JP5974851B2 - Sensor device - Google Patents

Description

  The present invention relates to a sensor device configured by, for example, MEMS (Micro Electro Mechanical Systems) or the like and detecting acceleration or the like.

  Conventionally, as a sensor device for detecting acceleration or the like, a pair of fixed electrodes are provided with movable electrodes movable in a predetermined direction, and two variable capacitors are formed between the fixed electrodes and the movable electrodes. A capacitance sensor is known (for example, Patent Document 1). In this type of capacitance sensor, when an external force (acceleration) acts in a direction in which the movable electrode can move, the movable electrode is displaced between the two fixed electrodes accordingly, and the capacitance of the two variable capacitance capacitors. Change each. In order to detect changes in the capacitances of these two variable capacitors, for example, in Patent Document 1, an external voltage whose polarity is inverted at a constant cycle is applied to a pair of fixed electrodes, and the amount of charge movement with respect to the movable electrode is determined. The CV conversion circuit is configured to detect and output.

JP 2004-294077 A

  However, in the circuit configuration as disclosed in Patent Document 1, when various noises act on the capacitance sensor, the influence of the noise appears in the output of the CV conversion circuit. Therefore, it is required to reduce the influence of such noise. .

  Therefore, for example, as a circuit example of a sensor device that reduces the influence of disturbance noise, it is conceivable to employ a circuit configuration as shown in FIG. In the circuit configuration of FIG. 8, the sensor unit 101 is provided with a movable electrode 106 supported by a spring between a pair of fixed electrodes 104 and 105, and the pair of fixed electrodes 104 and 105 and the movable electrode 106 An electrostatic capacitance sensor 103 having two variable capacitors 110 and 111 is provided. On the other hand, the signal processing circuit 102 that processes a signal output from the sensor unit 101 includes a CV conversion circuit 120, a sampling circuit 130, a dummy element 140, and an input common mode feedback (ICMFB) circuit 150. The CV conversion circuit 120 includes a fully differential operational amplifier 121, a pair of feedback capacitors Cf1 and Cf2, and a pair of switches Sf1 and Sf2 for discharging the feedback capacitors Cf1 and Cf2. The inverting input terminal of the fully differential operational amplifier 121 is connected to the movable electrode 106 of the capacitance sensor 103 via a line L1. The non-inverting input terminal of the fully differential operational amplifier 121 is connected to the dummy element 140. The dummy element 140 is composed of a capacitor 141 having substantially the same capacitance as the capacitance when the sensor unit 101 is viewed from the signal processing circuit 102, and one end is connected to the non-inverting input terminal of the fully differential operational amplifier 121. The other end is held at the reference voltage Vref. A line L2 that connects the non-inverting input terminal of the fully differential operational amplifier 121 and the dummy element 140 is extended to the sensor unit 101 so as to run in parallel with the line L1. Further, the sampling circuit 130 is a circuit that samples and outputs the differential output (Vop, Von) of the fully differential operational amplifier 121 at a predetermined timing.

  In the circuit configuration as described above, the fully differential operational amplifier 121 of the CV conversion circuit 120 acts so that the potential Vin of the inverting input terminal and the potential Vip of the non-inverting input terminal are the same potential. Further, an output common mode feedback circuit (not shown) acts so that the average value of the output signal Vop at the non-inverting output terminal and the output signal Von at the inverting output terminal becomes the reference voltage Vref. The input common mode feedback circuit 150 operates so that the average value of the potentials of the line L1 and the line L2 becomes the reference voltage Vref. When voltages having different polarities (for example, VH, VL, where VH> Vref, VL <Vref) are applied to the two terminals X1 and X2 of the capacitance sensor 103, electric charges are respectively applied to the two variable capacitors 110 and 111. Accumulated. At this time, if an external force (acceleration) is applied to the movable electrode 106 and is displaced from an intermediate position between the two fixed electrodes 104 and 105, it corresponds to a change ΔC in the capacitance of the two variable capacitors 110 and 111. To be accumulated in the feedback capacitors Cf1 and Cf2. As a result, output signals Vop and Von corresponding to the charge accumulation amounts of the feedback capacitors Cf1 and Cf2 are output from the inverting output terminal and the non-inverting output terminal of the fully differential operational amplifier 121, respectively, and the sampling circuit 7 outputs the output signal Vop. , Von are sampled and output, so that the change in capacitance ΔC according to the acceleration can be detected.

  In the case of such a circuit configuration, the lines L1 and L2 run side by side and are wired to the sensor unit 101. Therefore, when disturbance noise acts on the line L1, disturbance noise also acts on the line L2. Accordingly, since such disturbance noise is input as common mode noise to the fully differential operational amplifier 121, the disturbance noise can be canceled by the differential output (Vop, Von) of the fully differential operational amplifier 121.

  However, on the other hand, even with the circuit configuration as shown in FIG. 8, the voltage applied to the two terminals X1 and X2 of the capacitance sensor 103 is not stabilized at a constant value and does not synchronize with the operation of the CV conversion circuit 120. When it fluctuates due to a noise component, a variation (noise component) of the applied voltage appears as it is in the differential output (Vop, Von) of the fully differential operational amplifier 121, which causes a measurement error. There is. In particular, since the voltage applied to the capacitance sensor 103 in such a sensor device is supplied from an external power source of a device in which the sensor device is mounted, a general DC / DC converter is used for the external power source. Etc. is used, a relatively large ripple becomes a noise component, and it becomes impossible to accurately detect a change in capacitance when an external force (acceleration) is applied.

  FIG. 9 is a diagram for explaining the circuit operation when the noise component Vz is included in the voltage VH applied to the terminal X1 in the circuit of FIG. FIG. 9 illustrates a case where the voltage VH applied to the terminal X1 is Vref + Vc, and the noise component Vz is superimposed on the voltage VH. The voltage VL applied to the terminal X2 is Vref−Vc. At this time, if the capacitance C of the variable capacitor 110 is changed to C + ΔC and the capacitance C of the variable capacitor 111 is changed to C−ΔC, the charge Q = (C + ΔC) (Vc + Vz) is accumulated, and charge Q = − (C−ΔC) Vc is accumulated in the variable capacitor 111. Therefore, the charge transfer amount Q1 from the sensor unit 101 to the signal processing circuit 102 is Q1 = − (2ΔCVc + ΔCVz + CVz). On the other hand, since no charge accumulation occurs in the capacitor 141 of the dummy element 140, Q = 0. Therefore, the charge transfer amount Q2 from the capacitor 141 of the dummy element 140 is Q2 = 0.

  At this time, the charge Q = − (2ΔCVc + ΔCVz + CVz) / 2 is accumulated in the feedback capacitor Cf1 of the fully differential operational amplifier 121, and the charge Q = (2ΔCVc + ΔCVz + CVz) / 2 is accumulated in the feedback capacitor Cf2. The output signal Vop from the non-inverting output terminal of the fully differential operational amplifier 121 is Vop = − (2ΔCVc + ΔCVz + CVz) / 2Cf + Vref, and the output signal Von from the inverting output terminal is Von = (2ΔCVc + ΔCVz + CVz) / 2Cf + Vref. Cf is the capacitance of the feedback capacitors Cf1 and Cf2. Therefore, if the voltage VH applied to the terminal X1 includes the noise component Vz, the output signal Vop includes the noise component − (ΔCVz + CVz) / 2Cf, and the output signal Von includes the noise component ( ΔCVz + CVz) / 2Cf is included.

  Here, paying attention to the noise components included in the output signals Vop and Von, the change amount ΔC of the capacitance C is extremely small compared to the capacitances Cf of the feedback capacitors Cf1 and Cf2. That is, since ΔC / Cf << 1, the noise term of ΔCVz / 2Cf is an extremely small value and can be ignored. On the other hand, the remaining CVz / 2Cf noise term is a relatively large value and cannot be ignored. Therefore, in order to reduce the influence of the noise component Vz included in the voltage VH applied to the terminal X1, it is desirable to cancel and output the CVz / 2Cf noise term.

  Therefore, in order to solve the above problems, the present invention has an object to provide a sensor device capable of reducing and outputting a noise component even when the external voltage supplied from an external power source includes the noise component. To do.

  In order to achieve the above object, the present invention adopts as a solution means that a pair of fixed electrodes are provided across a movable electrode movable in a predetermined direction, and formed between each fixed electrode and the movable electrode. A capacitance sensor is formed by the first and second variable capacitors, and external voltages having different polarities supplied from an external power source are applied to the pair of fixed electrodes, whereby the first and second variable capacitors are applied. A sensor device having a sensor unit that outputs a signal corresponding to the amount of charge accumulated in each of the capacitance capacitors from the movable electrode, and a signal processing circuit that processes the signal from the sensor unit, the signal processing circuit, A first fixed capacitor having substantially the same capacitance as the first variable capacitor, and a second fixed capacitor having substantially the same capacitance as the second variable capacitor. A fully differential operational amplifier in which an output signal output from the sensor unit is input to one of the input terminals and a common terminal in which the first and second fixed capacitors are connected to each other is connected to the other input terminal. And a CV conversion circuit that CV-converts an output signal output from the sensor unit, and the other terminal different from the common terminal of the first fixed capacitor is applied to the first variable capacitor. A configuration in which the other terminal different from the common terminal in the second fixed capacitor is connected to the connection terminal for the external voltage applied to the second variable capacitor, connected to the external voltage connection terminal. It is in.

  In the above configuration, the signal processing circuit further includes a first switch group for inverting the polarity of the external voltage applied to the first variable capacitor at a predetermined cycle centered on the reference voltage, and the second variable capacitor. A second switch group that reverses the polarity at a predetermined period around the reference voltage so that the external voltage applied to the capacitor has a reverse polarity in relation to the external voltage applied to the first variable capacitor. The other terminal different from the common terminal in the first fixed capacitor is connected between the first variable capacitor and the first switch group, and the common terminal in the second fixed capacitor is You may employ | adopt the structure characterized by connecting the other different terminal between the 2nd variable capacitor and the 2nd switch group.

  Further, in the above configuration, it is more preferable to employ a configuration in which the first and second fixed capacitors are formed on a semiconductor chip different from the sensor unit.

  According to the present invention, even when the external voltage applied to the capacitance sensor includes a noise component, the same external voltage is also applied to the first fixed capacitor or the second fixed capacitor. As a result, such noise components can be reduced and output.

It is a circuit diagram which shows one structural example of a sensor device. It is a figure which shows the voltage waveform applied to the terminal of the both ends of an electrostatic capacitance sensor. It is a figure which shows an electric charge movement when an applied voltage changes from a 1st area to a 2nd area. It is a figure which shows an electric charge movement when an applied voltage changes from a 2nd area to a 3rd area. It is a figure which shows an electric charge movement when an applied voltage changes from a 3rd area to a 4th area. It is a figure which shows electric charge movement in the state in which the noise component is contained in the voltage supplied from an external power supply. It is a figure which shows an example of the other circuit structure of a sensor device. It is a figure which shows the circuit structure of the sensor device which reduces the influence of disturbance noise. FIG. 9 is a diagram illustrating an operation when noise is included in an applied voltage in the circuit of FIG. 8.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the embodiments described below, members that are common to each other are denoted by the same reference numerals, and redundant descriptions thereof are omitted.

  FIG. 1 is a circuit diagram showing a configuration of a sensor device 1 according to an embodiment of the present invention. This sensor device 1 mainly includes a sensor unit 2 formed on a MEMS chip by a MEMS process and a signal processing circuit 4 formed on a semiconductor chip by a CMOS process, and these are packaged into one. For example, it is configured as a device that can be mounted on an external device such as a smartphone or a tablet terminal.

  The sensor unit 2 is configured as a sensor that detects acceleration. The sensor unit 2 includes a capacitance sensor 3 that changes capacitance according to the acceleration when acceleration in a predetermined direction is applied. In the electrostatic capacitance sensor 3, the movable electrode 13 is disposed between the two fixed electrodes 11, 12, the first variable capacitor 31 including the fixed electrode 11 and the movable electrode 13, the fixed electrode 12 and the movable electrode 13 are movable. And a second variable capacitor 32 composed of the electrode 13. When acceleration is not acting on the sensor unit 2, the movable electrode 13 of the capacitance sensor 3 is held at an intermediate position between the two fixed electrodes 11 and 12 by a spring (not shown). Accordingly, the capacitances of the two variable capacitors 31 and 32 are equal to each other. On the other hand, when acceleration in a predetermined direction acts on the sensor unit 2, the movable electrode 13 of the capacitance sensor 3 is displaced from an intermediate position between the two fixed electrodes 11, 12, and the two variable capacitors 31, 32. The electrostatic capacitance C of the relative changes. When one capacitance C increases by ΔC, the other capacitance C decreases by ΔC. At this time, when external voltages having different polarities supplied from an external power source are applied to the pair of fixed electrodes 11 and 12, the electrostatic capacitance sensor 3 causes the static capacitance of the two variable capacitors 31 and 32. Charges corresponding to the electric capacities (C + ΔC) and (C−ΔC) are accumulated, and a signal corresponding to the charge amount is output. Note that the output terminal of the sensor unit 2 is connected to the movable electrode 13.

  The signal processing circuit 4 is a circuit that processes a signal output from the sensor unit 2, and includes a CV conversion circuit 5, a dummy element 6, a sampling circuit 7, and an input common mode feedback circuit 9. . The signal processing circuit 4 is separately provided with a control circuit for opening and closing various switches, which will be described later, but this is not shown.

  The dummy element 6 is an element for performing an operation simulating the capacitance sensor 3 in a state where no acceleration is applied to the sensor unit 2, and has two fixed capacitances that are fixed on the capacitance formed on the semiconductor chip. Capacitors 61 and 62 are provided. Of these two capacitors 61, 62, the first capacitor 61 is the first variable capacitor 31 in the state where the movable electrode 13 is in an intermediate position between the two fixed electrodes 11, 12 in the capacitance sensor 3. The electrostatic capacitance C is substantially the same as the electrostatic capacitance C. The second capacitor 62 is substantially the same as the capacitance C of the second variable capacitor 32 in the state where the movable electrode 13 is in the middle position between the two fixed electrodes 11 and 12 in the capacitance sensor 3. It has a capacitance C. In the present embodiment, the capacitances C of these two capacitors 61 and 62 are equal to each other.

  Here, in the present embodiment, the dummy element 6 is provided in the signal processing circuit 4, but instead of being provided in the signal processing circuit 4, for example, the capacitance sensor 3 and the sensor unit 2 configured by a MEMS structure are provided. The dummy element 6 may be formed with a substantially similar structure. However, in that case, it is necessary to form both the capacitance sensor 3 and the dummy element 6 in the sensor unit 2 by the MEMS process, and the size of the sensor unit 2 cannot be avoided. Therefore, the size of the sensor device 1 itself is increased, and there is a problem that the size becomes unsuitable for mounting on a small external device such as a smartphone. Therefore, in this embodiment, in order to avoid the occurrence of such a problem, the dummy element 6 described above is provided in the signal processing circuit 4, and the two capacitors 61 and 62 are formed on the semiconductor chip by the CMOS process. The element 6 is reduced in size, and the sensor device 1 is reduced in size.

  The CV conversion circuit 5 includes a fully differential operational amplifier 20 and is a circuit that performs CV conversion on an output signal output from the sensor unit 2. An inverting input terminal, which is one input terminal of the fully differential operational amplifier 20, is connected to the capacitance sensor 3 via a line L 1 connected to the movable electrode 13 provided in the sensor unit 2. Input the output signal. The other input terminal, the non-inverting input terminal, is connected to the common terminal 63 of the two capacitors 61 and 62 via the line L2 connected to the common terminal 63 to which the two capacitors 61 and 62 of the dummy element 6 are connected to each other. It is connected. The line L2 is further extended from the dummy element 6 so as to run in parallel with the line L1, and is wired so that the end thereof is positioned in the sensor unit 2 formed on the MEMS chip.

  A feedback capacitor Cf1 and a switch Sf1 are connected in parallel between the inverting input terminal and the non-inverting output terminal of the fully differential operational amplifier 20. The capacitor Cf1 is a capacitance Cf. The switch Sf1 is for discharging the electric charge accumulated in the capacitor Cf1, and is on / off controlled by a control circuit (not shown). A feedback capacitor Cf2 and a switch Sf2 are also connected in parallel between the non-inverting input terminal and the inverting output terminal of the fully differential operational amplifier 20. The capacitor Cf2 has the same capacitance Cf as the capacitor Cf1. The switch Sf2 is for discharging the electric charge accumulated in the capacitor Cf2, and is on / off controlled at the same timing as the switch Sf1 by a control circuit (not shown). The fully differential operational amplifier 20 operates so that the potential Vin of the inverting input terminal is equal to the potential Vip of the non-inverting input terminal, and the output of the non-inverting output terminal is output by an output common mode feedback circuit (not shown). It operates so that the average value of the signal Vop and the output signal Von of the inverting output terminal becomes the reference voltage Vref.

  An input common mode feedback circuit 9 is connected to the lines L1 and L2 that are input to the inverting input terminal and the non-inverting input terminal of the fully differential operational amplifier 20, respectively. The input common mode feedback circuit 9 is a circuit that controls the average value of the potential Vin of the inverting input terminal and the potential Vip of the non-inverting input terminal to be the reference voltage Vref. Therefore, by the action of the input common mode feedback circuit 9, the potential Vin of the inverting input terminal and the potential Vip of the non-inverting input terminal of the fully differential operational amplifier 20 are held at the reference voltage Vref.

  Such a CV conversion circuit 5 accumulates charges corresponding to the charge transfer amounts of the lines L1 and L2 in the feedback capacitors Cf1 and Cf2, and outputs an output signal Vop corresponding to the charge amounts accumulated in the feedback capacitors Cf1 and Cf2. , Von.

  The sampling circuit 7 is a circuit that samples and outputs the output signals Vop and Von output from the CV conversion circuit 5 at a predetermined timing. In this embodiment, an output correlated double sampling circuit is employed as the sampling circuit 7. The sampling circuit 7 includes capacitors Cp1 and Cp2 and switches Sp1 and Sp2 in order to sample and output the output signal Vop of the CV conversion circuit 5. One end of the capacitor Cp1 is connected to the non-inverting output terminal of the fully differential operational amplifier 20, and the other end is connected to one end of the switch Sp2. One end of the switch Sp1 is connected between the capacitor Cp1 and the switch Sp2, and the other end of the switch Sp1 is held at the reference voltage Vref. The other end of the switch Sp2 is an output terminal Voutp of the sampling circuit 7, and the output terminal Voutp is connected to the reference voltage Vref via the capacitor Cp2. In order to sample and output the output signal Von of the CV conversion circuit 5, the sampling circuit 7 includes capacitors Cn1 and Cn2 and switches Sn1 and Sn2. The connection state between the capacitors Cn1, Cn2 and the switches Sn1, Sn2 is the same as the connection state between the capacitors Cp1, Cp2 and the switches Sp1, Sp2.

  Such a sampling circuit 7 first samples the output signals Vop and Von from the CV conversion circuit 5 in the capacitors Cp1 and Cn1 by closing the switches Sp1 and Sn1 with the switches Sp2 and Sn2 open. As a result, charges based on the potential difference between the output signals Vop and Von and the reference voltage Vref are accumulated in the capacitors Cp1 and Cn1, respectively. Then, with the switches Sp2 and Sn2 open, the switches Sp1 and Sn1 are opened, the output signals Vop and Von from the CV conversion circuit 5 are sampled for the second time, and the switches Sp2 and Sn2 are closed in that state. As a result, output signals Voutp and Voutn are output. That is, the sampling circuit 7 performs the second sampling while holding the electric charges accumulated in the capacitors Cp1 and Cn1 at the first sampling, so that the output circuit Vop and Von at the second sampling can be compared with the capacitors. Output signals Voutp and Voutn reflecting the potential difference corresponding to the charges accumulated in Cp1 and Cn1 are output. By performing such sampling, it is possible to output the output signals Voutp and Voutn in which the offset voltage of the fully differential operational amplifier 20 in the CV conversion circuit 5 and kT / C noise are canceled. The switches Sp1, Sp2, Sn1, Sn2 in the sampling circuit 7 are on / off controlled by a control circuit (not shown).

  The signal processing circuit 4 is provided with connection terminals X1 and X2 for applying external voltages Vref, VH, and VL to the capacitance sensor 3 of the sensor unit 2. One fixed electrode 11 of the capacitance sensor 3 is connected to the connection terminal X1, and is applied to one fixed electrode 11 from three types of external voltages Vref, VH, and VL supplied from an external power source. A first switch group S10 including three switches S11, S12, and S13 for switching voltages is connected. The other fixed electrode 12 of the capacitance sensor 3 is connected to the connection terminal X2, and applied to the other fixed electrode 12 from three types of external voltages Vref, VH, and VL supplied from an external power source. A second switch group S20 including three switches S21, S22, and S23 for switching the voltage to be connected is connected. Here, Vref is a reference voltage, VH is a voltage (Vref + Vc) higher than the reference voltage Vref by a predetermined voltage Vc, and VL is a voltage (Vref−Vc) lower than the reference voltage Vref by a predetermined voltage Vc. These three types of voltages are generated by an external power source provided in a device on which the sensor device 1 is mounted and supplied to the signal processing circuit 4. When Vc = Vref, VH = 2Vref and VL = 0.

  Each switch S11, S12, S13 included in the first switch group S10 is alternatively switched to an on state (closed state) under the control of a control circuit (not shown). Similarly, each switch S21, S22, S23 included in the second switch group S20 is alternatively switched to an on state (closed state) in synchronization with the first switch group S10. In the present embodiment, by such switching control, a voltage whose polarity is switched at a constant cycle around the reference voltage Vref is applied to both ends of the capacitance sensor 3.

  The two capacitors 61 and 62 provided in the dummy element 6 have the other terminals 64 and 65 different from the common terminal 63 connected to the external voltage Vref, VH, and VL applied to the electrostatic capacitance sensor 3. Connected to X2. That is, the other terminal 64 of the capacitor 61 is connected to the connection terminal X1, and the other terminal 65 of the capacitor 62 is connected to the connection terminal X2. Therefore, the same external voltage as that of the variable capacitor 31 is applied to the capacitor 61, and the same external voltage as that of the variable capacitor 32 is applied to the capacitor 62.

  Next, a basic circuit operation for detecting a change in capacitance according to the acceleration acting on the capacitance sensor 3 in the sensor device 1 having the above configuration will be described. FIG. 2 is a diagram showing applied voltages to both ends (connection terminals X1, X2) of the capacitance sensor 3, where (a) shows the voltage applied to the terminal X1, and (b) shows the voltage applied to the terminal X2. The voltage is shown. In the present embodiment, the above-described first switch group S10 and second switch group S20 are synchronized with each other to perform on / off switching control, so that as shown in FIG. For X2, one cycle Ts consists of four sections, a first section Ta, a second section Tb, a third section Tc, and a fourth section Td, and the polarities of the second section Tb and the fourth section Td are reversed. Apply the voltage.

  In the first section Ta, the sensor device 1 sets the applied voltage of the two terminals X1 and X2 of the capacitance sensor 3 to the reference voltage Vref. At this time, no charge accumulation occurs in the two variable capacitors 31 and 32 of the capacitance sensor 3. Further, no charge accumulation occurs in the capacitors 61 and 62 of the dummy element 6. In the first section Ta, the switches Sf1 and Sf2 of the CV conversion circuit 5 are closed to discharge the feedback capacitors Cf1 and Cf2. Further, the switches Sp1 and Sn1 of the sampling circuit 7 are turned on (closed state), and the switches Sp2 and Sn2 are turned off (open state). Thus, if the fully differential operational amplifier 20 has an offset voltage, charges corresponding to the offset voltage can be stored in the capacitors Cp1 and Cn1.

  When the sensor device 1 makes a transition from the first section Ta to the second section Tb, the sensor device 1 opens the switches Sf1 and Sf2 of the CV conversion circuit 5 and applies a voltage to be applied to one terminal X1 of the capacitance sensor 3. The reference voltage Vref is switched to the voltage VH, and the voltage applied to the other terminal X2 is switched from the reference voltage Vref to the voltage VL. FIG. 3 is a diagram illustrating charge transfer when transitioning from the first section Ta to the second section Tb. As shown in FIG. 3, when the transition is made from the first section Ta to the second section Tb, charges are accumulated in the variable capacitors 31 and 32, respectively. At this time, acceleration acts on the capacitance sensor 3, and the capacitance C of the variable capacitor 31 changes to C + ΔC, and the capacitance C of the variable capacitor 32 changes to C−ΔC. Assume. In this case, the charge Q = (C + ΔC) Vc is accumulated in the variable capacitor 31, and the charge Q = − (C−ΔC) Vc is accumulated in the variable capacitor 32. On the other hand, since the same voltage as that of the capacitance sensor 3 is applied to the dummy element 6, the charge Q = CVc is accumulated in the capacitor 61 and the charge Q = −CVc is accumulated in the capacitor 62. As a result, the charge Q = −ΔCVc is accumulated in the feedback capacitor Cf1 of the CV conversion circuit 5, and the charge Q = ΔCVc is accumulated in the feedback capacitor Cf2. At this time, however, the charge is absorbed or supplied by the input common mode feedback circuit 9. Then, the output signal Vop = −ΔCVc / Cf + Vref is output from the non-inverting output terminal of the fully differential operational amplifier 20, and the output signal Von = ΔCVc / Cf + Vref is output from the inverting output terminal. As a result, the charge Qp corresponding to the potential difference −ΔCVc / Cf between the output signal Vop and the potential Va = Vref of the output-side node Na is accumulated in the capacitor Cp1 of the sampling circuit 7. The capacitor Cn1 stores a charge Qn corresponding to the potential difference ΔCVc / Cf between the output signal Von and the potential Vb = Vref of the output-side node Nb.

  Next, when transitioning from the second section Tb to the third section Tc, the sensor device 1 first opens the switches Sp1 and Sn1 of the sampling circuit 7, and stores the charges accumulated in the capacitors Cp1 and Cn1. Thereafter, the voltage applied to both ends X1 and X2 of the capacitance sensor 3 is switched to the reference voltage Vref again. FIG. 4 is a diagram illustrating charge transfer when transitioning from the second section Tb to the third section Tc. When both ends X1 and X2 of the capacitance sensor 3 return to the reference voltage Vref again in the third section Tc, the charge movement for setting the charges accumulated in the two variable capacitors 31 and 32 in the second section Tb to zero. Happens. Similarly, in the dummy element 6, charge transfer for reducing the charge accumulated in the capacitors 61 and 62 to 0 occurs. As a result, as shown in FIG. 4, the feedback capacitors Cf1 and Cf2 each have a charge Q = 0, and the output signals Vop and Von of the fully differential operational amplifier 20 both become the reference voltage Vref. At this time, since the capacitors Cp1 and Cn1 of the sampling circuit 7 remain in the state where charges Qp and Qn are accumulated, the potentials Va and Vb of the nodes Na and Nb on the output side of the capacitors Cp1 and Cn1 are Va = ΔCVc / Cf + Vref, Vb = −ΔCVc / Cf + Vref.

  Next, when transitioning from the third section Tc to the fourth section Td, the sensor device 1 switches the voltage applied to one terminal X1 of the capacitance sensor 3 from the reference voltage Vref to the voltage VL, and the other terminal X2 Is switched from the reference voltage Vref to the voltage VH. FIG. 5 is a diagram showing the charge transfer when the transition is from the third section Tc to the fourth section Td. As shown in FIG. 5, when the transition is made from the third section Tc to the fourth section Td, charges are accumulated again in the variable capacitors 31 and 32, respectively. That is, the charge Q = − (C + ΔC) Vc is accumulated in the variable capacitor 31, and the charge Q = (C−ΔC) Vc is accumulated in the variable capacitor 32. On the other hand, since the same voltage as that of the capacitance sensor 3 is applied to the dummy element 6, the charge Q = −CVc is accumulated in the capacitor 61 and the charge Q = CVc is accumulated in the capacitor 62. As a result, the charge Q = ΔCVc is accumulated in the feedback capacitor Cf1 of the CV conversion circuit 5, and the charge Q = −ΔCVc is accumulated in the feedback capacitor Cf2, so that the non-inverted output of the fully differential operational amplifier 20 is obtained. An output signal Vop = ΔCVc / Cf + Vref is output from the terminal, and an output signal Von = −ΔCVc / Cf + Vref is output from the inverted output terminal. At this time, the capacitors Cp1 and Cn1 of the sampling circuit 7 remain in a state where charges Qp and Qn are accumulated. Therefore, the potential Va of the node Na on the output side of the capacitor Cp1 is Va = 2ΔCVc / Cf + Vref, and the potential Vb of the node Nb on the output side of the capacitor Cn1 is Vb = −2ΔCVc / Cf + Vref. Therefore, the potentials Va and Vb on the output side of the capacitors Cp1 and Cn1 in the fourth section Td correspond to the capacitance displacement ΔC of the capacitance sensor 3 as compared with the potentials Va and Vb in the third section Tc. The potential becomes a component amplified twice.

  The sensor device 1 samples the output signals Vop and Von output from the CV conversion circuit 5 in the fourth section Td with the capacitors Cp1 and Cn1 of the sampling circuit 7, and then closes the switches Sp2 and Sn2 to thereby sample the sampling circuit 7 Output signals Voutp and Voutn. At this time, charges are accumulated in the capacitors Cp2 and Cn2 by closing the switches Sp2 and Sn2, so that the output signals Voutp and Voutn have potentials corresponding to the charge accumulation amounts of the capacitors Cp2 and Cn2. Therefore, by repeating the operation from the first section Ta to the fourth section Td a plurality of times while the electric charges are accumulated in the capacitors Cp2 and Cn2, the output signals Voutp and Voutn are gradually changed in the fourth section Td described above. It approaches the values of the potentials Va and Vb of the output side nodes Na and Nb. The differential output (Voutp−Voutn) output from the sampling circuit 7 after repeating the operations of the first section Ta to the fourth section Td a plurality of times is 4ΔCVc / Cf, and thus acts on the capacitance sensor 3. Capacitance change according to the acceleration being detected can be detected. Such a differential output (Voutp−Voutn) is a signal in which the offset voltage of the fully differential operational amplifier 20 is canceled.

  Next, in the sensor device 1 as described above, an operation when the external voltage applied to the capacitance sensor 3 is not stable at a constant value and noise components such as ripples are included will be described. FIG. 6 is a diagram illustrating charge transfer when the voltage VH supplied from the external power source includes the noise component Vz and transitions from the first section Ta to the second section Tb. In this case, VH = Vref + Vc + Vz, and when the transition is made from the first section Ta to the second section Tb, the noise component Vz acts on the variable capacitor 31, so that the charge Q = (C + ΔC) (Vc + Vz) is accumulated. Since no noise component Vz acts on 32, charge Q = − (C−ΔC) Vc is accumulated. As a result, charge transfer of the charge amount Q1 occurs from the sensor unit 2. Here, the amount of charge Q1 moving from the sensor unit 2 is Q1 = (2ΔCVc + CVz + ΔCVz).

  On the other hand, since the same voltage as that of the capacitance sensor 3 is applied to the dummy element 6, the noise component Vz acts on the capacitor 61 and the charge Q = C (Vc + Vz) is accumulated, and the noise component Vz is accumulated on the capacitor 62. The charge Q = −CVc is accumulated without acting. As a result, charge transfer of the charge amount Q2 occurs from the dummy element 6. Here, the amount of charge Q2 moving from the dummy element 6 is Q2 = CVz. The charge amount Q2 cancels the charge amount component of CVz contained in the charge amount Q1 moving from the sensor unit 2. This is because the fully differential operational amplifier 20 operates to amplify the input difference. Therefore, the charge Q = −ΔC (2Vc + Vz) / 2 is accumulated in the feedback capacitor Cf1 of the CV conversion circuit 5, and the charge Q = ΔC (2Vc + Vz) / 2 is accumulated in the feedback capacitor Cf2. The output signal Vop = −ΔC (2Vc + Vz) / 2Cf + Vref is output from the non-inverting output terminal of the fully differential operational amplifier 20, and the output signal Von = ΔC (2Vc + Vz) / 2Cf + Vref is output from the inverting output terminal.

  Here, paying attention to the noise component included in the output signals Vop and Von, although the term (ΔCVz) of the product of the change amount ΔC of the capacitance C and the noise component Vz is included, the movable electrode 13 moves. The product term (CVz) of the initial electrostatic capacitance C and the noise component Vz in a state of not being included is not included. That is, in this embodiment, a small noise component (ΔCVz) that does not significantly affect the measurement value is output while being included in the output signals Vop and Von, but has a large influence on the measurement value. A large value noise component (CVz) that causes an error can be canceled and output from the output signals Vop and Von. Therefore, even when the external voltage VH supplied from the external power source includes the noise component Vz, the noise component Vz can be reduced and output.

  In the above, the case where the noise component Vz is included in the voltage VH is illustrated, but the same applies to the case where the noise component Vz is included in the voltage VL. When the noise component Vz is included in the reference voltage Vref, it is possible to output the output signals Vop and Von in which the noise component Vz is completely canceled.

  In the present embodiment, as described above, the line L2 is wired so as to reach the sensor unit 2 in parallel with the line L1 from the dummy element 6, so that when disturbance noise acts on the line L1, the line L2 In the same way, disturbance noise is applied. Accordingly, since such disturbance noise is input as common mode noise to the fully differential operational amplifier 20, the disturbance noise can be canceled by the differential output (Vop, Von) of the fully differential operational amplifier 20. Therefore, the sensor device 1 is a device that can remove not only noise included in the external voltage but also disturbance noise acting on the lines L1 and L2.

  As described above, the sensor device 1 of the present embodiment includes the fixed capacitors 61 and 62 having substantially the same capacitance as the variable capacitors 31 and 32 of the capacitance sensor 3, and the first fixed capacitor. The other terminal 64 of 61 is connected to the connection terminal X1 of the external voltages VH, Vref, and VL applied to the first variable capacitor 31, and the other terminal 65 of the second fixed capacitor 62 is connected to the second The external voltage VH, Vref, VL applied to the variable capacitor 32 is connected to the connection terminal X2. Therefore, even if the external voltage VH, VL applied to the capacitance sensor 3 includes the noise component Vz, it is also applied to the fixed capacitors 61, 62, so the variable capacitor 31 , 32 can cancel the noise term (CVz) formed by the product of the capacitance C and the noise component Vz, and output with reduced noise components can be performed. When the external voltage Vref includes a noise component Vz, the output can be performed with the noise component completely removed.

  In addition, the signal processing circuit 4 of the sensor device 1 switches the external voltage applied to the variable capacitor 31 to the first switch group S10 for switching the voltages VH and VL whose polarities are inverted at a predetermined cycle around the reference voltage Vref. And the voltages VL, VH whose polarities are inverted at a predetermined cycle centered on the reference voltage Vref so that the external voltage applied to the variable capacitor 32 has a reverse polarity in relation to the external voltage applied to the variable capacitor 31. And the other terminal 64 of the fixed capacitor 61 is connected between the variable capacitor 31 and the first switch group S10, and the fixed capacitor 62 has a second switch group S20. The other terminal 65 is connected between the variable capacitor 32 and the second switch group S20. Therefore, even if any of the external voltages VH and VL applied to the capacitance sensor 3 includes the noise component Vz, it is possible to perform output with the noise component reduced.

  Furthermore, the sensor device 1 of the present embodiment has a configuration in which the fixed capacitors 61 and 62 included in the dummy element 6 are formed on a semiconductor chip different from the sensor unit 2. Therefore, it is possible to form the dummy element 6 that reduces the noise component with a relatively small size without increasing the size of the sensor unit 2 having the MEMS structure, and there is an advantage that the sensor device 1 can be reduced in size.

(Modification)
As mentioned above, although several embodiment regarding this invention was described, this invention is not limited to the thing of the content mentioned above, A various modification is applicable. Hereinafter, some modified examples will be described.

  First, another circuit configuration will be described. FIG. 7 is a diagram illustrating an example of another circuit configuration of the sensor device 1. The sensor unit 2 in this circuit configuration includes three capacitance sensors 3a, 3b, and 3c, and these three capacitance sensors 3a, 3b, and 3c are in three axial directions X, Y, and Z that are orthogonal to each other. It is the structure which detects an acceleration. A control circuit (not shown) controls on / off of the switch groups S10 to S60 provided at both ends of the three capacitance sensors 3a, 3b, and 3c, so that one of the three capacitance sensors 3a, 3b, and 3c is controlled. One is shifted to the measurement mode in a time division manner, and external voltages VH, Vref, and VL supplied from an external power source are applied. A reference voltage Vref is applied to both ends of the capacitance sensor that is not in the measurement mode. Therefore, in this sensor device 1, the accelerations in the three axial directions X, Y, and Z orthogonal to each other can be measured in a time division manner. The dummy element 6 is provided with six capacitors 61, 62, 71, 72, 81, 82. The capacitors 61 and 62 have substantially the same capacitance as that of the X-axis variable capacitors CX1 and CX2, respectively, and one end of these capacitors 61 and 62 applies external voltages VH, Vref, and VL to the capacitance sensor 3a. Are connected to connection terminals X1 and X2. Capacitors 71 and 72 have substantially the same capacitance as the Y-axis variable capacitors CY1 and CY2, respectively, and one end of these capacitors 71 and 72 applies external voltages VH, Vref, and VL to the capacitance sensor 3b. Connected to the connection terminals Y1, Y2. Further, the capacitors 81 and 82 have substantially the same capacitance as each of the Z-axis variable capacitor CZ1 and the fixed capacitor CZ2, and one end of each of these capacitors 81 and 82 is connected to the external voltage VH, Vref, Connected to connection terminals Z1 and Z2 for applying VL. In such a circuit configuration, even if any of the three capacitance sensors 3a, 3b, 3c is in the measurement mode, if the external voltage VH, Vref, VL includes the noise component Vz, the noise There is an advantage that an output with reduced components can be performed.

  Secondly, in the above embodiment, the case where the external voltages VH, Vref, and VL are mainly supplied from the external power source and applied to the capacitance sensor as it is in the sensor device 1 is exemplified. However, the present invention is not limited to this, and a voltage control circuit such as a regulator or a buffer amplifier is separately provided in the sensor device 1, and a voltage supplied from an external power supply is input to the voltage control circuit and applied to the capacitance sensor. You may generate | occur | produce a voltage. That is, when the external power supply generates a voltage to be supplied to the sensor device 1 using a DC / DC converter or the like, the supply voltage includes a high-frequency noise component, and the noise component is It is difficult to remove with a voltage control circuit such as a regulator or buffer amplifier provided in the device 1. Therefore, even when the voltage generated by the voltage control circuit is applied to the capacitance sensor, the noise component included in the external voltage can be reduced by adopting the circuit configuration of the above embodiment. It is.

  Thirdly, in the above-described embodiment, the case where the sensor device 1 measures acceleration is illustrated, but the measurement target of the sensor device 1 is not necessarily limited to acceleration, for example, it measures angular velocity. It is also possible to measure pressure.

  Fourthly, in the above-described embodiment, the case where the output correlated double sampling circuit is adopted as the sampling circuit 7 is illustrated, but a sampling circuit having other configurations may be adopted.

  1: sensor device, 2: sensor unit, 3: capacitive sensor, 4: signal processing circuit, 5: CV conversion circuit, 11, 12: fixed electrode, 13: movable electrode, 31, 32: variable capacitor, 61 62: Fixed capacitor.

Claims (3)

A pair of fixed electrodes is provided across a movable electrode movable in a predetermined direction, and a capacitance sensor is configured by first and second variable capacitors formed between each fixed electrode and the movable electrode, By applying an external voltage having a different polarity supplied from an external power source to the pair of fixed electrodes, a signal corresponding to the amount of charge accumulated in each of the first and second variable capacitance capacitors is transmitted. A sensor unit that outputs from a movable electrode;
A signal processing circuit for processing a signal from the sensor unit;
A sensor device comprising:
The signal processing circuit includes:
A first fixed capacitor having substantially the same capacitance as the first variable capacitor;
A second fixed capacitor having substantially the same capacitance as the second variable capacitor;
An output signal output from the sensor unit is input to one input terminal, and a fully-differential operational amplifier is connected to the other input terminal and a common terminal to which the first and second fixed capacitors are connected to each other. A CV conversion circuit for CV converting the output signal output from the sensor unit;
With
The other terminal different from the common terminal in the first fixed capacitor is connected to a connection terminal of an external voltage applied to the first variable capacitor, and the common terminal in the second fixed capacitor A sensor device, wherein the other terminal different from the above is connected to a connection terminal of an external voltage applied to the second variable capacitor. The signal processing circuit includes:
A first switch group for reversing the polarity of an external voltage applied to the first variable capacitor at a predetermined cycle centered on a reference voltage;
A second inversion of polarity at a predetermined cycle centered on a reference voltage so that the external voltage applied to the second variable capacitor has a reverse polarity in relation to the external voltage applied to the first variable capacitor. A group of switches,
Further comprising
The other terminal different from the common terminal in the first fixed capacitor is connected between the first variable capacitor and the first switch group, and the common in the second fixed capacitor. 2. The sensor device according to claim 1, wherein the other terminal different from the terminal is connected between the second variable capacitor and the second switch group.   The sensor device according to claim 1, wherein the first and second fixed capacitors are formed on a semiconductor chip different from the sensor unit.

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