JP5496515B2 - Acceleration sensor circuit and three-axis acceleration sensor circuit - Google Patents

Description

  The present invention relates to an acceleration sensor that detects acceleration based on a change in capacitance.

  Acceleration sensors are used in a wide range of fields such as automobile airbags and game machines. These acceleration sensors are small in size and have a frequency characteristic that extends from near 0 to a low frequency band of several thousand Hz.

  The seismic reflection method, a type of geophysical exploration, is a method of artificially generating seismic waves, capturing reflected waves that bounce off the ground using a geophone installed on the ground surface, and analyzing the results to elucidate the underground structure. It is. This geophone is provided with an acceleration sensor, and detects vibration caused by reflected waves as acceleration. In the seismic reflection method, about 1000 geophones are installed to detect the underground structure. In the future, it is assumed that about 1 million geophones are installed at a grid interval of about 10 m.

JP-A-11-258265

  In the seismic reflection method, the acceleration sensor needs to detect an acceleration of about 120 dB as a dynamic range. That is, the noise level in the S / N ratio is required to be 1 to 10 μG / √Hz or less. However, for example, current vehicle control acceleration sensors only detect acceleration of about 60 to 80 dB as the dynamic range, and the noise level at the S / N ratio is about 100 μG / √Hz, which is necessary for the reflection seismic exploration method. Does not have the dynamic range.

  Some acceleration sensors have been put into practical use as exploration systems using MEMS elements. However, they are very expensive, and considering that 1 million pieces will be installed in the future, an acceleration sensor using a cheaper MEMS element is required.

  In view of the above points, an object of the present invention is to provide an acceleration sensor circuit having a wide dynamic range, high sensitivity, efficiency, and low cost.

The acceleration sensor circuit of the present invention includes a first capacitor whose capacitance changes according to the detected acceleration,
A second capacitor whose capacitance fluctuates in a direction opposite to the first capacitor due to the acceleration;
A first amplifier whose degree of amplification with respect to an AC signal is determined by a ratio between the capacitance of the first capacitor and the capacitance of the second capacitor;
And a second amplifier that generates a differential signal between an input AC signal input to the first amplifier and an amplified AC signal amplified by the first amplifier.

In the acceleration sensor circuit of the present invention, the amplification factor of the first amplifier is determined by the capacitance of the first capacitor and the capacitance of the second capacitor, and the capacitance of each capacitor varies in a direction opposite to the detected acceleration. Therefore, the acceleration can be calculated based on the amplification degree of the first amplifier. Further, the acceleration can be calculated from the difference value between the amplified signal and the input AC signal of the signal before amplification.

  In the acceleration sensor circuit of the present invention, the ratio of the difference signal value to the input AC signal value may change with the detected acceleration. In the acceleration sensor circuit of the present invention, when the ratio of the differential signal value to the input AC signal value is positive, the acceleration is set as the acceleration in the first direction, and the ratio of the differential signal value to the input AC signal value is A means for detecting acceleration in two positive and negative directions may be further provided by setting the acceleration in a second direction that is opposite to the first direction in the negative case. By configuring in this way, it is possible to obtain a positive value and a negative value as the ratio of the differential signal value to the input AC signal value, and the acceleration applied to the acceleration sensor circuit between the first direction and the second direction. It can be detected in two directions.

  The frequency of the input AC signal may be set to a frequency band in which white noise is dominant in a noise component generated by the amplifier. As a result, noise other than white noise generated in the amplifier can be avoided, and noise generated in the acceleration sensor circuit can be reduced.

  The acceleration sensor circuit is preferably configured on a MEMS (Micro Electro Mechanical Systems). By configuring the acceleration sensor circuit with MEMS, an inexpensive, lightweight, and small acceleration sensor circuit can be realized.

  The three acceleration sensor circuits of the present invention may be a three-axis acceleration sensor circuit having each of the three-axis direction acceleration sensors for detecting acceleration.

  According to the present invention, an acceleration sensor circuit having a wide dynamic range, high sensitivity, efficiency, and low cost can be provided.

It is a figure which shows the structural example of an acceleration sensor circuit. It is a figure which shows the example of the change in the time t of the output voltage Vin of an alternating current bias oscillation circuit, and the output voltage vB of an operational amplifier. It is a figure which shows the flowchart of the operation example of an acceleration detection. It is a figure which shows the example of a plot of the output value of an acceleration sensor circuit. It is an example of an image figure when acceleration is applied to a capacitive acceleration sensor and a movable electrode is displaced by x. It is a figure which shows the example of the relationship between the frequency of the alternating current bias input into an operational amplifier, and the generated noise. It is a figure which shows the example of the operational amplifier circuit which is a non-inverting amplifier circuit. It is a general view of an example of a 3-axis acceleration sensor.

<First Embodiment>
Embodiments of an acceleration sensor circuit according to the present invention will be described below with reference to the drawings. The configuration of the following embodiment is an exemplification, and the acceleration sensor circuit of the present invention is not limited to the configuration of the embodiment.

<< Configuration example of acceleration sensor circuit >>
FIG. 1 is a diagram illustrating a configuration example of the acceleration sensor circuit A1. The acceleration sensor circuit A1 includes an AC bias oscillation circuit 1, a capacitive acceleration sensor 2, an operational amplifier OP1, an impedance Z1, an impedance Z2, an operational amplifier OP2, an impedance Z3, and an A / D converter 3. CPU4. The acceleration sensor circuit A1 shown in FIG. 1 is preferably configured as a MEMS element.

  The AC bias oscillation circuit 1 outputs a square wave signal according to the clock signal transmitted from the CPU 4. The voltage output from the AC bias oscillation circuit 1 is defined as Vin (unit V (volt)).

  The capacitive acceleration sensor 2 includes a capacitor C1 (corresponding to a “first capacitor”) and a capacitor C2 (corresponding to a “second capacitor”). Capacitor C1 and capacitor C2 are configured such that the capacitance changes when acceleration is applied to capacitive acceleration sensor 2. For example, when acceleration is applied to the capacitive acceleration sensor 2, the capacitance of the capacitor C1 increases while the capacitance of the capacitor C2 decreases (details will be described later). . Assume that the capacitance of the capacitor C1 is C1 (unit F (Faraday)), and similarly, the capacitance of the capacitor C2 is C2 (unit F). The impedances z1 and z2 are shown as follows.

オペアンプＯＰ１（「第１の増幅器」に相当）は、自身のフィードバックに容量式加速度センサ２に含まれるコンデンサＣ２を含む。オペアンプＯＰ１は、容量式加速度センサ２に含まれるコンデンサＣ１を通過した信号（「第１の増幅器に入力される入力交流信号」に相当）を入力信号とし、増幅する。オペアンプＯＰ１の出力信号（「第１の増幅器にて増幅される増幅交流信号」に相当）の電圧ｖＡ（単位Ｖ）は、以下のように示される。

The operational amplifier OP1 (corresponding to “first amplifier”) includes a capacitor C2 included in the capacitive acceleration sensor 2 in its own feedback. The operational amplifier OP1 amplifies the signal that has passed through the capacitor C1 included in the capacitive acceleration sensor 2 (corresponding to “input AC signal input to the first amplifier”) as an input signal. The voltage vA (unit V) of the output signal of the operational amplifier OP1 (corresponding to “amplified AC signal amplified by the first amplifier”) is expressed as follows.

オペアンプＯＰ２（「第２の増幅器」に相当）とインピータンスＺ３とは、オペアンプＯＰ１にて増幅された信号と、インピータンスＺ２とを通過した信号を加算する加算回路である。このオペアンプＯＰ２の出力信号（「差分信号」に相当）の電圧ｖＢは、以下のように示される。ただし、インピータンスＺ１及びインピータンスＺ２のインピータンスを同じ値にしてＺ（単位Ω）、インピータンスＺ３のインピータンスをＺｆ（単位Ω）とする。

The operational amplifier OP2 (corresponding to “second amplifier”) and the impedance Z3 are addition circuits that add the signal amplified by the operational amplifier OP1 and the signal that has passed through the impedance Z2. The voltage vB of the output signal (corresponding to “difference signal”) of the operational amplifier OP2 is expressed as follows. However, the impedances of the impedance Z1 and the impedance Z2 are set to the same value, Z (unit Ω), and the impedance of the impedance Z3 is set to Zf (unit Ω).

ここで、Ｚｆ＝Ｚの場合、オペアンプＯＰ２の出力電圧ｖＢは、オペアンプＯＰ１の入力電圧と出力電圧との差分となる。

Here, when Zf = Z, the output voltage vB of the operational amplifier OP2 is the difference between the input voltage and the output voltage of the operational amplifier OP1.

The A / D converter 3 measures the output voltage vB of the operational amplifier OP2. CPU4 is A / D
The output voltage Vout is calculated based on the measured value of the converter 3. The CPU 4 controls the output frequency of the AC bias oscillation circuit 1.

<< Example of acceleration detection operation >>
FIG. 2 shows an example of a change in time t between the output voltage Vin of the AC bias oscillation circuit 1 and the output voltage vB of the operational amplifier OP2. FIG. 2 shows an example in which the AC bias oscillation circuit 1 operates at a frequency of 20 kHz.

  From (Equation 1), when the capacitance of the capacitor C1 is larger than the capacitance of the capacitor C2 (C1> C2), vB has the same phase as the output voltage Vin of the AC bias oscillation circuit 1. When the capacitance C1 of the capacitor C1 and the capacitance C2 of the capacitor C2 are equal (C1 = C2), vB indicates a straight line. When the capacitance C1 of the capacitor C1 is smaller than the capacitance of the capacitor C2 (C1 <C2), vB has an opposite phase to the output voltage Vin of the AC bias oscillation circuit 1.

  FIG. 3 shows a flowchart of an operation example of acceleration detection. In FIG. 3, it is assumed that the AC bias oscillation circuit 1 has a frequency of 20 kHz (a period of 50 μsec).

  When detecting the rising of the output voltage Vin (hereinafter referred to as input voltage) of the AC bias oscillation circuit 1, the CPU 4 starts a rising detection timer (OP1).

  For example, when the rising edge detection timer counts 12 μs (OP2), the CPU 4 reads vB from the A / D converter 3 and temporarily stores it (OP3). At this time, the time t = t1, and the value of vB is v1 (FIG. 2).

  For example, when the rising edge detection timer counts 37 μsec after 25 μsec (half cycle) from time t1 (OP4), the CPU 4 reads vB from the A / D converter 3 and temporarily stores it (OP5). At this time, time t = t2, and the value of vB is v2 (FIG. 2).

  After reading v1 and v2, the CPU calculates Vout. Vout is obtained by the following equation.

ここで、Ｖｉｎ（ｔ１）は時刻ｔ＝ｔ１における入力電圧を示す。Ｖｉｎ（ｔ２）は時刻ｔ＝ｔ２における入力電圧を示す。Ｃ１（ｔ１）は時刻ｔ＝ｔ１におけるコンデンサＣ１の静電容量を示す。Ｃ１（ｔ２）は時刻ｔ＝ｔ２におけるコンデンサＣ１の静電容量を示す。Ｃ２（ｔ１）は時刻ｔ＝ｔ１におけるコンデンサＣ２の静電容量を示す。Ｃ２（ｔ２）は時刻ｔ＝ｔ２におけるコンデンサＣ２の静電容量を示す。

Here, Vin (t1) indicates an input voltage at time t = t1. Vin (t2) indicates an input voltage at time t = t2. C1 (t1) represents the capacitance of the capacitor C1 at time t = t1. C1 (t2) indicates the capacitance of the capacitor C1 at time t = t2. C2 (t1) indicates the capacitance of the capacitor C2 at time t = t1. C2 (t2) indicates the capacitance of the capacitor C2 at time t = t2.

  t2−t1 = 25 μsec is t1−t2 << T, where T is the period of vibration (detection wave) detected by the acceleration sensor, so C1 (t1) = C1 (t2), C2 (t1) = C2 (t2). Therefore,

となる。ＣＰＵ４は、Ｖｉｎの立ち上がりを基準にしてｔ１、ｔ２をカウントするので、図２より、常に、Ｖｉｎ（ｔ１）−Ｖｉｎ（ｔ２）＞０となる。従って、（式２）より
Ｃ１＞Ｃ２の場合、Ｖｏｕｔ＞０、
Ｃ１＝Ｃ２の場合、Ｖｏｕｔ＝０、
Ｃ１＜Ｃ２の場合、Ｖｏｕｔ＜０
となる。

It becomes. Since the CPU 4 counts t1 and t2 on the basis of the rise of Vin, Vin (t1) −Vin (t2)> 0 is always satisfied from FIG. Therefore, when C1> C2 from (Equation 2), Vout> 0,
When C1 = C2, Vout = 0,
When C1 <C2, Vout <0
It becomes.

  After calculating Vout, the CPU 4 records the value of Vout (OP6). Since the flow shown in FIG. 3 is performed every time the rising of the input voltage Vin is detected, in the case of FIGS. 2 and 3, Vout is calculated every 50 μsec (Vout sampling frequency 20 kHz).

  FIG. 4 is a diagram illustrating a plot example of Vout. When Vout is plotted, a waveform of a detection wave detected by the acceleration sensor circuit is obtained. In the case of FIG. 4, it can be seen that the detected wave has a frequency of approximately 250 Hz.

  From (Equation 2), when C1> C2, Vout takes a positive value, and when C1 <C2, Vout takes a negative value. Thus, for example, in the example of the output result shown in FIG. 4, when an AC power supply with an output voltage of 5 V is used, the output voltage −5 with respect to the input voltage 0 (V) <Vin (V) <5 V (V). An output voltage in the range of (V) <Vout <5 (V) that is twice the range of the input voltage Vin can be obtained. In other words, the acceleration sensor circuit A1 can detect the negative amplitude in addition to the positive amplitude of the detection wave by obtaining Vout by (Equation 2). In addition, when a 5V AC power supply is used, the power supply can be shared with the computer, resulting in a very efficient circuit configuration.

<< Relationship between capacitive acceleration sensor and acceleration >>
FIG. 5 is a diagram illustrating a conceptual configuration example of the capacitive acceleration sensor 2. For example, as shown in FIG. 5, the capacitive acceleration sensor 2 can be modeled by a movable silicon vibrator (movable electrode) supported by a beam between two fixed electrodes. A capacitor C1 (FIG. 1) and a capacitor C2 (FIG. 1) are formed by the two fixed electrodes and the movable electrode therebetween.

  In FIG. 5, d indicates the distance between the electrode plates of the fixed electrode and the movable electrode when the capacitances of the capacitor C1 and the capacitor C2 are equal. d1 is the distance between the electrode plates of the capacitor C1 after the acceleration a is applied. Similarly, d2 is the distance between the electrode plates of the capacitor C2 after the acceleration a is applied. When acceleration is applied to the MEMS element constituting the acceleration sensor circuit A1, the movable electrode moves. When the movable electrode moves, the interval of the electrostatic gap between the movable electrode and the fixed electrode changes, and at the same time, the capacitances of the capacitor C1 and the capacitor C2 change. Generally, the capacitance of a capacitor is expressed by the following equation.

ここで、ε［Ｆ／ｍ］は誘電率、Ｓ［ｍ²］は極板面積、ｄ［ｍ］は極板間距離である
。

Here, ε [F / m] is the dielectric constant, S [m ² ] is the electrode plate area, and d [m] is the distance between the electrode plates.

  For example, when the movable electrode moves by x, the distance between the plates of the capacitor C1 is reduced, and the distance between the plates of the capacitor C2 is increased. In this case, from (Equation 3), the value of the capacitance C1 of the capacitor C1 increases, and the value of the capacitance C2 of the capacitor C2 decreases. Therefore, the capacitances of the capacitors C1 and C2 of the capacitive acceleration sensor 2 change in directions opposite to each other when acceleration is applied.

  On the other hand, the acceleration is obtained from the following equation.

（式４）はニュートンの第２法則であり、ｆｉは慣性力［Ｎ・ｓ］、錘の質量をｍ［Ｋｇ］、加速度をａ［ｍ／ｓ²］とする。（式５）はフックの法則であり、ｆｒは復元力［
Ｎ］、ばね定数をｋ［Ｎ／ｍ］、変位をｘ［ｍ］とする。

(Equation 4) is Newton's second law, where fi is the inertial force [N · s], the mass of the weight is m [Kg], and the acceleration is a [m / s ² ]. (Equation 5) is Hooke's law, and fr is the restoring force [
N], the spring constant is k [N / m], and the displacement is x [m].

  When the inertial force fi and the restoring force fr are balanced, in FIG. 5, the relationship between the displacement x of the movable electrode and the acceleration a is expressed by the following equation from (Equation 4) and (Equation 5).

図５において、加速度ａが容量式加速度センサ２に印加され、可動電極がｘだけ動いた場合、ｄ１、ｄ２、及びｘとは以下の関係にあとする。

In FIG. 5, when acceleration a is applied to the capacitive acceleration sensor 2 and the movable electrode moves by x, d1, d2, and x are in the following relationship.

尚、ｄ＞＞ｘであるとする。

Note that d >> x.

  From (Formula 1), (Formula 3), (Formula 6) and (Formula 7),

（式８）より、オペアンプＯＰ２の出力電圧ｖＢと加速度ａとはほぼ線形の関係にあることがわかる。従って、（式１）及び（式８）より、加速度センサ回路Ａ１の出力電圧Ｖｏｕｔは加速度センサ回路Ａ１に印加される加速度ａにしたがって変化する。つまり、加
速度センサ回路Ａ１の増幅度（入力電圧Ｖｉｎに対する出力電圧Ｖｏｕｔの比）に変化があった場合（特にＶｏｕｔの出力に検出波が認められた場合）は、加速度センサ回路Ａ１に加速度が印加されたことを検出できる。

From (Equation 8), it can be seen that the output voltage vB of the operational amplifier OP2 and the acceleration a have a substantially linear relationship. Therefore, from (Equation 1) and (Equation 8), the output voltage Vout of the acceleration sensor circuit A1 varies according to the acceleration a applied to the acceleration sensor circuit A1. That is, when there is a change in the amplification degree of the acceleration sensor circuit A1 (ratio of the output voltage Vout to the input voltage Vin) (particularly when a detection wave is recognized in the output of Vout), acceleration is applied to the acceleration sensor circuit A1. Can be detected.

  Since x << L holds between the displacement ‘x’ and the electrode length ‘L’, it can be considered that the movable electrode always moves parallel to the fixed electrode.

<< Noise reduction of acceleration sensor circuit >>
Signals and noise can be expressed by S / N ratio. In order to detect acceleration with higher sensitivity, it is necessary to detect a signal (S: Signal) largely and to reduce noise (N: Noise).

In the acceleration sensor circuit A1 of FIG. 1, the following points are considered in order to reduce noise.
(1) Reduction of Brian Noise Brian noise is noise generated when gas molecules contained in the air collide with the movable electrode of the capacitive acceleration sensor 2. In order to reduce this noise, the inside of the element of the capacitive acceleration sensor 2 is evacuated.
(2) Reduction of noise generated by operational amplifier Some commercially available operational amplifiers are designed to reduce noise. Even if such an operational amplifier is used, noise peculiar to the semiconductor used in the operational amplifier is generated. It is necessary to consider this noise.

The noise spectrum generated in an operational amplifier includes the following.
White noise (white noise): Pn (f) = C (constant noise independent of frequency)
1 / f noise: Pn (f) ∝1 / f (noise inversely proportional to frequency)
Here, Pn (f) is the power spectrum density of the noise source, and indicates an execution value in a certain frequency band.

  White noise is generated in any way and cannot be reduced. However, 1 / f noise is noise that varies with frequency.

FIG. 6 is a diagram illustrating an example of the relationship between the frequency of the AC bias input to a certain operational amplifier and the generated noise. This relationship varies depending on the operational amplifier used. FIG. 6 shows that 1 / f noise is generated in the low frequency region. Further, if the frequency exceeds a certain frequency, 1 / f noise can be avoided (in the case of FIG. 6, the region exceeding 100 Hz). Therefore, the frequency of the output voltage Vin of the AC bias oscillation circuit 1 is a sufficiently high frequency that can avoid 1 / f noise.
(3) Noise generated by irregular thermal oscillation of electrons in the resistor The noise voltage Vn (unit V) generated in the resistor is expressed as follows.

ここで、ｋはボルツマン係数、Ｔは抵抗の胴体温度（単位Ｋ）、Ｒは抵抗値（単位Ω）、Δｆは帯域幅を示す。

Here, k is a Boltzmann coefficient, T is a resistance body temperature (unit K), R is a resistance value (unit Ω), and Δf is a bandwidth.

  FIG. 7 is a diagram illustrating an example of an operational amplifier circuit which is a non-inverting amplifier circuit. When determining the amplification factor of this operational amplifier, the following equation is used.

例えば、Ｒ１＝１Ｋ（Ω）、Ｒ２＝９Ｋ（Ω）を用いた場合をケース１とする。Ｒ１＝１０Ｋ（Ω）、Ｒ２＝９０Ｋ（Ω）を用いた場合をケース２とする。ケース１とケース２とでは、（式１０）における増幅度は１０と同じ値になる。しかし、ノイズレベルでは、（式９）から、ケース２はケース１の√１０倍（＝３．１６倍）大きくなる。

For example, Case 1 is a case where R1 = 1K (Ω) and R2 = 9K (Ω) are used. Case 2 uses R1 = 10K (Ω) and R2 = 90K (Ω). In Case 1 and Case 2, the amplification degree in (Equation 10) is the same value as 10. However, at the noise level, from (Equation 9), Case 2 is √10 times (= 3.16 times) larger than Case 1.

  Therefore, in order to reduce the thermal noise generated by the resistance, it is desired to use a resistance element having a small resistance value and to reduce the number of resistance elements that are sources of thermal noise as much as possible. In the acceleration sensor circuit A1 of FIG. 1, the number of elements constituting the circuit is reduced. Furthermore, it is possible to employ capacitors instead of resistance elements for the impedances Z1 to Z3 in the acceleration sensor circuit A1 of FIG. Generation of thermal noise can be avoided by using capacitors as the impedances Z1 to Z3 in the acceleration sensor circuit A1.

<Operational effects of the first embodiment>
According to the first embodiment, the difference voltage ratio between the input signal for inputting the acceleration to the operational amplifier OP1 and the signal amplified by the operational amplifier OP1 can be obtained as vB. Since the measured value of the A / D converter 3 is a differential voltage, the work to be processed by the CPU is reduced and the efficiency is high.

  In the first embodiment, Vout can take a positive value and a negative value because it is obtained by the output voltage Vout = v1−v2. Usually, for example, when it is desired to obtain the output voltage Vout in both the positive and negative regions, a power source for obtaining the output result in the positive region as the input voltage Vin and a power source for obtaining the output result in the negative region It is necessary to prepare. However, according to the circuit configuration of the first embodiment, the single AC bias oscillation circuit 1 can obtain the output voltage Vout in both the positive and negative regions, so that the efficiency is high.

  According to the acceleration sensor circuit A1 of the first embodiment, since the number of circuit elements constituting the circuit is small, power consumption can be reduced. Furthermore, since the number of circuit elements is small, noise generated in the circuit elements can be suppressed to a low level, and the sensitivity of the acceleration sensor circuit A1 can be increased.

  According to the first embodiment, the acceleration sensor element is evacuated, the AC power supply is used at a frequency at which 1 / f noise can be avoided, and the number of circuit elements is small, so that noise can be reduced. By reducing the noise, the S / N ratio increases and the sensitivity can be increased.

  If the first embodiment is configured by MEMS, an efficient, wide dynamic range, high sensitivity, inexpensive, lightweight, and small acceleration sensor circuit can be realized.

<Modification>
A three-axis acceleration sensor can be configured by providing three acceleration sensor circuits A1 of the first embodiment.

  FIG. 8 is an overall view of an example of a three-axis acceleration sensor. An acceleration sensor circuit A1 is arranged for each of three orthogonal axes (X axis, Y axis, and Z axis). The acceleration in the three-axis direction can be detected by the output voltages from the three acceleration sensor circuits A1.

DESCRIPTION OF SYMBOLS 1 AC bias oscillation circuit 2 Capacitance type acceleration sensor 3 A / D converter 4 CPU
A1 acceleration sensor circuit C1, C2 capacitors OP1, OP2 operational amplifiers Z1, Z2, Z3, impedance

Claims (6)

An AC bias oscillation circuit that outputs a square-wave AC signal at a predetermined period;
A first capacitor whose capacitance changes according to the detected acceleration;
A second capacitor whose capacitance fluctuates in a direction opposite to the first capacitor due to the acceleration;
A first amplifier in which an amplification factor for an AC signal output from the AC bias oscillation circuit is determined by a ratio between a capacitance of the first capacitor and a capacitance of the second capacitor;
A second amplifier that generates a differential signal between an input AC signal input to the first amplifier and an amplified AC signal amplified by the first amplifier;
Predetermined from the first time during the first period, the phase of the AC signal output from the AC bias oscillation circuit is inverted from the phase of the AC signal at the first time. At a second time after the lapse of time, the difference signal generated by the second amplifier is read, and the amount of change from the difference signal at the first time to the difference signal at the second time is calculated as the first time . Computing means for calculating a value up to twice the range of the input voltage from the power supply voltage of the amplifier of 2 ;
An acceleration sensor circuit comprising:   The acceleration sensor circuit according to claim 1, wherein a ratio of the difference signal value to the input AC signal value changes with the detected acceleration. When the ratio of the differential signal value to the input AC signal value is positive, the acceleration is set to the acceleration in the first direction, and when the ratio of the differential signal value to the input AC signal value is negative, the acceleration is set to the first direction. The acceleration sensor circuit according to claim 1, further comprising means for detecting acceleration in two positive and negative directions by setting the acceleration in a second direction opposite to the second direction.   4. The acceleration sensor circuit according to claim 1, wherein the frequency of the input AC signal is set to a frequency band in which white noise is dominant in a noise component generated by the amplifier. 5.   5. The acceleration sensor circuit according to claim 1, wherein the acceleration sensor circuit is configured on a MEMS (Micro Electro Mechanical Systems). An AC bias oscillation circuit that outputs a square-wave AC signal at a predetermined period;
A first capacitor whose capacitance varies depending on the detected acceleration;
A second capacitor whose capacitance fluctuates in a direction opposite to the first capacitor due to the acceleration;
A first amplifier in which an amplification factor for an AC signal output from the AC bias oscillation circuit is determined by a ratio between a capacitance of the first capacitor and a capacitance of the second capacitor;
A second amplifier that generates a differential signal between an input signal input to the first amplifier and an amplified AC signal amplified by the first amplifier;
Three sensor circuits that respectively detect acceleration in three axial directions,
For each of the three sensor circuits, the first time during the predetermined period and the phase of the AC signal output from the AC bias oscillation circuit are inverted from the phase of the AC signal at the first time. A differential signal generated by the second amplifier at a second time after a lapse of a predetermined time from the first time, and the second time from the differential signal at the first time. Calculating means for calculating the amount of change to the difference signal of the second amplifier with a value up to twice the range of the input voltage from the power supply voltage of the second amplifier ;
A three-axis acceleration sensor circuit comprising:

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