Acceleration sensor

Provided is an acceleration sensor capable of realizing a simultaneous operation method of signal detection and servo control in place of a time-division processing method, by an MEMS process in which a manufacturing variation is large.The acceleration sensor is an MEMS capacitive acceleration sensor and has capacitive elements for signal detection and capacitive elements for servo control different from the capacitive elements for the signal detection. A voltage to generate force in a direction reverse to a detection signal of acceleration by the capacitive elements for the signal detection is applied to the capacitive elements for the servo control. Further, the acceleration sensor includes a variable capacity unit compensating for a mismatch of capacity values of the capacitive elements for the servo control at an ASIC side, detects a leak signal due to the mismatch of the capacity values in an ASIC, controls a capacity value of the variable capacity unit, on the basis of a detection result, compensates for an influence of the mismatch of the capacity values, and executes a normal signal detection/servo control simultaneous operation.

TECHNICAL FIELD

The present invention relates to an acceleration sensor and particularly, to a micro electromechanical systems (MEMS) capacitive acceleration sensor.

BACKGROUND ART

A sensor for a seismic reflection survey to explore for oil or natural gas excites an artificial earthquake and grasps reflected waves of seismic waves thereof reflected on a stratum as acceleration, after multiple sensors are scattered and disposed to become a predetermined two-dimensional array, on aground surface of the stratum in which resources are predicted to be buried. The sensor for the seismic reflection survey is used to analyze acceleration data received all at once by a group of sensors disposed two-dimensionally, check a state of the stratum, and determine presence or absence of the resources such as the oil and the natural gas. An acceleration sensor for the sensor for the seismic reflection survey is requested to realize both a low noise characteristic enabling detection of an acceleration signal having an extraordinarily low noise as compared with sensors of other fields and low consumption power necessary for controlling multiple sensors at the same time.

An acceleration sensor manufactured using MEMS technology can be miniaturized dramatically as compared with the conventional acceleration sensor and is expected as a solution to realize the above request.

PTL 1 and PTL 2 disclose an MEMS capacitive acceleration sensor that has a structure in which an MEMS capacitive element is used commonly for signal detection and for application of servo force to generate force in a direction reverse to a detection signal and an area is reduced. In this structure, a method of performing signal detection and servo control alternately is used in time-division processing to use the MEMS capacitive element commonly. In the time-division processing, a method of including resetting between the signal detection and the servo control is also used.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

In the time-division processing method described in PTL 1 or PTL 2, there are the following problems.

(1) When the time-division processing is executed, an internal operation speed doubles (the method of performing the signal detection and the servo control alternately) or quadruples (the method of including the resetting between the signal detection and the servo control) to maintain a signal processing band. For this reason, consumption power of an analog circuit such as an amplifier, a filter, and an A/D converter, a logic circuit, and a servo control unit (D/A converter) may double or quadruple.

(2) When time-division switching is performed, a sampling noise (kT/C noise; k is a Boltzmann constant) is generated by a switching operation for switching and a noise density increases. This is a theoretical phenomenon and is not avoided. As a result, a noise of sensor increases.

(3) When the time-division processing is executed, it is necessary to increase a servo voltage or increase an MEMS capacity value for servo to secure effective servo force. In the case of the former, a design of a high-voltage low-noise circuit is difficult or is originally impossible due to a breakdown voltage of a MOS transistor of a semiconductor process. In the case of the latter, a merit of reducing an area by sharing the MEMS capacity for the detection and the servo by the time-division processing may be lost.

An object of the present invention is to resolve the problems of the time-division processing method described above and provide an acceleration sensor for realizing a simultaneous operation method of signal detection and servo control in place of the time-division processing method, thereby realizing a novel acceleration sensor of low consumption power and a low noise. Particularly, emphasis is placed on compensating for a mismatch of capacity values between MEMS capacitive elements, such that the acceleration sensor can be realized by an MEMS process in which a manufacturing variation is large. As a result, effects of (1) applicability of a low-cost MEMS process using a cheap manufacturing device and (2) improvement of a yield of an MEMS sensor can be expected and productivity can be improved.

The above and other object and novel characteristics of the present invention will be apparent from the description of the present specification and the accompanying drawings.

Solution to Problem

To resolve the above problems, in the present invention, an MEMS capacitive acceleration sensor is configured to include a first MEMS capacitor pair for signal detection; a second MEMS capacitor pair for servo control in which one electrode of each capacitor is connected to one electrode of each capacitor in the first capacitor pair and to which a servo voltage to generate force in a direction reverse to a detection signal of acceleration by the first MEMS capacitor pair is applied; a charge amplifier which is connected to electrodes of the first MEMS capacitor pair and the second MEMS capacitor pair connected to each other and forming one weight and converts a charge change on the weight into a voltage change; an A/D converter which digitizes a voltage change signal of an output of the charge amplifier; a 1-bit quantizer which quantizes a servo value to generate force in a direction reverse to displacement of the weight by the acceleration, generated from an output of the A/D converter, with 1 bit; a 1-bit D/A converter which converts an output of the 1-bit quantizer into an analog servo voltage and applies the analog servo voltage to the second MEMS capacitor pair; a correlation detection unit which correlates the output of the A/D converter and the output of the 1-bit quantizer and outputs a signal proportional to a mismatch ΔC of capacity values in the second MEMS capacitor pair; a control unit which outputs a capacity control value to cancel an influence by the mismatch ΔC of the capacity values on an input node of the charge amplifier, on the basis of the output of the correlation detection unit; and a variable capacity unit which is inserted between an output node of a driver outputting the output of the 1-bit quantizer at voltage amplitude more suppressed than amplitude of the servo voltage and the input node of the charge amplifier and of which a capacity is controlled according to the capacity control value of the output of the control unit.

In addition, to resolve the above problems, in the present invention, an MEMS capacitive acceleration sensor is configured to include a first MEMS capacitor pair for signal detection; a second MEMS capacitor pair for servo control in which one electrode of each capacitor is connected to one electrode of each capacitor in the first capacitor pair to configure a positive-side weight (movable electrode portion) and to which a servo voltage to generate force in a direction reverse to a detection signal of acceleration by the first MEMS capacitor pair is applied; a third MEMS capacitor pair for signal detection; a fourth MEMS capacitor pair for servo control in which one electrode of each capacitor is connected to one electrode of each capacitor in the third capacitor pair to configure a negative-side weight (movable electrode portion) and to which a servo voltage to generate force in a direction reverse to a detection signal of acceleration by the third MEMS capacitor pair is applied; a first charge amplifier which is connected to the positive-side weight and converts a charge change on the positive-side weight into a voltage change; a second charge amplifier which is connected to the negative-side weight and converts a charge change on the negative-side weight into a voltage change; an A/D converter which digitizes voltage change signals of differential outputs of the first and second charge amplifiers; a 1-bit quantizer which quantizes a servo value to generate force in a direction reverse to displacement of the weight by the acceleration, generated from an output of the A/D converter, with 1 bit; a 1-bit D/A converter which converts an output of the 1-bit quantizer into an analog servo voltage and applies the analog servo voltage to the second MEMS capacitor pair and the fourth MEMS capacitor pair; a correlation detection unit which correlates the output of the A/D converter and the output of the 1-bit quantizer and outputs a signal proportional to a mismatch ΔC of capacity values in the second MEMS capacitor pair and the fourth MEMS capacitor pair; a control unit which outputs differential capacity control values to cancel an influence by the mismatch ΔC of the capacity values on input nodes of the first and second charge amplifiers, on the basis of the output of the correlation detection unit; a first variable capacity unit which is inserted between an output node of a driver outputting the output of the 1-bit quantizer at voltage amplitude more suppressed than amplitude of the servo voltage and the input node of the first charge amplifier and of which a capacity is controlled according to the differential capacity control values of the output of the control unit; and a second variable capacity unit which is inserted between the output node of the driver outputting the output of the 1-bit quantizer at the voltage amplitude more suppressed than the amplitude of the servo voltage and the input node of the second charge amplifier and of which a capacity is controlled according to the differential capacity control values of the output of the control unit.

Advantageous Effects of Invention

According to a representative effect of the invention disclosed in the present application, an acceleration sensor for realizing a simultaneous operation method of signal detection and servo control in place of a time-division processing method can be provided by an MEMS process in which a manufacturing variation is large.

DESCRIPTION OF EMBODIMENTS

In embodiments described below, the invention will be described in a plurality of sections or embodiments when required as a matter of convenience. However, these sections or embodiments are not irrelevant to each other unless otherwise stated and one relates to the entire or part of the other as a modification, details, or a supplementary explanation thereof. In addition, in the embodiments described below, when referring to the number of elements (including the number of pieces, values, amounts, ranges, and the like), the number of the elements is not limited to a specific number unless otherwise stated or except for the case in which the number is apparently limited to a specific number in principle and the number larger or smaller than the specified number is also applicable.

In addition, in the embodiments described below, it goes without saying that components (including element steps and the like) are not always indispensable unless otherwise stated or except for the case in which the components are apparently indispensable in principle. Similarly, in the embodiments described below, when shapes of the components, a positional relation thereof, and the like are mentioned, the substantially approximate and similar shapes and the like are included therein unless otherwise stated or except for the case in which it is conceivable that they are not apparently excluded in principle. The same is applicable to the numerical values and the ranges described above.

Hereinafter, the individual embodiments based on the outlines of the embodiments will be described in detail on the basis of the drawings. Throughout the drawings illustrating the embodiments, the same members are denoted with the same reference numerals or associated reference numerals in principle and repetitive description thereof is omitted. In addition, in the embodiments described below, description of the same or equivalent portions is not repeated in principle, except for when necessary in particular.

First Embodiment

An acceleration sensor according to a first embodiment will be described usingFIG. 1.FIG. 1is a diagram illustrating an example of a configuration of the acceleration sensor.

In the acceleration sensor, a mechanical portion is configured using micro electro mechanical systems (MEMS) and a circuit portion is configured using an application specific integrated circuit (ASIC). The acceleration sensor is not limited thereto. For example, the acceleration sensor is used for an MEMS capacitive acceleration sensor detecting vibration acceleration extremely smaller than gravity, as a sensor for a seismic reflection survey to explore for oil or natural gas.

First, a configuration will be described. The MEMS includes a capacitor pair101aand101bfor signal detection and a capacitor pair102aand102bfor servo control. One electrode of each of the four capacitors is connected mechanically and electrically and forms one weight (movable electrode portion)100. The weight is connected to an inversion input terminal of an operational amplifier103aconfiguring a charge amplifier103. The charge amplifier103is configured using the operational amplifier103a, a feedback capacitor103b, and a feedback resistor103c. A bias voltage VBis connected to a non-inversion input terminal of the operational amplifier103a. VBmay be a ground potential or may not be the ground potential. An output (that is, an output of the operational amplifier103a) of the charge amplifier103is input to an amplifier104, an output of the amplifier104is input to an analog filter105, and an output of the analog filter105is input to an A/D converter106. An output of the A/D converter106is input to a demodulator107. A modulation clock is also input to the demodulator107. An output of the demodulator107is input to a servo control unit108, an output of the servo control unit108is input to a 1-bit quantizer109, and an output of the 1-bit quantizer109is input to a 1-bit D/A converter110. Differential outputs of the 1-bit D/A converter110are connected to electrodes (electrodes not configuring the weight) of the capacitor pair102aand102bfor the servo control, respectively. In addition, the modulation clock and an inversion clock thereof are input to drivers112aand112bfor the capacitor pair for the signal detection, respectively, and outputs of the drivers are connected to electrodes (electrodes not configuring the weight) of the capacitor pair101aand101bfor the signal detection, respectively. The output of the A/D converter106is input to a correlation detection unit113and the output of the 1-bit quantizer109is also input to the correlation detection unit113. An output of the correlation detection unit113is input to a control unit114and an output of the control unit114functions as a control signal and controls a capacity value of a variable capacity unit115. The output of the 1-bit quantizer109is also input to a driver116for a variable capacity and an output of the driver116for the variable capacity is connected to one terminal of the variable capacity unit115. The other terminal of the variable capacity unit115is connected to the weight (movable electrode portion)100(that is, the inversion input terminal of the operational amplifier103a). In addition, the output of the 1-bit quantizer109is input to a digital low-pass filter111and an output of the digital low-pass filter111becomes an output of the acceleration sensor. The ASIC includes elements from the charge amplifier103to the 1-bit D/A converter110. The variable capacity unit115is also mounted in the ASIC.

Next, an operation will be described. If acceleration is applied from the outside to the acceleration sensor, inertial force is generated in the weight100to be moved. As a result, the weight100is displaced. Since the weight100is also the electrodes of the capacitor pair101aand101bfor the signal detection, capacity values of the capacitor pair are changed by the displacement. For example, inFIG. 1, if the acceleration is applied in a rightward direction and the weight100is displaced in the rightward direction, a distance between the movable electrode (that is, the weight100) of the capacitor101bfor the signal detection and a fixed electrode thereof decreases and a capacity change value of +ΔCDis obtained and a distance between the movable electrode (that is, the weight100) of the capacitor101afor the signal detection and a fixed electrode thereof increases and a capacity change value of −ΔCDis obtained. An application direction and an amount of the acceleration can be detected on the basis of the capacity change values (+ΔCDand −ΔCD) in the capacitor pair101aand101bfor the signal detection. A pair structure of the capacitor pair101aand101bfor the detection is a structure for various known objects not described in detail, for example, for canceling in-phase components of the capacity values. For the convenience of description, parallel plate capacitors are used in the MEMS configuration in the above description andFIG. 1. However, the same mechanism is realized in other types of capacitors. Therefore, the present invention is not limited to the MEMS of the parallel plate capacitor type.

A modulation clock voltage and an inversion clock voltage thereof are applied to the capacitor pair101aand101bfor the detection via the drivers112aand112bfor the capacitor pair for the signal detection, respectively. As a result, the capacity change of ΔCDis converted into a charge change. The charge change is converted into a voltage change by the charge amplifier103of an initial step in the ASIC. The charge amplifier103has a configuration of a so-called operational amplifier inversion amplifier of a capacitive type and input capacitors are the capacitor pair101aand101bfor the signal detection at the MEMS side and a feedback capacitor is the feedback capacitor103bat the ASIC side. However, the feedback resistor103chaving a high resistance value is inserted into a feedback path in parallel. This is to secure a direct-current feed path to compensate for an input leak current of the operational amplifier103a. Meanwhile, measures using a reset switch in a portion of the feedback resistor103care known conventionally. However, in this case, there is a problem in that a noise density of a sampling noise by the reset switch is high. Since a thermal noise by the feedback resistor103chaving the high resistance value used in this method is sufficiently suppressed in the vicinity of a desired frequency (that is, a frequency of the modulation clock), by a low-pass filter characteristic by the feedback resistor103cand the feedback capacitor103b, there is no problem.

A signal converted into a voltage by the charge amplifier103is amplified by the amplifier104, a noise or an unnecessary signal component thereof is suppressed by the analog filter105, and the signal is converted into a digital value by the A/D converter106. The demodulator107is a digital multiplier of two inputs and multiplies the output of the A/D converter106and the modulation clock and performs synchronous detection for the modulation clock. As a result, a value proportional to the displacement of the weight100is obtained in an output of the demodulator107. A series of modulation/demodulation processing is equivalent to a so-called “chopper system”. As a result, an influence by a loud 1/f noise generated by the charge amplifier103, the amplifier104, the analog filter105, and the A/D converter106can be avoided. The servo control unit108is a circuit that receives the displacement value of the weight100demodulated by the demodulator107, determines a servo value to generate force in a direction reverse to a detection signal, on the basis of the value, and outputs the servo value to the 1-bit quantizer109. Particularly, a digital integration operation may be included in signal processing in the servo control unit108and control may be performed such that the displacement of the weight100becomes zero.

As a result, because the capacity value changes of the capacitor pair101aand101bfor the signal detection are minimized, linearity of the acceleration sensor can be increased. (That is, because the servo force is applied to almost cancel the displacement of the weight and the displacement of the weight remains in an extremely narrow range, detection is enabled in a region where a relation of the acceleration and the displacement shows the linearity).

In addition, a differentiation (or difference) operation may be included in the signal processing in the servo control unit108, phase compensation may be performed, and a servo control loop may be stabilized. At this time, logic of general PID control can be applied.

The 1-bit quantizer109quantizes the servo value determined and output by the servo control unit108with 1 bit. For example, if an input of the 1-bit quantizer109is 0 or more, the 1-bit quantizer109outputs +1 and if the input is negative, the 1-bit quantizer109outputs −1. The 1-bit D/A converter110receives a digital value (±1) of 1 bit quantized by the 1-bit quantizer109, converts the digital value into an analog voltage (for example, ±5 V or 10 V/0 V), and applies the analog voltage to the fixed electrodes of the capacitor pair102aand102bfor the servo control. As a result, electrostatic force in a direction reverse to the detected acceleration signal can be applied to the weight100. In a steady state, net force acting on the weight100and the displacement of the weight100become almost zero. As such, the 1-bit quantizer109is inserted, so that the subsequent D/A converter can be configured as the 1-bit D/A converter110. Since the 1-bit D/A converter is easily mounted in terms of a circuit, the 1-bit D/A converter is advantageous to low consumption power. In addition, a capacity unit for servo control can be simplified. In the output of the 1-bit quantizer109, because a high frequency component (that is, a quantization error noise-shaped (diffused) to a high frequency side by sigma-delta control of a servo loop) is suppressed by the digital low-pass filter111, a final output of the acceleration sensor can be made to have a low noise. By the above configuration, in this acceleration sensor, the signal detection and the servo control are performed at the same time.

In the present invention, an output signal of the A/D converter106and an output signal of the 1-bit quantizer109are correlated by the correlation detection unit113. This is to detect a mismatch ΔC of capacity values in the capacitor pair102aand102bfor the servo control. If ΔC exists, a servo leak signal proportional to ΔC is generated in the output of the charge amplifier103, the servo leak signal is amplified hereinafter by the amplifier104, a high frequency component of the servo leak signal is suppressed to a certain degree by the analog filter105, and the servo leak signal is converted into a digital value by the A/D converter106. The same waveform as a waveform of an output signal of the 1-bit quantizer109and an inversion waveform thereof are differentially applied to the capacitor pair102aand102bfor the servo control via the 1-bit D/A converter110. For this reason, the servo leak signal has almost the same waveform as the waveform of the output signal of the 1-bit quantizer109. That is, the servo leak signal is proportional to ΔC and has the same waveform as the waveform of the output signal of the 1-bit quantizer109. Focusing on the above fact, the servo leak signal included in the output of the A/D converter106can be detected. To do this, the output signal of the A/D converter106and the output signal of the 1-bit quantizer109are correlated in the correlation detection unit113, as described above. As a result, a direct-current or low frequency signal proportional to ΔC can be included in the output of the correlation detection unit113. Therefore, the subsequent control unit114executes a digital integration operation or a phase compensation operation according to necessity, with respect to the output signal of the correlation detection unit113, and determines and outputs a capacity control value. The capacity control value is a digital value. Thereby, a capacity value of the variable capacity unit115is controlled. An object of the variable capacity unit115is to cancel an influence by ΔC on an input node (that is, an inversion input terminal node of the operational amplifier103a) of the charge amplifier103. To do this, it is considered that a capacitor of the same capacity value as the mismatch ΔC of the capacitor pair102aand102bfor the servo control is inserted between the node and one output of the 1-bit D/A converter110. However, this method has two problems.

First, the output of the 1-bit D/A converter110has high voltage amplitude of about 10 V. For this reason, when a voltage amplitude level is input to the variable capacity unit115mounted in the ASIC as it is, a waveform may be clipped by an electrostatic discharge damage protection element (ESD element) connected to the input terminal of the ASIC to do so. Finally, a desired voltage is not transmitted to the variable capacity unit115. In this acceleration sensor, because a semiconductor process of a low voltage is applied to the ASIC for low consumption power, an input voltage range in which the clipping is not generated is smaller than output amplitude of the 1-bit D/A converter110. For this reason, the waveform clipping is generated.

Second, in the case of the above method, it may be necessary to adjust the capacity values to the same capacity values with extremely high precision. For example, in the case in which ΔC is the mismatch of about 1% and it is necessary to cancel ΔC with precision of 0.01%, if it is assumed that the capacity values of the capacitor pair102aand102bfor the servo control are 5 pF, a variable capacity unit capable of being controlled with 5 pF×0.01/100=0.5 fF increments is necessary. Capacity value increment precision is unrealistic in view of processing dimension precision of a semiconductor process or a parasitic capacity value.

Therefore, in the present invention, a lower end of the variable capacity unit115is connected to the output of the driver116for the variable capacity, not the output of the 1-bit D/A converter110. The output of the 1-bit D/A converter110has high voltage amplitude (for example, ±5 V or 10 V/0 V) to generate sufficient electrostatic force. However, the driver116for the variable capacity is inserted, so that output amplitude of the output signal of the driver116for the variable capacity can be decreased while the output signal is made to have the same waveform (that is, the same waveform as the waveform of the output signal of the 1-bit quantizer109) as the waveform of the output signal of the 1-bit D/A converter110. For example, if the output voltage amplitude of the 1-bit D/A converter110is set to ±V and the output amplitude of the driver116for the variable capacity is set to V/α (α>1), a capacity value necessary for canceling error charges generated by ΔC is αΔC and can be multiplied by α. That is, the capacity value increment precision can also be multiplied by α. For example, α=20 is set, so that the capacity value increment precision needed in the previous example increases from 0.5 fF to 10 fF. It can be considered that the variable capacity unit115of the capacity value increment precision of the above degree can be sufficiently realized by a realistic semiconductor process.

Even when the servo leak signal is weak, the servo leak signal disturbs the servo loop remarkably and causes a noise increase or an unjust oscillation. For this reason, it is absolutely necessary to cancel the influence of the mismatch ΔC in the capacitor pair102aand102bfor the servo control with high precision, as described above. An integration operation is included in the control unit114, so that negative feedback control can be executed such that ΔC and the servo leak signal become zero. A negative feedback control loop is hereinafter called an “MEMS capacity compensation loop”.

In addition, the acceleration sensor can be configured by using a 1.5-bit (three values) quantizer or a multi-bit quantizer in place of the 1-bit quantizer109according to this embodiment and using a 1.5-bit (three values) D/A converter or a multi-bit D/A converter in place of the 1-bit D/A converter110.

Effect of First Embodiment

As described above, according to the acceleration sensor in the first embodiment, the signal detection and the servo control are performed at the same time and the servo leak signal according to the mismatch of the capacity values of the MEMS capacitive elements can be canceled with high precision. Therefore, the noise increase or the unjust oscillation according to the servo leak signal can be prevented. For this reason, even when an MEMS process in which a manufacturing variation is large is used, a capacitive MEMS acceleration sensor of a low noise and low consumption power can be realized. The servo leak signal can be canceled by a digital operation subject to be easily mounted and a dedicated A/D converter is not necessary.

Second Embodiment

FIG. 2illustrates a second embodiment of the present invention. Since a basic portion of this embodiment is the same as that of the first embodiment, a difference with the first embodiment will be described. In this embodiment, a correlation detection unit113is connected to an output of a charge amplifier103, not an output of an A/D converter106, as in the first embodiment. For this reason, the correlation detection unit113correlates a servo leak signal included in the output of the charge amplifier103with an output signal of a 1-bit quantizer109and detects the servo leak signal. Differently from the first embodiment of the present invention, the servo leak signal of the output of the charge amplifier103is detected directly not via an amplifier104, an analog filter105, and the A/D converter106. Therefore, because there is no delay of the servo leak signal by each analog circuit block, an MEMS capacity compensation loop is easily stabilized and a parameter design of a control unit114is facilitated. Instead, a point that it is necessary to realize the correlation detection unit113with an analog circuit and a point that an A/D converter is necessary separately in the control unit114become demerits as compared with the first embodiment.

Effect of Second Embodiment

As described above, according to an acceleration sensor in the second embodiment, signal detection and servo control are performed at the same time and the servo leak signal according to a mismatch of capacity values of MEMS capacitive elements can be canceled with high precision. Therefore, a noise increase or an unjust oscillation according to the servo leak signal can be prevented. For this reason, even when an MEMS process in which a manufacturing variation is large is used, a capacitive MEMS acceleration sensor of a low noise and low consumption power can be realized. Since the servo leak signal of the output of the charge amplifier is directly detected, a stabilization design is easy.

Third Embodiment

FIG. 3illustrates a third embodiment of the present invention. Since a basic portion of this embodiment is the same as that of the first embodiment, a difference with the first embodiment will be described. In this embodiment, a correlation detection unit113is connected to an output of an amplifier104, not an output of an A/D converter106, as in the first embodiment. For this reason, the correlation detection unit113correlates a servo leak signal included in the output of the amplifier104with an output signal of a 1-bit quantizer109and detects the servo leak signal. Therefore, a delay amount of the servo leak signal is small as compared with the case of the first embodiment and a parameter design of a control unit114is facilitated. In addition, because the servo leak signal is amplified by gain of the amplifier104, a noise specification required in the correlation detection unit113or the control unit114can be alleviated as compared with the case of the second embodiment.

Effect of Third Embodiment

As described above, according to an acceleration sensor in the third embodiment, signal detection and servo control are performed at the same time and the servo leak signal according to a mismatch of capacity values of MEMS capacitive elements can be canceled with high precision. Therefore, a noise increase or an unjust oscillation according to the servo leak signal can be prevented. For this reason, even when an MEMS process in which a manufacturing variation is large is used, a capacitive MEMS acceleration sensor of a low noise and low consumption power can be realized.

Fourth Embodiment

FIG. 4illustrates a fourth embodiment of the present invention. Since a basic portion of this embodiment is the same as that of the first embodiment, a difference with the first embodiment will be described. In this embodiment, a delay unit417is inserted between a driver116for a variable capacity and a variable capacity unit115. An object of the delay unit417is to simulate a response delay due to a limitation of the driving capability of a 1-bit D/A converter110. The response delay of the 1-bit D/A converter110is determined by a time constant to be a product of output resistance thereof and capacity values of capacitors102aand102bfor servo control and becomes a primary low-pass filter characteristic. Therefore, the delay unit417is configured as a primary low-pass filter having almost the same time constant, so that an output waveform of the 1-bit D/A converter110can be simulated. As a result, cancellation of the servo leak signal disclosed in the first embodiment can be performed completely.

Effect of Fourth Embodiment

As described above, according to an acceleration sensor in the fourth embodiment, signal detection and servo control are performed at the same time and a servo leak signal according to a mismatch of capacity values of MEMS capacitive elements can be canceled with high precision. Therefore, a noise increase or an unjust oscillation according to the servo leak signal can be prevented. For this reason, even when an MEMS process in which a manufacturing variation is large is used, a capacitive MEMS acceleration sensor of a low noise and low consumption power can be realized. Since delay compensation of the 1-bit D/A converter is performed by the delay unit, the servo leak signal can be canceled with higher precision.

Fifth Embodiment

An acceleration sensor according to a fifth embodiment will be described usingFIG. 5.FIG. 5is a diagram illustrating an example of a configuration of the acceleration sensor. The fifth embodiment corresponds to the case in which the first embodiment of the present invention is applied to a “differential MEMS” configuration.

In the acceleration sensor, a mechanical portion is configured using micro electro mechanical systems (MEMS) and a circuit portion is configured using an application specific integrated circuit (ASIC). The acceleration sensor is not limited thereto. For example, the acceleration sensor is used for an MEMS capacitive acceleration sensor detecting vibration acceleration extremely smaller than gravity, as a sensor for a seismic reflection survey to explore for oil or natural gas.

First, a configuration will be described. The MEMS includes a capacitor pair501aand501bfor positive-side signal detection, a capacitor pair501cand501dfor negative-side signal detection, a capacitor pair502aand502bfor positive-side servo control, and a capacitor pair502cand502dfor negative-side servo control. One electrode of each of the four capacitors501a,501b,502a, and502bis connected mechanically and electrically and forms a positive-side weight (movable electrode portion)500a. In addition, one electrode of each of the four capacitors501c,501d,502c, and502dis connected mechanically and electrically and forms a negative-side weight (movable electrode portion)500b. The positive-side weight500ais connected to an inversion input terminal of an operational amplifier503aconfiguring a differential charge amplifier503. A positive side of the differential charge amplifier503is configured using the operational amplifier503a, a feedback capacitor503b, and a feedback resistor503c. A bias voltage VBis connected to a non-inversion input terminal of the operational amplifier503a. VBmay be a ground potential or may not be the ground potential. Likewise, a negative side of the differential charge amplifier503is configured using an operational amplifier503d, a feedback capacitor503e, and a feedback resistor503f.

Differential outputs (that is, an output of the operational amplifier503aand an output of the operational amplifier503d) of the differential charge amplifier503are input to a differential amplifier504, outputs of the differential amplifier504are input to a differential analog filter505, and outputs of the differential analog filter505are input to a differential A/D converter506. An output of the differential A/D converter506is input to a demodulator507. A modulation clock is also input to the demodulator507. An output of the demodulator507is input to a servo control unit508, an output of the servo control unit508is input to a 1-bit quantizer509, and an output of the 1-bit quantizer509is input to a 1-bit D/A converter510. A positive side of differential outputs of the 1-bit D/A converter510is connected to electrodes (electrodes not configuring the weight) of the capacitor502afor the positive-side servo control and the capacitor502cfor the negative-side servo control. Meanwhile, a negative side of the differential outputs of the 1-bit D/A converter510is connected to electrodes (electrodes not configuring the weight) of the capacitor502bfor the positive-side servo control and the capacitor502dfor the negative-side servo control. In addition, the modulation clock and an inversion clock thereof are input to drivers512aand512bfor the capacitor pair for the signal detection, respectively. An output of the driver512afor the capacitor pair for the signal detection is connected to electrodes (electrodes not configuring the weight) of the capacitor501afor the positive-side signal detection and the capacitor501dfor the negative-side signal detection and an output of the driver512bfor the capacitor pair for the signal detection is connected to electrodes (electrodes not configuring the weight) of the capacitor501bfor the positive-side signal detection and the capacitor501cfor the negative-side signal detection.

The output of the differential A/D converter506is input to a correlation detection unit513and the output of the 1-bit quantizer509is also input to the correlation detection unit513. An output of the correlation detection unit513is input to a control unit514and differential outputs of the control unit514function as control signals and control capacity values of variable capacity units515aand515b. The output of the 1-bit quantizer509is also input to a driver516for a variable capacity and an output of the driver516for the variable capacity is connected to one terminal of the variable capacity unit515aand one terminal of the variable capacity unit515b. The other terminal of the variable capacity unit515ais connected to the positive-side weight (movable electrode portion)500a(that is, the inversion input terminal of the operational amplifier503a) and the other terminal of the variable capacity unit515bis connected to the negative-side weight (movable electrode portion)500b(that is, the inversion input terminal of the operational amplifier503d). In addition, the output of the 1-bit quantizer509is input to a digital low-pass filter511and an output of the digital low-pass filter511becomes an output of the acceleration sensor. The ASIC includes elements from the differential charge amplifier503to the 1-bit D/A converter510. The variable capacity units515aand515bare also mounted in the ASIC.

Next, an operation will be described. If acceleration is applied from the outside to the acceleration sensor, inertial forces of the same direction and the same magnitude are generated in the weights500aand500bto be moved. As a result, the weights500aand500bare displaced by the same amount in the same direction. Since the weight500ais also the electrodes of the capacitor pair501aand501bfor the positive-side signal detection, capacity values of the capacitor pair are changed by the displacement. For example, inFIG. 5, if the acceleration is applied in a rightward direction and the weight500ais displaced in the rightward direction, a distance between the movable electrode (that is, the weight500a) of the capacitor501bfor the positive-side signal detection and a fixed electrode thereof decreases and a capacity change value of +ΔCDis obtained and a distance between the movable electrode (that is, the weight500a) of the capacitor501afor the positive-side signal detection and a fixed electrode thereof increases and a capacity change value of −αCDis obtained. An application direction and an amount of the acceleration can be detected on the basis of the capacity change values (+ΔCDand −ΔCD) in the capacitor pair501aand501bfor the positive-side signal detection.

Likewise, because the weight500bis displaced by the same amount in the rightward direction, a distance between the movable electrode (that is, the weight500b) of the capacitor501dfor the negative-side signal detection and a fixed electrode thereof decreases and a capacity change value of +ΔCDis obtained and a distance between the movable electrode (that is, the weight500b) of the capacitor501cfor the negative-side signal detection and a fixed electrode thereof increases and a capacity change value of −ΔCDis obtained. An application direction and an amount of the acceleration can be detected on the basis of the capacity change values (+ΔCDand −ΔCD) in the capacitor pair501cand501dfor the negative-side signal detection. A pair structure of the capacitor pair501aand501bfor the positive-side detection or a pair structure of the capacitor pair501cand501dfor the negative-side detection is a structure for various known objects not described in detail, for example, for canceling in-phase components of the capacity values. For the convenience of description, parallel plate capacitors are used in the MEMS configuration in the above description andFIG. 5. However, the same mechanism is realized in other types of capacitors. Therefore, the present invention is not limited to the MEMS of the parallel plate capacitor type.

A modulation clock voltage and an inversion clock voltage thereof are applied to the capacitor pair501aand501bfor the positive-side detection via the drivers512aand512bfor the capacitor pair for the signal detection, respectively. As a result, the capacity change of ΔCDin the capacitor pair501aand501bfor the positive-side detection is converted into a charge change. Meanwhile, the inversion clock voltage and the modulation clock voltage are applied to the capacitor pair501cand501dfor the negative-side detection via the drivers512band512afor the capacitor pair for the signal detection, respectively. That is, methods of applying the clock are opposite in the capacitor pair for the positive-side detection and the capacitor pair for the negative-side detection. As a result, the capacity change of ΔCDin the capacitor pair501cand501dfor the negative-side detection is converted into a charge change of a sign (the same magnitude) opposite to a sign of the charge change of the positive side. In other words, ΔCDis generated in phase with respect to a “differential MEMS” structure, by the application of the acceleration from the outside. However, because the applied clocks are “differential”, the charge changes generated by the positive-side MEMS and the negative-side MEMS become differential signals. The differential charge signal is converted into a differential voltage signal by the differential charge amplifier503of an initial step in the ASIC. The differential charge amplifier503has a configuration in which so-called operational amplifier inversion amplifiers of a capacitive type are disposed in bi-parallel. At the positive side, input capacitors are the capacitor pair501aand501bfor the positive-side signal detection at the MEMS side and a feedback capacitor is the feedback capacitor503bat the ASIC side. However, the feedback resistor503chaving a high resistance value is inserted into a feedback path in parallel. This is to secure a direct-current feed path to compensate for an input leak current of the operational amplifier503a. Meanwhile, measures using a reset switch in a portion of the feedback resistor503care known conventionally. However, in this case, there is a problem in that a noise density of a sampling noise by the reset switch is high. Since a thermal noise by the feedback resistor503chaving the high resistance value used in this method is sufficiently suppressed in the vicinity of a desired frequency (that is, a frequency of the modulation clock) by a low-pass filter characteristic by the feedback resistor503cand the feedback capacitor503b, there is no problem. Likewise, at the negative side, input capacitors are the capacitor pair501cand501dfor the negative-side signal detection at the MEMS side and a feedback capacitor is the feedback capacitor503eat the ASIC side. However, the feedback resistor503fhaving a high resistance value is inserted into a feedback path in parallel. The reason is as described above.

A signal converted into a differential voltage by the differential charge amplifier503is amplified by the differential amplifier504, a noise or an unnecessary signal component thereof is suppressed by the differential analog filter505, and the signal is converted into a digital value by the differential A/D converter506. The demodulator507is a digital multiplier of two inputs and multiplies the output of the A/D converter506and the modulation clock and performs synchronous detection for the modulation clock. As a result, values proportional to the displacements of the weights500aand500bare obtained in an output of the demodulator507. A series of modulation/demodulation processing is equivalent to a so-called “chopper system”. As a result, an influence by a loud 1/f noise generated by the differential charge amplifier503, the differential amplifier504, the differential analog filter505, and the differential A/D converter506can be avoided.

The servo control unit508is a circuit that receives the displacement values of the weights500aand500bdemodulated by the demodulator507, determines servo values to generate forces in directions reverse to detection signals, on the basis of the values, and outputs the servo values to the 1-bit quantizer509. Particularly, a digital integration operation may be included in signal processing in the servo control unit508and control may be performed such that the displacements of the weights500aand500bbecome zero. As a result, because the capacity value changes of the capacitor pair501aand501bfor the positive-side signal detection and the capacitor pair501cand501dfor the negative-side signal detection are minimized, linearity of the acceleration sensor can be increased. In addition, a differentiation (or difference) operation may be included in the signal processing in the servo control unit508, phase compensation may be performed, and a servo control loop may be stabilized. At this time, logic of general PID control can be applied.

The 1-bit quantizer509quantizes the servo value determined and output by the servo control unit508with 1 bit. For example, if an input of the 1-bit quantizer509is 0 or more, the 1-bit quantizer509outputs +1 and if the input is negative, the 1-bit quantizer509outputs −1. The 1-bit D/A converter510receives a digital value (±1) of 1 bit quantized by the 1-bit quantizer509and converts the digital value into an analog voltage (for example, ±5 V or 10 V/0 V). A positive-side output voltage of the 1-bit D/A converter510is applied to the fixed electrodes of the capacitor pair502afor the positive-side servo control and the capacitor pair502cfor the negative-side servo control and a negative-side output voltage of the 1-bit D/A converter510is applied to the fixed electrodes of the capacitor pair502bfor the positive-side servo control and the capacitor pair502dfor the negative-side servo control. As a result, electrostatic forces in directions reverse to the detected acceleration signals can be applied to the weights500aand500b. Differently from the case of the capacitors for the signal detection, an output of the 1-bit D/A converter510is applied to the capacitors for the servo control “in phase” with respect to a “differential MEMS” structure. As a result, the acceleration signals generated in phase with respect to the “differential MEMS” structure are canceled in phase. In a steady state, net force acting on the weights500aand500band the displacements of the weights500aand500bbecome almost zero. As such, the 1-bit quantizer509is inserted, so that a subsequent D/A converter can be configured as the 1-bit D/A converter510. Since the 1-bit D/A converter is easily mounted in terms of a circuit, the 1-bit D/A converter is advantageous to low consumption power. In addition, a capacity unit for servo control can be simplified. In an output of the 1-bit quantizer509, because a high frequency component (that is, a quantization error noise-shaped (diffused) to a high frequency side by sigma-delta control of a servo loop) is suppressed by the digital low-pass filter511, a final output of the acceleration sensor can be made to have a low noise. By the above configuration, in this acceleration sensor, the signal detection and the servo control are performed at the same time in the differential MEMS structure.

In the present invention, an output signal of the A/D converter506and an output signal of the 1-bit quantizer509are correlated by the correlation detection unit513. This is to detect a mismatch ΔC (ΔC=capacity value of capacitor502afor positive-side servo control+capacity value of capacitor502dfor negative-side servo control−capacity value of capacitor502bfor positive-side servo control−capacity value of capacitor502cfor negative-side servo control) of capacity values in the capacitor pair502aand502bfor the positive-side servo control and the capacitor pair502cand502dfor the negative-side servo control. If ΔC exists, a servo leak signal proportional to ΔC is generated in the output of the differential charge amplifier503, the servo leak signal is amplified hereinafter by the differential amplifier504, a high frequency component of the servo leak signal is suppressed to a certain degree by the differential analog filter505, and the servo leak signal is converted into a digital value by the differential A/D converter506. The same waveform as a waveform of an output signal of the 1-bit quantizer509and an inversion waveform thereof are differentially applied to the capacitor pair502aand502bfor the positive-side servo control and the capacitor pair502cand502dfor the negative-side servo control via the 1-bit D/A converter510. For this reason, the servo leak signal has almost the same waveform as the waveform of the output signal of the 1-bit quantizer509. That is, the servo leak signal is proportional to ΔC and has the same waveform as the waveform of the output signal of the 1-bit quantizer509. Focusing on the above fact, the servo leak signal included in the output of the differential A/D converter506can be detected. To do this, the output signal of the differential A/D converter506and the output signal of the 1-bit quantizer509are correlated in the correlation detection unit513, as described above. As a result, a direct-current or low frequency signal proportional to ΔC can be included in the output of the correlation detection unit513.

Therefore, the subsequent control unit514executes a digital integration operation or a phase compensation operation according to necessity, with respect to the output signal of the correlation detection unit513, and determines and outputs differential capacity control values. The differential capacity control values are digital values. Thereby, capacity values of the variable capacity units515aand515bare controlled. The differential capacity control values may be complementary to each other (that is, signs are opposite and magnitudes are the same). An object of the variable capacity units515aand515bis to cancel an influence by ΔC on differential input nodes (that is, an inversion input terminal node of the operational amplifier503aand an inversion input terminal node of the operational amplifier503d) of the differential charge amplifier503. To do this, it is considered that a capacitor of the same capacity value as the mismatch ΔC in the capacitor pair502aand502bfor the positive-side servo control and the capacitor pair502cand502dfor the negative-side servo control is inserted between the node and one output of the 1-bit D/A converter510. However, this method has two problems.

First, the output of the 1-bit D/A converter510has high voltage amplitude of about 10 V. For this reason, when a voltage amplitude level is input to the variable capacity unit515aor515bmounted in the ASIC as it is, a waveform may be clipped by an electrostatic discharge damage protection element (ESD element) connected to the input terminal of the ASIC to do so. Finally, a desired voltage is not transmitted to the variable capacity unit515aor515b. In this acceleration sensor, because a semiconductor process of a low voltage is applied to the ASIC for low consumption power, an input voltage range in which clipping is not generated is smaller than output amplitude of the 1-bit D/A converter510. For this reason, the waveform clipping is generated.

Second, in the case of the above method, it may be necessary to adjust the capacity values to the same capacity values with extremely high precision. For example, in the case in which ΔC is the mismatch of about 1% and it is necessary to cancel ΔC with precision of 0.01%, if it is assumed that the capacity values in the capacitor pair502aand502bfor the positive-side servo control and the capacitor pair502cand502dfor the negative-side servo control are 5 pF, a variable capacity unit capable of being controlled with 5 pF×0.01/10=0.5 fF increments is necessary. Capacity value increment precision is unrealistic in view of processing dimension precision of a semiconductor process or a parasitic capacity value.

Therefore, in the present invention, lower ends of the variable capacity units515aand515bare connected to the output of the driver516for the variable capacity, not the output of the 1-bit D/A converter510. The output of the 1-bit D/A converter510has high voltage amplitude (for example, ±5 V or 10 V/0 V) to generate sufficient electrostatic force. However, the driver516for the capacity is inserted, so that output amplitude of the output signal of the driver516for the capacity can be decreased while the output signal is made to have the same waveform (that is, the same waveform as the waveform of the output signal of the 1-bit quantizer509) as the waveform of the output signal of the 1-bit D/A converter510. For example, if the output voltage amplitude of the 1-bit D/A converter510is set to ±V and the output amplitude of the driver516for the capacity is set to V/α (α>1), a capacity value necessary for canceling error charges generated by ΔC is αΔC and can be multiplied by α. That is, the capacity value increment precision can also be multiplied by α. For example, α=20 is set, so that the capacity value increment precision needed in the previous example increases from 0.5 fF to 10 fF. It is considered that the variable capacity units515aand515bof the capacity value increment precision of the above degree can be sufficiently realized by a realistic semiconductor process.

Even when the servo leak signal is weak, the servo leak signal disturbs the servo loop remarkably and causes a noise increase or an unjust oscillation. For this reason, it is absolutely necessary to cancel the influence of the mismatch ΔC in the capacitor pair502aand502bfor the positive-side servo control and the capacitor pair502cand502dfor the negative-side servo control with high precision, as described above. An integration operation is included in the control unit514, so that negative feedback control can be executed such that ΔC and the servo leak signal become zero. A negative feedback control loop is hereinafter called an “MEMS capacity compensation loop”.

The “differential MEMS” structure disclosed in this embodiment has three great advantages. First, because a signal amount doubles with respect to the same acceleration signal, a double circuit noise can be allowed. That is, consumption power of a circuit can be reduced to ¼ theoretically. Second, because the acceleration sensor is not affected by an in-phase noise (power noise of the differential charge amplifier) of the circuit, a noise can be reduced. Third, even when the capacity values for the servo control are changed by the displacements of the weights, in ΔC (ΔC=capacity value of capacitor502afor positive-side servo control+capacity value of capacitor502dfor negative-side servo control−capacity value of capacitor502bfor positive-side servo control−capacity value of capacitor502cfor negative-side servo control), the changes are canceled and the acceleration sensor is not affected by the changes. That is, in the case of the differential MEMS structure, because ΔC is regarded as a static value determined by an initial MEMS manufacturing variation, a band of an MEMS capacity compensation loop can be narrowed and a design of the control unit514is facilitated. As a result, a noise can be further reduced.

Effect of Fifth Embodiment

As described above, according to the acceleration sensor in the fifth embodiment, the signal detection and the servo control are performed at the same time and the servo leak signal according to the mismatch of the capacity values of the MEMS capacitive elements can be canceled with high precision. Therefore, the noise increase or the unjust oscillation according to the servo leak signal can be prevented. For this reason, even when an MEMS process in which a manufacturing variation is large is used, a capacitive MEMS acceleration sensor of a low noise and low consumption power can be realized. As compared with the first embodiment, because the MEMS has the differential structure, a mounting area increases. However, a noise can be further reduced.

Sixth Embodiment

FIG. 6illustrates a sixth embodiment of the present invention. Since a basic portion of this embodiment is the same as that of the fifth embodiment, a difference with the fifth embodiment will be described. In this embodiment, a correlation detection unit513is connected to an output of a differential charge amplifier503, not an output of a differential A/D converter506, as in the fifth embodiment. For this reason, the correlation detection unit513correlates a servo leak signal included in the output of the differential charge amplifier503with an output signal of a 1-bit quantizer509and detects the servo leak signal. Differently from the fifth embodiment of the present invention, the servo leak signal of the output of the differential charge amplifier503is detected directly not via a differential amplifier504, a differential analog filter505, and the differential A/D converter506. Therefore, because there is no delay of the servo leak signal by each analog circuit block, an MEMS capacity compensation loop is easily stabilized and a parameter design of a control unit514is facilitated. Instead, a point that it is necessary to realize the correlation detection unit513with an analog circuit and a point that an A/D converter is necessary separately in the control unit514become demerits as compared with the fifth embodiment.

Effect of Sixth Embodiment

As described above, according to an acceleration sensor in the sixth embodiment, signal detection and servo control are performed at the same time and the servo leak signal according to a mismatch of capacity values of MEMS capacitive elements can be canceled with high precision. Therefore, a noise increase or an unjust oscillation according to the servo leak signal can be prevented. For this reason, even when an MEMS process in which a manufacturing variation is large is used, a capacitive MEMS acceleration sensor of a low noise and low consumption power can be realized. Since the servo leak signal of the output of the differential charge amplifier is directly detected, a stabilization design is easy.

Seventh Embodiment

FIG. 7illustrates a seventh embodiment of the present invention. Since a basic portion of this embodiment is the same as that of the fifth embodiment, a difference with the fifth embodiment will be described. In this embodiment, a delay unit717is inserted between a driver516for a variable capacity and variable capacity units515aand515b. An object of the delay unit717is to simulate a response delay due to a limitation of the driving capability of a 1-bit D/A converter510. The response delay of the 1-bit D/A converter510is determined by a time constant to be a product of output resistance thereof and capacity values of capacitors502aand502bfor positive-side servo control and capacitors502cand502dfor negative-side servo control and becomes a primary low-pass filter characteristic. Therefore, the delay unit717is configured as a primary low-pass filter having almost the same time constant, so that an output waveform of the 1-bit D/A converter510can be simulated. As a result, cancellation of the servo leak signal disclosed in the fifth embodiment can be performed completely.

Effect of Seventh Embodiment

As described above, according to an acceleration sensor in the seventh embodiment, signal detection and servo control are performed at the same time and the servo leak signal according to a mismatch of capacity values of MEMS capacitive elements can be canceled with high precision. Therefore, a noise increase or an unjust oscillation according to the servo leak signal can be prevented. For this reason, even when an MEMS process in which a manufacturing variation is large is used, a capacitive MEMS acceleration sensor of a low noise and low consumption power can be realized. Since delay compensation of the 1-bit D/A converter is performed by the delay unit, the servo leak signal can be canceled with higher precision.

Eighth Embodiment

FIG. 8illustrates an eighth embodiment of the present invention. Since a basic portion of this embodiment is the same as that of the fifth embodiment, a difference with the fifth embodiment will be described. In this embodiment, an inverter818is inserted between a driver516for a variable capacity and a variable capacity unit515b. As a result, because a voltage applied to a lower end of a variable capacity unit515aand a voltage applied to a lower end of the variable capacity unit515bare logically inverted, a control signal from a control unit514can be used commonly for the variable capacity unit515aand the variable capacity unit515b.

Effect of Eighth Embodiment

As described above, according to an acceleration sensor in the eighth embodiment, signal detection and servo control are performed at the same time and a servo leak signal according to a mismatch of capacity values of MEMS capacitive elements can be canceled with high precision. Therefore, a noise increase or an unjust oscillation according to the servo leak signal can be prevented. For this reason, even when an MEMS process in which a manufacturing variation is large is used, a capacitive MEMS acceleration sensor of a low noise and low consumption power can be realized.

Ninth Embodiment

An acceleration sensor according to a ninth embodiment will be described usingFIG. 9.FIG. 9is a diagram illustrating an example of a configuration of the acceleration sensor. Differently from the first embodiment, a variable resistance unit is used and an MEMS capacity is compensated for in an amplifier of a next step, not a charge amplifier of an initial step.

In the acceleration sensor, a mechanical portion is configured using micro electro mechanical systems (MEMS) and a circuit portion is configured using an application specific integrated circuit (ASIC). The acceleration sensor is not limited thereto. For example, the acceleration sensor is used for an MEMS capacitive acceleration sensor detecting vibration acceleration extremely smaller than gravity, as a sensor for a seismic reflection survey to explore for oil or natural gas.

First, a configuration will be described. The MEMS includes a capacitor pair101aand101bfor signal detection and a capacitor pair102aand102bfor servo control. One electrode of each of the four capacitors is connected mechanically and electrically and forms one weight (movable electrode portion)100. The weight is connected to an inversion input terminal of an operational amplifier103aconfiguring a charge amplifier103. The charge amplifier103is configured using the operational amplifier103a, a feedback capacitor103b, and a feedback resistor103c. A bias voltage VBis connected to a non-inversion input terminal of the operational amplifier103a. VBmay be a ground potential or may not be the ground potential. An output (that is, an output of the operational amplifier103a) of the charge amplifier103is input to an amplifier904. The amplifier904is an operational amplifier inversion amplification circuit of a resistance feedback type and includes an operational amplifier904a, an input resistor904b, and a feedback resistor904c. The bias voltage VBis connected to a non-inversion input terminal of the operational amplifier904a. VBmay be a ground potential or may not be the ground potential. An output (that is, an output of the operational amplifier904a) of the amplifier904is input to an analog filter105and an output of the analog filter105is input to an A/D converter106. An output of the A/D converter106is input to a demodulator107. A modulation clock is also input to the demodulator107. An output of the demodulator107is input to a servo control unit108, an output of the servo control unit108is input to a 1-bit quantizer109, and an output of the 1-bit quantizer109is input to a 1-bit D/A converter110. Differential outputs of the 1-bit D/A converter110are connected to electrodes (electrodes not configuring the weight) of the capacitor pair102aand102bfor the servo control, respectively. In addition, the modulation clock and an inversion clock thereof are input to drivers112aand112bfor the capacitor pair for the signal detection, respectively, and outputs of the drivers are connected to electrodes (electrodes not configuring the weight) of the capacitor pair101aand101bfor the signal detection, respectively. The output of the A/D converter106is also input to a correlation detection unit113and the output of the 1-bit quantizer109is also input to the correlation detection unit113. An output of the correlation detection unit113is input to a control unit114and an output of the control unit114functions as a control signal and controls a resistance value of a variable resistance unit915. The output of the 1-bit quantizer109is also input to a driver916, an output of the driver916is input to a delay unit917, and an output of the delay unit917is connected to one terminal of the variable resistance unit915. The other terminal of the variable resistance unit915is connected to an inversion input terminal node of the operational amplifier904a. In addition, the output of the 1-bit quantizer109is input to a digital low-pass filter111and an output of the digital low-pass filter111becomes an output of the acceleration sensor. The ASIC includes elements from the charge amplifier103to the 1-bit D/A converter110.

Next, an operation will be described. If acceleration is applied from the outside to the acceleration sensor, inertial force is generated in the weight100to be moved. As a result, the weight100is displaced. Since the weight100is also the electrodes of the capacitor pair101aand101bfor the signal detection, capacity values of the capacitor pair are changed by the displacement. For example, inFIG. 9, if the acceleration is applied in a rightward direction and the weight100is displaced in the rightward direction, a distance between the movable electrode (that is, the weight100) of the capacitor101bfor the signal detection and a fixed electrode thereof decreases and a capacity change value of +ΔCDis obtained and a distance between the movable electrode (that is, the weight100) of the capacitor101afor the signal detection and a fixed electrode thereof increases and a capacity change value of −ΔCD) is obtained. An application direction and an amount of the acceleration can be detected on the basis of the capacity change values (+ΔCDand −ΔCD) in the capacitor pair101aand101bfor the signal detection. A pair structure of the capacitor pair101aand101bfor the detection is a structure for various known objects not described in detail, for example, for canceling in-phase components of the capacity values. For the convenience of description, parallel plate capacitors are used in the MEMS configuration in the above description andFIG. 9. However, the same mechanism is realized in other types of capacitors. Therefore, the present invention is not limited to the MEMS of the parallel plate capacitor type.

A modulation clock voltage and an inversion clock voltage thereof are applied to the capacitor pair101aand101bfor the detection via the drivers112aand112bfor the capacitor pair for the signal detection, respectively. As a result, the capacity change of ΔCDis converted into a charge change. The charge change is converted into the voltage change by the charge amplifier103of an initial step in the ASIC. The charge amplifier103has a configuration of a so-called operational amplifier inversion amplifier of a capacitive type and input capacitors are the capacitor pair101aand101bfor the signal detection at the MEMS side and a feedback capacitor is the feedback capacitor103bat the ASIC side. However, the feedback resistor103chaving a high resistance value is inserted into a feedback path in parallel. This is to secure a direct-current feed path to compensate for an input leak current of the operational amplifier103a. Meanwhile, measures using a reset switch in a portion of the feedback resistor103care known conventionally. However, in this case, there is a problem in that a noise density of a sampling noise by the reset switch is high. Since a thermal noise by the feedback resistor103chaving the high resistance value used in this method is sufficiently suppressed in the vicinity of a desired frequency (that is, a frequency of the modulation clock) by a low-pass filter characteristic by the feedback resistor103cand the feedback capacitor103b, there is no problem.

A signal converted into a voltage by the charge amplifier103is amplified by the amplifier904, a noise or an unnecessary signal component thereof is suppressed by the analog filter105, and the signal is converted into a digital value by the A/D converter106. The demodulator107is a digital multiplier of two inputs and multiplies the output of the A/D converter106and the modulation clock and performs synchronous detection for the modulation clock. As a result, a value proportional to the displacement of the weight100is obtained in an output of the demodulator107. A series of modulation/demodulation processing is equivalent to a so-called “chopper system”. As a result, an influence by a loud 1/f noise generated by the charge amplifier103, the amplifier904, the analog filter105, and the A/D converter106can be avoided. The servo control unit108is a circuit that receives the displacement value of the weight100demodulated by the demodulator107, determines a servo value to generate force in a direction reverse to a detection signal, on the basis of the value, and outputs the servo value to the 1-bit quantizer109. Particularly, a digital integration operation may be included in signal processing in the servo control unit108and control may be performed such that the displacement of the weight100becomes zero. As a result, because the capacity value changes of the capacitor pair101aand101bfor the signal detection are minimized, linearity of the acceleration sensor can be increased. In addition, a differentiation (or difference) operation may be included in the signal processing in the servo control unit108, phase compensation may be performed, and a servo control loop may be stabilized. At this time, logic of general PID control can be applied.

The 1-bit quantizer109quantizes the servo value determined and output by the servo control unit108with 1 bit. For example, if an input of the 1-bit quantizer109is 0 or more, the 1-bit quantizer109outputs +1 and if the input is negative, the 1-bit quantizer109outputs −1. The 1-bit D/A converter110receives a digital value (±1) of 1 bit quantized by the 1-bit quantizer109, converts the digital value into an analog voltage (for example, ±5 V or 10 V/0 V), and applies the analog voltage to the fixed electrodes of the capacitor pair102aand102bfor the servo control. As a result, electrostatic force in a direction reverse to the detected acceleration signal can be applied to the weight100. In a steady state, net force acting on the weight100and the displacement of the weight100become almost zero. As such, the 1-bit quantizer109is inserted, so that a subsequent D/A converter can be configured as the 1-bit D/A converter110. Since the 1-bit D/A converter is easily mounted in terms of a circuit, the 1-bit D/A converter is advantageous to low consumption power. In addition, a capacity unit for servo control can be simplified. In an output of the 1-bit quantizer109, because a high frequency component (that is, a quantization error noise-shaped (diffused) to a high frequency side by sigma-delta control of a servo loop) is suppressed by the digital low-pass filter111, a final output of the acceleration sensor can be made to have a low noise. By the above configuration, in this acceleration sensor, the signal detection and the servo control are performed at the same time.

In the present invention, an output signal of the A/D converter106and an output signal of the 1-bit quantizer109are correlated by the correlation detection unit113. This is to detect a mismatch ΔC of capacity values in the capacitor pair102aand102bfor the servo control. If ΔC exists, a servo leak signal proportional to ΔC is generated in the output of the charge amplifier103, the servo leak signal is amplified hereinafter by the amplifier904, a high frequency component of the servo leak signal is suppressed to a certain degree by the analog filter105, and the servo leak signal is converted into a digital value by the A/D converter106. The same waveform as a waveform of an output signal of the 1-bit quantizer109and an inversion waveform thereof are differentially applied to the capacitor pair102aand102bfor the servo control via the 1-bit D/A converter110. For this reason, the servo leak signal has almost the same waveform as the waveform of the output signal of the 1-bit quantizer109. That is, the servo leak signal is proportional to ΔC and has the same waveform as the waveform of the output signal of the 1-bit quantizer109. Focusing on the above fact, the servo leak signal included in the output of the A/D converter106can be detected. To do this, the output signal of the A/D converter106and the output signal of the 1-bit quantizer109are correlated in the correlation detection unit113, as described above. As a result, a direct-current or low frequency signal proportional to ΔC can be included in the output of the correlation detection unit113.

Therefore, the subsequent control unit114executes a digital integration operation or a phase compensation operation according to necessity, with respect to the output signal of the correlation detection unit113, and determines and outputs a resistance control value. The resistance control value is a digital value. Thereby, a resistance value of the variable resistance unit915is controlled. An object of the variable resistance unit915is to cancel an influence by ΔC on an inversion input terminal node of the operational amplifier904aof the amplifier904, using a method of a so-called “operational amplifier subtracter”. However, differently from the other embodiments of the present invention, because the influence of ΔC is canceled via the charge amplifier103, it is necessary to apply a voltage waveform having compensated for a primary high-pass filter characteristic by the feedback capacitor103band the feedback resistor103cof the charge amplifier103and a low-pass filter characteristic by a finite band of the operational amplifier103ato a lower end of the variable resistance unit915. For this reason, it is necessary to insert the delay unit917. Similarly to the other embodiments, an integration operation is included in the control unit114, so that negative feedback control can be executed such that ΔC and the servo leak signal become zero.

Effect of Ninth Embodiment

As described above, according to the acceleration sensor in the ninth embodiment, the signal detection and the servo control are performed at the same time and the servo leak signal according to the mismatch of the capacity values of the MEMS capacitive elements can be canceled with high precision. Therefore, a noise increase or an unjust oscillation according to the servo leak signal can be prevented. For this reason, even when an MEMS process in which a manufacturing variation is large is used, a capacitive MEMS acceleration sensor of a low noise and low consumption power can be realized.

In the acceleration sensors according to the first to ninth embodiments described above, the acceleration sensors may be configured by using a 1.5-bit (three values) quantizer or a multi-bit quantizer in place of the 1-bit quantizers109and509and correspondingly using a 1.5-bit (three values) D/A converter or a multi-bit D/A converter in place of the 1-bit D/A converters110and510.

For the acceleration sensor according to each embodiment, first, it is considered that each of the MEMS of the mechanical portion and the ASIC of the circuit portion is formed on an individual semiconductor substrate and is configured as an individual semiconductor chip. However, in the future, it is considered that the MEMS and the ASIC are formed on the same semiconductor chip.

The acceleration sensor according to each embodiment configures a one-dimensional acceleration sensor. For example, when the acceleration sensor is used for a sensor for a seismic reflection survey to explore for oil or natural gas, a configuration of the one-dimensional acceleration sensor is used. For example, when the acceleration sensor is used for a three-dimensional acceleration sensor, a configuration in which three modules of one-dimensional acceleration sensors are formed for individual dimensions or three MEMSs are connected to a common ASIC is considered.

Also, it is considered that the MEMS capacitive acceleration sensor illustrated in each embodiment is used for a vehicle in the future.

<<Configurations of Correlation Detection Unit, Control Unit, Variable Capacity Unit, and Variable Resistance Unit>>

FIG. 10illustrates one example of the correlation detection unit113(513), the control unit114(514), and the variable capacity unit115(515aand515b) applied in the first embodiment (fifth embodiment). The correlation detection unit113can be realized by a digital multiplier1001and a digital low-pass filter1002connected to an output thereof. In addition, the control unit114can be realized by a digital integrator1003and a phase compensation unit1004connected to an output thereof. The phase compensation unit1004includes a differentiation (or difference) operation, generates a zero point to advance a phase by the differentiation (or difference) operation, and stabilizes servo control. The variable capacity unit115is controlled by a digital value of an output of the phase compensation unit1004. The variable capacity unit115includes binary capacitive elements1005a,1005b,1005c, and1005dconnected in parallel and switches1006connected to the individual capacitive elements, in the case of 4-bit control, for example. According to the 4-bit control value, the switches1006connected to the individual binary capacitive elements are selectively turned on. As a result, a parallel capacity value varies.

FIG. 11illustrates one example of the correlation detection unit113(513), the control unit114(514), and the variable capacity unit115(515aand515b) applied in the second embodiment (sixth embodiment). The correlation detection unit113can be realized by an analog multiplier1101and an analog low-pass filter1102connected to an output thereof. In addition, the control unit114can be realized by an analog integrator1103and an A/D converter1104connected to an output thereof. The phase compensation unit1004ofFIG. 10may be inserted after the A/D converter1104and stability of the servo control may be increased. The variable capacity unit115is controlled by a digital value of an output of the A/D converter1104or an output of the phase compensation unit1004. The variable capacity unit115is the same as that inFIG. 10.

FIG. 12illustrates another example of the correlation detection unit113(513), the control unit114(514), and the variable capacity unit115(515aand515b) applied in the second embodiment (sixth embodiment). Similarly toFIG. 11, the correlation detection unit113can be realized by the analog multiplier1101and the analog low-pass filter1102connected to the output thereof. In addition, the control unit114can be realized by a comparator1203and an up/down counter1204connected to an output thereof. Here, the comparator1203executes a function of a 1-bit A/D converter and the up/down counter1204executes a function of a digital integrator. However, because a servo value is quantized with 1 bit by the comparator1203, responsiveness and stability of an MEMS capacity compensation loop are inferior to those in the configurations ofFIGS. 10 and 11. Instead, because both the comparator and the up/down counter are simple circuits, a circuit design is facilitated. The variable capacity unit115is controlled by a digital value (count value) of an output of the up/down counter. The variable capacity unit115is the same as that inFIG. 10.

FIG. 13illustrates one example of the correlation detection unit113, the control unit114, and the variable resistance unit915applied in the ninth embodiment. The correlation detection unit113can be realized by the digital multiplier1001and the digital low-pass filter1002connected to the output thereof. In addition, the control unit114can be realized by the digital integrator1003and the phase compensation unit1004connected to the output thereof. The phase compensation unit1004includes a differentiation (or difference) operation, generates a zero point to advance a phase by the differentiation (or difference) operation, and stabilizes servo control. The variable resistance unit915is controlled by a digital value of an output of the phase compensation unit1004. The variable resistance unit915includes binary resistive elements1305a,1305b,1305c, and1305dconnected in parallel and switches1306connected to the individual capacitive elements, in the case of 4-bit control, for example. According to the 4-bit control value, the switches1306connected to the individual binary resistive elements are selectively turned on. As a result, a parallel resistance value varies.

FIG. 17is a diagram illustrating a configuration example of a digital control voltage divider inFIG. 16.

FIG. 18is a diagram illustrating another configuration example of the digital control voltage divider inFIG. 16.

FIG. 19is a diagram illustrating a configuration example of a delay unit of an acceleration sensor according to the present invention.

FIG. 20is a diagram illustrating another configuration example of the delay unit of the acceleration sensor according to the present invention.

FIG. 21is a diagram illustrating a configuration example of an analog multiplier in a correlation detection unit of the acceleration sensor according to the present invention.

FIG. 22is a diagram illustrating another configuration example of the analog multiplier in the correlation detection unit of the acceleration sensor according to the present invention.

REFERENCE SIGNS LIST