it gets higher than now. With the above negative feedback control, the capacitance value C DIG  ultimately converges to an appropriate capacitance value C DIG_FINAL , and the output common mode voltage level V CMO  of the CV conversion amplifier is sufficiently close to V DD /2. 
     Next, an operation in the normal operation period (normal mode) will be explained. In the normal mode, the CV conversion amplifier converts the MEMS capacitance change ΔC caused by the signal such as the acceleration applied to the sensor into the voltage signal ΔV by employing the appropriate capacitance value C DIG_FINAL  as a correction capacitance. The voltage signal ΔV is the output differential voltage of the CV conversion amplifier. When C DIG_FINAL  is employed, the output common mode voltage level of the CV conversion amplifier is set near V DD /2, and thus an input signal to a sufficiently large sensor can be normally converted to a voltage signal. 
     In the normal mode, the differential voltage detection switches  7   a  and  7   b  in the ON state, the output of the operational amplifier  3   a  of the CV conversion amplifier is connected to the normal phase input terminal of the A/D converter  9  via the filter  18   a , and the output of the operational amplifier  3   b  is connected to the reversed phase input terminal of the A/D converter  9  via the filter  18   b . Accordingly, the output differential voltage of CV conversion amplifier is converted into the digital value through the A/D converter  9 . 
       FIG. 4  illustrates an implementation example of the digitally controlled variable capacitors  2   a  and  2   b . This is a configuration of a so-called binary capacitor array, and  FIG. 4  is an example in which it is implemented with 5 bits. Capacitive elements  26 ,  27 ,  28 ,  29  and  30  have capacitance values of 16 Cu, 8 Cu, 4 Cu, 2 Cu, and Cu and are selected by capacitor selection switches ( 21   a ,  21   b ), ( 22   a ,  22   b ), ( 23   a ,  23   b ), ( 24   a ,  24   b ), and ( 25   a ,  25   b ), respectively. It is controlled whether the capacitor selection switches ( 21   a ,  21   b ), ( 22   a ,  22   b ), ( 23   a ,  23   b ), ( 24   a ,  24   b ), and ( 25   a ,  25   b ) are turned on or off in accordance with D CAL [4], D CAL [3], D CAL [2], D CAL [1], and D CAL [0]. Accordingly, a variable capacitance value of n*Cu can be implemented in accordance with a D CAL  [4:0] value n (n=0 to 31) which is a signal indicated by the digital value. 
     The adjustment mode can be executed before the normal mode. For example, the adjustment mode can be executed when the capacitive sensor is shipped or just after the capacitive sensor is powered on. Further, the adjustment mode may be executed twice or more. For example, when the adjustment mode is periodically executed, it is possible to cope with a temporal signal change such as aging. 
     After the digitally controlled variable capacitors  2   a  and  2   b  are set through the adjustment mode, a physical quantity such as acceleration is measured through the normal mode. A detection principle in the normal mode of the present embodiment is a continuous time type and basically similar to a non-continuous time type (switch type) CV conversion amplifier described in  FIG. 1 . Here, since the digitally controlled variable capacitors  2   a  and  2   b  are connected, the output common mode voltage level V CMO  (V OUTP +V OUTN )/2) of (Math. 4) is changed as in (Math. 5). If the digitally controlled variable capacitors  2   a  and  2   b  are set so that a second term and a third term on the right side of (Math. 5) are negated by the adjustment mode, the output common mode voltage level V CMO  of the CV conversion amplifier is sufficiently close to the center voltage V DD /2. 
     
       
         
           
             
               
                 
                   
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                     5 
                   
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     In the scheme of the present embodiment, unlike the analog correction scheme using the common mode feedback circuit of the related art, the analog circuit need not operate continuously in the common mode voltage control, and thus low power consumption can be implemented. Further, since the feedback control can be implemented digitally, it is possible to implement the control unit with a small area and low power consumption as compared with the analog scheme. 
     In the first embodiment, the A/D converter  9  is shared by switching the adjustment mode and the normal mode through the switch, and in the adjustment mode, the voltage in which the common mode voltage level of the output voltage of the CV conversion amplifier is reflected is used as the input voltage of the A/D converter  9 , and in the normal mode, the voltage in which the change in the capacitance value of the detection MEMS capacitive element  1  is used as the input voltage of the A/D converter  9 . However, it is also possible to install two or more A/D converters for the respective modes instead of sharing the A/D converter  9 . 
     Second Embodiment 
       FIG. 5  illustrates a second embodiment of the present invention. A difference from the first embodiment lies in that the filters  18   a  and  18   b  are not installed, and switches  13   a  and  13   b  are installed instead of the resistive element  5   a  (the resistance value R F ) and  5   b  (the resistance value R F ) installed between the inverting input terminal and the output terminal of the operational amplifiers  3   a  and  3   b . Accordingly, it operates as a so-called non-continuous time type (switch type) CV conversion amplifier. 
     In the description of the second and later embodiments, the same parts as or parts having similar functions to those of the first embodiment are denoted by the same reference numerals, and duplicate description will be omitted, and the description will proceed with different parts. 
     As illustrated in  FIG. 5 , in the second embodiment, the feedback capacitive element  4   a  (capacitance value C F ) and  4   b  (capacitance value C F ) and the switches  13   a  and  13   b  are installed between the inverting input terminals and the output terminals of the operational amplifiers  3   a  and  3   b . The switches  13   a  and  13   b  are connected to the clock signal ϕ 1 . In the configuration of the second embodiment, the digitally controlled variable capacitors  2   a  (a capacitance value C DIG ) and  2   b  (a capacitance value C DIG ) are connected in parallel to the detection MEMS capacitive elements  1   a  and  1   b  as in the first embodiment. 
     The outputs of the operational amplifiers  3   a  and  3   b  of the CV conversion amplifier are connected to the differential input terminal of the A/D converter  9  via the differential voltage detection switches  7   a  and  7   b . The common mode voltage detection resistors  6   a  and  6   b  are inserted in series between the outputs of the operational amplifiers  3   a  and  3   b  via the common mode voltage generation switches  17   a  and  17   b , and the common mode voltage detection resistors  6   a  and  6   b  are connected to the normal phase input terminal of the A/D converter  9  via the common mode voltage detection switch  8   a . Further, the reversed phase input terminal of the A/D converter  9  is connected to the voltage V B  via the common mode voltage detection switch  8   b . Further, the output of the A/D converter  9  is input to the switch  10 , and the output of the switch  10  serves as the sensor output. The output of the A/D converter  9  is input to the digital control unit  12  via the switch  11 . The output D CAL  of the digital control unit  12  is input to the digitally controlled variable capacitors  2   a  and  2   b.    
     Next, an operation of the circuit of  FIG. 5  will be described. The detection MEMS capacitive elements  1   a  and  1   b  have a similar configuration as in the first embodiment, and if the capacitance value change caused by the change amount of the plate interval is ΔC, a capacitance value of the detection MEMS capacitive element  1   a  is C+ΔC, and a capacitance value of the detection MEMS capacitive element  1   b  is C−ΔC. 
       FIG. 6  illustrates waveforms of a carrier clock  40 , a signal  41  applied to the digitally controlled variable capacitors  2   a  and  2   b , and a clock signal  42  used in the present embodiment. In the clock signal  42 , the switches  13   a  and  13   b  are turned on when a high potential signal (for example, V DD ) is input and turned off when a low potential signal (for example, GND) is input. Further, a phase of ϕ′ CHOP_B  of the signal  41  applied to the digitally controlled variable capacitors  2   a  and  2   b  is inverted from ϕ CHOP  of the carrier clock  40 , and the voltage level is decided in accordance with the common mode voltage compensation range. 
     Here, a high voltage value of the carrier clock ϕ CHOP  is W AR , a high voltage value of ϕ′ CHOP_B  connected to the digitally controlled variable capacitors  2   a  and  2   b  is indicated by V′ CAR , and a voltage of the node connected to the inverting input terminal of the first operational amplifier is V B =V DD /2. 
     Next, an operation in a period (the adjustment mode) in which the capacitance value C DIG  is decided will be described with reference to  FIG. 5 . In the adjustment mode, the common mode voltage generation switches  17   a  and  17   b  are turned on. Accordingly, similarly to the first embodiment, the output common mode voltage level V CMO  of the CV conversion amplifier (=the center voltage level of the output of each of the operational amplifiers  3   a  and  3   b ) is generated in the joint node  200  of the common mode voltage detection resistors  6   a  and  6   b . Further, in the adjustment mode period, the common mode voltage detection switches  8   a  and  8   b  are turned on, and the output common mode voltage level V CMO  of the CV conversion amplifier generated at the joint node  200  is input to the normal phase input terminal of the A/D converter  9 , and the voltage V B  is input to the reversed phase input terminal of the A/D converter  9 . In the adjustment mode period, since the differential voltage detection switches  7   a  and  7   b  are turned off, the outputs of the operational amplifiers  3   a  and  3   b  of the CV conversion amplifier are not input to the A/D converter  9 . The A/D converter  9  converts a difference voltage between the voltage of the normal phase input terminal and the voltage of the voltage of the reversed phase input terminal, that is, a difference between the output common mode voltage levels V CMO  and V B  of the CV conversion amplifier, that is, V CMO −V DD /2 into a digital value. Here, V B  is V DD /2, but other voltage values may be used as well. 
     The digital value is supplied to the digital control unit  12 . Similarly to the case of the first embodiment, with the negative feedback control, the capacitance value C DIG  ultimately converges to an appropriate capacitance value C DIG_FINAL , and the output common mode voltage level V CMO  of the CV conversion amplifier is sufficiently close to V DD /2. 
     Next, an operation of in normal operation period (normal mode) will be described. In the normal operation period, the CV conversion amplifier converts the MEMS capacitance change ΔC caused by the signal such as the acceleration applied to the sensor into the voltage signal ΔV by employing the appropriate capacitance value C DIG_FINAL  set in the adjustment mode as a correction capacitance. The voltage signal ΔV is the output differential voltage of the CV conversion amplifier. When C DIG_FINAL  is employed, the output common mode voltage level of the CV conversion amplifier is set near V DD /2, and thus an input signal to a sufficiently large sensor can be normally converted to a voltage signal. 
     During the period of the normal mode, the differential voltage detection switches  7   a  and  7   b  in the ON state, the output of the operational amplifier  3   a  of the CV conversion amplifier is connected to the normal phase input terminal of the A/D converter  9  via the filter  18   a , and the output of the operational amplifier  3   b  is connected to the reversed phase input terminal of the A/D converter  9  via the filter  18   b . Accordingly, the output differential voltage of CV conversion amplifier is converted into the digital value through the A/D converter  9 . As an implementation example of the digitally controlled variable capacitors  2   a  and  2   b , the configuration of  FIG. 4  can be used like the embodiment 1. 
     The present scheme is a non-continuous time type (switch type) detection scheme, unlike the analog correction scheme using the common mode feedback circuit of the related art, the scheme of the present embodiment can implement low power consumption because it is not necessary for the analog circuit to constantly operate continuously in the common mode voltage control. Further, since the feedback control can be implemented digitally, it is possible to implement the control unit with a small area and low power consumption as compared with the analog scheme. 
     Third Embodiment 
       FIG. 7  illustrates a third embodiment of the present invention. In the present embodiment, the capacitive MEMS  1  of the first embodiment is replaced with a capacitive MEMS  19 . The capacitive MEMS  19  includes four detection MEMS capacitive elements  1   a ,  1   b ,  1   c , and  1   d . In a case in which servo control is performed, an MEMS capacitive element for applying servo force may be further installed. In this case, of course, the present invention is effective as well. 
     A structure of a first pair including the detection MEMS capacitive elements  1   a  and  1   b  and a structure of a second pair including a pair of detection MEMS capacitive elements  1   c  and  1   d  are designed to be as identical as possible to each other. 
     Further, movable electrodes of the detection MEMS capacitive elements  1   a ,  1   b ,  1   c , and  1   d  are operational amplifier side electrodes of the capacitive elements, unlike the first to the second embodiments. On the other hand, the fixed electrodes of the detection MEMS capacitive elements  1   a ,  1   b ,  1   c , and  1   d  are connected to the carrier clock ϕ CHOP , the inversed carrier clock ϕ CHOP_B , the inversed carrier clock ϕ CHOP_B , and the carrier clock ϕ CHOP , respectively. 
     The movable electrode of the detection MEMS capacitive element  1   a , the movable electrode of the detection MEMS capacitive element  1   b , the movable electrode of the detection MEMS capacitive element  1   c , and the movable electrode of the detection MEMS capacitive element  1   d  are mechanically coupled to move as one body and functions as one weight (a mass body). When the signal such as the acceleration is not applied to the sensor, since no force such as inertial force acts on the weight, the weight, that is, the movable electrode of the detection MEMS capacitive element  1   a , the movable electrode of the detection MEMS capacitive element  1   b , the weight, that is, the movable electrode of the detection MEMS capacitive element  1   c , and the movable electrode of the detection MEMS capacitive element  1   d  are positioned at initial positions. 
     When positioned at the initial positions, since an electrode structure is designed so that a distance between the movable electrode and the fixed electrode of the detection MEMS capacitive element  1   a  is equal to a distance between the movable electrode and the fixed electrode of the detection MEMS capacitive element  1   b , the capacitance values of the detection MEMS capacitive element  1   a  and the detection MEMS capacitive element  1   b  are equal to each other, but practically, the capacitance values of the detection MEMS capacitive element  1   a  and the detection MEMS capacitive element  1   b  are not equal to each other due to parasitic capacitance, a manufacturing variation of MEMS, or the like, the capacitance value of the detection MEMS capacitive element  1   a  is indicated by C+C DC +C DC2 , and the capacitance value of the detection MEMS capacitive element  1   b  is indicated by C−C DC −C DC2 . 
     Similarly, when positioned at the initial positions, since an electrode structure is designed so that a distance between the movable electrode and the fixed electrode of the detection MEMS capacitive element  1   c  is equal to a distance between the movable electrode and the fixed electrode of the detection MEMS capacitive element  1   d , the capacitance values of the detection MEMS capacitive element  1   c  and the detection MEMS capacitive element  1   d  are equal to each other, but practically, the capacitance values of the detection MEMS capacitive element  1   c  and the detection MEMS capacitive element  1   d  are not equal to each other due to parasitic capacitance, a manufacturing variation of MEMS, or the like, the capacitance value of the detection MEMS capacitive element  1   c  is indicated by C—C DC +C DC2 , and the capacitance value of the detection MEMS capacitive element  1   d  is indicated by C+C DC −C DC2 . 
     In other words, the capacitance values Ca, Cb, Cc, and Cd of the respective MEMS capacitive elements  1   a ,  1   b ,  1   c , and  1   d  can be indicated as follows without loss of generality.
 
 Ca=C+C   DC   +C   DC2   , Cb=C−C   DC   −C   DC2  
 
 Cc=C−C   DC   +C   DC2   , Cd=C+C   DC   −C   DC2  
 
     Here, C DC  is a component affecting the deviation of the center value in the MEMS capacitance variation, and C DC2  is a component that does not affect the deviation of the center value in the MEMS capacitance variation. The reason will be described. 
     In the MEMS capacitive element  1   a ,  1   b ,  1   c , and  1   d , since the carrier clock ϕ CHOP  is applied to  1   a  and  1   d , and the inversed carrier clock ϕ CHOP_B  is applied to  1   b  and  1   c , the following charges are induced to the input nodes of the operational amplifiers  3   a  and  3   b.  
 
( Ca−Cb ) V   CAR =2( C   DC   +C   DC2 ) V   CAR  
 
−( Cc−Cd ) V   CAR =2( C   DC   −C   DC2 ) V   CAR  
 
     However, it is assumed that there is no capacitance change ΔC caused by an acceleration signal. As can be seen from the above formula, since C DC2  is output as the differential signal, it does not affect the deviation of the center voltage level, but because C DC  is output as the common mode signal, it causes a deviation of the center voltage level. Therefore, in the following description, C DC2  is not included in each capacitance value, but, of course, the present invention is effective regardless of the presence or absence of C DC2 . 
     If a signal such as acceleration is applied to the sensor, the weight receives force such as an inertial force proportional to the signal such as the acceleration, and thus the weight, that is, the positions of the movable electrode of the detection MEMS capacitive element  1   a , the movable electrode of the detection MEMS capacitive element  1   b , the movable electrode of the detection MEMS capacitive element  1   c , and the movable electrode of the detection MEMS capacitive element  1   d  are displaced as one body in proportion to the signal such as the acceleration. Accordingly, if the movable electrode of the detection MEMS capacitive element  1   a  is displaced to approach the fixed electrode of the detection MEMS capacitive element  1   a , the movable electrode of the detection MEMS capacitive element  1   b  conversely gets away from the fixed electrode of the detection MEMS capacitive element  1   b  by the same displacement amount. Further, if the movable electrode of the detection MEMS capacitive element  1   a  is displaced to get away from the fixed electrode of the detection MEMS capacitive element  1   a , the movable electrode of the detection MEMS capacitive element  1   b  conversely approaches the fixed electrode of the detection MEMS capacitive element  1   b  by the same displacement amount. Similarly, if the movable electrode of the detection MEMS capacitive element  1   c  is displaced to approach the fixed electrode of the detection MEMS capacitive element  1   c , the movable electrode of the detection MEMS capacitive element  1   d  conversely gets away from the fixed electrode of the detection MEMS capacitive element  1   d  by the same displacement amount. Further, if the movable electrode of the detection MEMS capacitive element  1   c  is displaced to get away from the fixed electrode of the detection MEMS capacitive element  1   c , the movable electrode of the detection MEMS capacitive element  1   d  conversely approaches the fixed electrode of the detection MEMS capacitive element  1   d  by the same displacement amount. 
     If the displacement amount, that is, the capacitance value change according to the change amount of the plate interval is indicated by ΔC, the capacitance value of the detection MEMS capacitive element  1   a  becomes C+C DC +ΔC, the capacitance value of the detection MEMS capacitive element  1   b  becomes C−C DC −ΔC, the capacitance value of the detection MEMS capacitive element  1   c  becomes C—C DC +ΔC, and the capacitance value of the detection MEMS capacitive element  1   d  becomes C+C DC −ΔC. 
     Therefore, the following charges are induced to the respective input nodes of operational amplifiers  3   a  and  3   b , respectively.
 
( Ca−Cb ) V   CAR =2( C   DC   +ΔC ) V   CAR  
 
−( Cc−Cd ) V   CAR =2( C   DC   −ΔC ) V   CAR  
 
     C DC  causes a deviation of the center voltage level of the output of the operational amplifier of the CV conversion amplifier, but since the values of the digitally controlled variable capacitors  2   a  and  2   b  are adjusted during the adjustment mode described above, it is possible to compensate C DC  and compensate the deviation of the center voltage level. 
     Except for the MEMS configuration described above, a configuration and an operation of the present embodiment are similar to those of the first embodiment. In other words, in the present embodiment, the values of the digitally controlled variable capacitors  2   a  and  2   b  are adjusted during the adjustment mode so that the deviations of the center voltage levels of the outputs of the operational amplifiers  3   a  and  3   b  of the CV conversion amplifier caused by C DC  due to the variation of the detection MEMS capacitive element  1  is compensated. 
       FIG. 8  illustrates waveforms of a carrier clock  40 , a signal  41  applied to the digitally controlled variable capacitors  2   a  and  2   b , and an inversed carrier clock  43  used in the present embodiment. Further, a phase of ϕ′ CHOP_B  of the signal  41  applied to the digitally controlled variable capacitors  2   a  and  2   b  is inverted from ϕ CHOP  of the carrier clock  40 , and the voltage level is decided in accordance with the common mode voltage compensation range. Further, a phase of ϕ CHOP_B  of the inversed carrier clock  43  is inverted from ϕ CHOP  of the carrier clock  40 , and the voltage level is the same. 
     Fourth Embodiment 
       FIG. 9  illustrates a fourth embodiment of the present invention. In the present embodiment, the operational amplifiers  3   a  and  3   b  used in the pseudo differential CV conversion amplifier of the third embodiment are replaced with a fully differential operational amplifier  14 . In this case, a common mode feedback circuit (CMFB) that performs control such that the output common mode voltage level V CMO  of the fully differential operational amplifier  14  (=(V OUTP +V OUTN )/2, where V OUTP  and V OUTN  are a normal phase output voltage and a reversed phase output voltage of the fully differential operational amplifier  14 ) has a desired voltage level (for example, V DD /2). Therefore, in a case in which the fully differential operational amplifier is used, the output common mode voltage level V CMO  can be set to be around V DD /2, but in compensation, an input common mode voltage level V CMI  of the fully differential operational amplifier (=(V INP +V INN )/2, where V INP  and V INN  are a normal phase input voltage and a reversed phase input voltage of the fully differential operational amplifier  14  remarkably deviates from a desired voltage level. 
     Next, a configuration will be described. The capacitive MEMS  19  includes four detection MEMS capacitive elements  1   a ,  1   b ,  1   c , and  1   d  (each having a capacitance value C), and one of electrodes is a movable electrode which can mechanically move. A structure of a first pair including the detection MEMS capacitive elements  1   a  and  1   b  and a structure of a second pair including a pair of detection MEMS capacitive elements  1   c  and  1   d  are designed to be as identical as possible to each other. 
     The movable electrodes of the detection MEMS capacitive elements  1   a ,  1   b ,  1   c , and  1   d  are connected to first and second input terminals of the fully differential operational amplifier  14  via differential voltage detection switches  87   a  and  87   b . On the other hand, fixed electrodes of detection MEMS capacitive elements  1   a ,  1   b ,  1   c , and  1   d  are connected to the carrier clock ϕ CHOP , the inversed carrier clock ϕ CHOP_B , the inversed carrier clock ϕ CHOP_B , and the carrier clock ϕ CHOP , respectively. 
     Common mode voltage detection resistors  86   a  and  86   b  are inserted in series between common mode voltage generation switches  817   a  and  817   b  between the first and second input terminals and the output terminal of the fully differential operational amplifier  14 , and further, a joint node  800  of the common mode voltage detection resistors  86   a  and  86   b  is connected to the first input terminal of the fully differential operational amplifier  14  via a common mode voltage detection switch  88   a . Further, the second input terminal of the fully differential operational amplifier  14  is connected to the voltage V B  via a common mode voltage detection switch  88   b.    
     The feedback capacitive elements  4   a  (a capacitance value C F ) and  4   b  (a capacitance value C F ) and the resistive element  5   a  (a resistance value R F ) and  5   b  (a resistance value R F ) are installed between the first and second input terminals and the output terminal of the fully differential operational amplifier  14 . Further, in the present configuration, the digitally controlled variable capacitors  2   a  (a capacitance value C DIG ) and  2   b  (a capacitance value C DIG ) are connected in parallel with the detection MEMS capacitive elements  1   a ,  1   b ,  1   c , and  1   d. ϕ′   CHOP_B  obtained by inverting the carrier clock ϕ CHOP  and changing a level appropriately is connected to the digitally controlled variable capacitors  2   a  and  2   b.    
     First and second outputs of the fully differential operational amplifier  14  are connected to first and second inputs of a common mode detection circuit  15 . The common mode detection circuit  15  is a circuit that outputs an average voltage value of the two output voltages of the fully differential operational amplifier  14 . An output of the common mode detection circuit  15  is connected to the non-inverting input terminal of an operational amplifier  16 . A feedback circuit including the common mode detection circuit  15  and the operational amplifier  16  constitutes the common mode feedback circuit (CMFB) described above. 
     In the fully differential operational amplifier, since the fluctuation of the average output voltage is a problem of an amplification operation due to an element mismatch, it is necessary to install the CMFB. The CMFB detects the average output voltage through the common mode detection circuit  15  such as resistance voltage division, and amplifies a difference voltage between the average output voltage and a reference voltage V B  using the operational amplifier  16 . When a difference voltage W CM  is input to a gate of a tail current source of the fully differential operational amplifier  14 , a negative feedback is given, and control is performed such that the output common mode voltage level V CMO  of the fully differential operational amplifier  14  is kept constant. 
     Further, the first and second outputs of the fully differential operational amplifier  14  of the CV conversion amplifier produce a balanced differential output on the basis of V CMO . The differential outputs are connected to the differential input terminal of A/D converter  9  via the filters  18   a  and  18   b.    
     Further, an output of the A/D converter  9  is input to the switch  10 , and the output of the switch  10  serves as the sensor output. An output of the A/D converter  9  is input to the digital control unit  12  via the switch  11 . The output D CAL  of the digital control unit  12  is input to the digitally controlled variable capacitors  2   a  and  2   b.    
     Next, an operation will be described. A configuration of the capacitive MEMS  19  is similar to that of the third embodiment. Similar to the third embodiment, when positioned at the initial positions, the capacitance values of the detection MEMS capacitive element  1   a  and the detection MEMS capacitive element  1   b  are equal to each other, but practically, the capacitance values of the detection MEMS capacitive element  1   a  and the detection MEMS capacitive element  1   b  are not equal to each other due to parasitic capacitance, a manufacturing variation of MEMS, or the like, the capacitance value of the detection MEMS capacitive element  1   a  is indicated by C+C DC +C DC2 , and the capacitance value of the detection MEMS capacitive element  1   b  is indicated by C−C DC −C DC2 . Similarly, the capacitance value of the detection MEMS capacitive element  1   c  is indicated by C−C DC +C DC2 , and the capacitance value of the detection MEMS capacitive element  1   d  is indicated by C+C DC −C DC2 . Here, C DC  causes a deviation of the center voltage level of the output of the operational amplifier of the CV conversion amplifier, but C DC2  does not affect the deviation of the center voltage level. Therefore, in the following description, C DC2  is not included in each capacitance value. 
     When the signal such as the acceleration is applied to the sensor, If the displacement amount according to the acceleration, that is, the capacitance value change according to the change amount of the plate interval is indicated by ΔC, the capacitance value of the detection MEMS capacitive element  1   a  becomes C+C DC +ΔC, the capacitance value of the detection MEMS capacitive element  1   b  becomes C−C DC −ΔC, the capacitance value of the detection MEMS capacitive element  1   c  becomes C—C DC +ΔC, and the capacitance value of the detection MEMS capacitive element  1   d  becomes C+C DC −ΔC. 
     Waveforms of a carrier clock  40 , a signal  41  applied to the digitally controlled variable capacitors  2   a  and  2   b , and an inversed carrier clock  43  used in the present embodiment are identical to those illustrated in  FIG. 8 . 
     Next, an operation in a period (the adjustment mode) in which the capacitance value C DIG  is determined will be described. In the adjustment mode period, the common mode voltage generation switches  817   a  and  817   b  are turned on. Accordingly, an average voltage of the normal phase side input voltage of the CV conversion amplifier (the first input voltage of the fully differential operational amplifier  14 ) and the reversed phase side input voltage (the second input voltage of the fully differential operational amplifier  14 ), that is, the input common mode voltage level V CMI  of the CV conversion amplifier is generated at the joint node  800  of the common mode voltage detection resistors  86   a  and  86   b.    
     Further, in the adjustment mode period, the common mode voltage detection switches  88   a  and  88   b  are turned on, the input common mode voltage level V CMI  of the CV conversion amplifier generated at the joint node  800  is input to the first input terminal of the fully differential operational amplifier  14 , and the voltage V B  is input to the second input terminal of the fully differential operational amplifier  14 . In the adjustment mode period, the differential voltage detection switches  87   a  and  87   b  are turned off. 
     The A/D converter  9  converts a difference voltage between the voltage of the normal phase input terminal and the voltage of the voltage of the reversed phase input terminal, that is, a difference between the output common mode voltage levels V CMO  and V B  of the CV conversion amplifier, that is, V CMO −V DD /2 into a digital value. Here, V B  is V DD /2, but other voltage values may be used as well. 
     Since the value of the difference voltage which is the output of the fully differential operational amplifier  14  is not affected by the output common mode voltage level V CMO , the common mode feedback circuit (CMFB) including the common mode detection circuit  15  and the like may operate even in the adjustment mode. 
     The digital value is supplied to the digital control unit  12 . In a case in which the digital value is positive, it means that the input common mode voltage level V CMI  of the current CV conversion amplifier is higher than V DD /2. In this case, the digital control unit  12  updates the currently output digital compensation value D CAL  to a larger value and outputs it. On the other hand, in a case in which the digital value is negative, it means that the input common mode voltage level V CMI  of the current CV conversion amplifier is lower than V DD /2. In this case, the digital control unit  12  updates the currently output digital compensation value D CAL  to a smaller value and outputs it. 
     Further, the digitally controlled variable capacitors  2   a  and  2   b  convert the digital compensation value D CAL  supplied from the digital control unit  12  into the capacitance value C DIG . In a case in which the current input common mode voltage level V CMO  of the CV conversion amplifier is higher than V DD /2, D CAL  increases, and thus the capacitance value C DIG  increases as well. As a result, the input common mode voltage level V CMI  of the CV conversion amplifier moves in a direction in which it gets lower than now. On the other hand, in a case in which the current common mode voltage level V CMI  of the CV conversion amplifier is lower than V DD /2, D CAL  decreases, and thus the capacitance value C DIG  decreases as well. As a result, the input common mode voltage level V CMO  of the CV conversion amplifier moves in a direction in which it gets higher than now. With the above negative feedback control, the capacitance value C DIG  ultimately converges to an appropriate capacitance value C DIG_FINAL , and the output common mode voltage level V CMO  of the CV conversion amplifier is sufficiently close to V DD /2. 
     Next, an operation in the normal operation period (normal mode) will be described. In the normal mode, the CV conversion amplifier converts the MEMS capacitance change ΔC caused by the signal such as the acceleration applied to the sensor into the voltage signal ΔV by employing the appropriate capacitance value C DIG_FINAL  as a correction capacitance. The voltage signal ΔV is the output differential voltage of the CV conversion amplifier. When C DIG_FINAL  is employed, the input common mode voltage level of CV conversion amplifier is set near V DD /2, and the output common mode voltage level is kept constant by the common mode feedback circuit including the common mode detection circuit  15  and the operational amplifier  16 , and thus an input signal to a sufficiently large sensor can be normally converted to a voltage signal. 
     In the normal operation period, the differential voltage detection switches  7   a  and  7   b  are turned on, the first output of the fully differential operational amplifier  14  of the CV conversion amplifier is connected to the normal phase input terminal of the A/D converter  9  via the filter  18   a , and the second output of the operational amplifier  14  is connected to the reversed phase input terminal of the A/D converter  9  via the filter  18   b . Accordingly, the output differential voltage of CV conversion amplifier is converted into the digital value through the A/D converter  9 . 
     As an implementation example of the digitally controlled variable capacitors  2   a  and  2   b , the configuration of  FIG. 4  can be used. In the present scheme, in a case in which the deviation of the input common mode voltage level is corrected in the fully differential operational amplifier using the common mode feedback circuit, unlike the analog correction scheme, the analog circuit need not operate continuously in the common mode voltage control, and thus low power consumption can be implemented. Further, since the feedback control can be implemented digitally, it is possible to implement the control unit with a small area and low power consumption as compared with the analog scheme. 
     As various kinds of switches in the embodiments described above, various switches such as a complementary type switch in which an NMOS and a PMOS are connected in parallel, a switch including only an NMOS, and a switch including only a PMOS can be used. For the sake of convenience of description, in any case, it is assumed that the clock signal of controlling the switch is in the ON state in a case in which it is a high voltage and in the OFF state in a case in which it is a low voltage. 
     According to the embodiments described above in detail, since the detection of the common mode voltage is performed by performing the resistance voltage division of the differential output signal of the CV amplifier, the common mode component can be detected with a high degree of accuracy. Further, since the A/D converter originally included in the digital electrostatic capacitive sensor can be used, there is not necessary to use a new A/D converter when the adjustment voltage is converted into the digital value. The adjustment value can be held digitally, and unlike the adjustment by the analog circuit, it is not necessary to operate constantly, and it is possible to perform the adjustment with low power consumption. In addition to the normal sensor output operation, for example, it is possible to reduce the influence of the deviation of the adjustment value caused by the temperature fluctuation or the like by searching for the adjustment value once in every 100 times. 
     The CV conversion amplifiers and the capacitive sensors of the present invention described in the above embodiments detect, for example, the acceleration, the angular velocity, or the like, and output a sensor output signal corresponding thereto. This sensor output signal can be used in a system that performs posture control of automobiles, motorcycles, agricultural machines, or the like, secures driving stability, and prevents sideslip such as electronic stability control (ESC) or sensor systems for resource exploration. 
     In this specification or the like, the terms “electrode” and “wiring” do not functionally limit the components thereof. For example, the “electrode” may be used as a part of the “wiring,” or vice versa. Further, the terms “electrode” and “wiring” also include a case in which a plurality of “electrodes” or a plurality of “wirings” are integrally formed or the like. 
     The present invention is not limited to the embodiments described above but includes various modifications. For example, it is possible to replace a part of a configuration of a certain embodiment with a configuration of another embodiment, and it is also possible to add a configuration of another embodiment to a configuration of a certain embodiment. It is also possible to perform addition, deletion, and replacement of configurations of other embodiments on a part of the configurations of each embodiment. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used for acceleration sensors or the like. 
     REFERENCE SIGNS LIST 
     
         
           1  capacitive MEMS 
           1   a ,  1   b ,  1   c , and  1   d  detection MEMS capacitive element 
           2   a ,  2   b  digitally controlled variable capacitor 
           3   a ,  3   b  operational amplifier 
           4   a ,  4   b  feedback capacitive element 
           5   a ,  5   b  resistive element 
           6   a ,  6   b  resistive element 
           7   a ,  7   b ,  8   a ,  8   b  switch 
           9  A/D converter 
           10 ,  11  switch 
           12  digital control unit 
           13   a ,  13   b  switch 
           14  fully differential operational amplifier 
           15  common mode detection circuit 
           16  operational amplifier 
           17   a ,  17   b  switch 
           18   a ,  18   b  filter 
           19  capacitive MEMS 
           21   a ,  21   b ,  22   a ,  22   b ,  23   a ,  23   b ,  24   a ,  24   b ,  25   a ,  25   b  capacitor selection switch 
           26 ,  27 ,  28 ,  29 ,  30  capacitive element 
           40  carrier clock 
           41  signal applied to digitally controlled variable capacitors  2   a  and  2   b    
           42  clock signal 
           43  inversed carrier clock