Abstract:
An object of the invention is to provide a physical quantity sensor capable of producing a highly accurate physical quantity detection signal. The physical quantity sensor has an oscillator for converting an externally applied physical quantity into an electrical signal, an oscillation circuit which makes the oscillator oscillate, and a detector circuit for detecting a to-be-detected signal output from the oscillator by using a detection signal output from the oscillation circuit, includes a delta-sigma modulator, preceding the detector circuit, for delta-sigma modulating either one of the detection signal output from the oscillation circuit and the to-be-detected signal output from the oscillator, and for outputting a modulated signal, a variable voltage source capable of varying an output voltage, and a control unit for controlling the output voltage of the variable voltage source, and wherein the delta-sigma modulator performs the delta-sigma modulation by using a feedback signal created based on the output voltage.

Description:
TECHNICAL FIELD 
     The present invention relates to a physical quantity sensor that uses delta-sigma modulation. 
     BACKGROUND 
     It is known in the art to provide a gyro sensor using a piezoelectric crystal oscillator for use as a physical quantity sensor for attitude control of a car navigation system, robot, etc. (for example, refer to patent document 1). 
       FIG. 1  is a diagram showing one example of a prior art gyro sensor. 
     As shown in  FIG. 1 , gyro sensor  1  comprises an oscillation circuit  3 , which includes a crystal oscillator  2  having detection electrodes  5  and  6 , and a detection circuit  10  for detecting a Coriolis force based on detection signals supplied from detection electrodes  5  and  6 . Detection electrodes  5  and  6  are formed on a detection tine of crystal oscillator  2  and, based on the outputs from the driving electrodes formed on a driving tine of crystal oscillator  2 , oscillation circuit  3  performs binarization and outputs a detection clock CL in the form of a rectangular wave. 
     Crystal oscillator  2  continues to oscillate with a constant amplitude under the control of oscillation circuit  3 ; if, at this time, crystal oscillator  2  is rotated with an angular velocity ω, a Coriolis force F proportional to the angular velocity ω acts at right angles to the direction of vibration of the driving tine of crystal oscillator  2 . Then, due to the stress induced by the Coriolis force F, crystal oscillator  2  is set into vibration at a frequency equal to the drive frequency, as a result of which electrical charges due to the piezoelectric effect are set up on detection electrodes  5  and  6  formed on the detection tine. 
     These charges cause detection currents I 1  and I 2 , very small currents of opposite phases, to flow in detection electrodes  5  and  6 , respectively. I/V conversion circuits  11  and  12  in detection circuit  10  convert detection currents I 1  and I 2  into detection voltages V 10  and V 11 , respectively, and a differential amplifier  13  amplifies the difference between detection voltages V 10  and V 11 , and thus produces a difference output V 12 . A synchronous detection circuit  14  takes difference output V 12  as input, performs synchronous detection by synchronizing the timing with the detection clock CL output as a rectangular wave from oscillation circuit  3 , and produces a detection output V 13 . A low-pass filter (LPF)  15  cuts off the AC component of detection output V 13 , and outputs an angular velocity detection signal V 14  which is a DC voltage proportional to the angular velocity. 
       FIG. 2  is a diagram showing signal examples in synchronous detection circuit  14 . 
       FIG. 2( a )  shows the case where the difference output V 12  of the differential amplifier  13  is input to the synchronous detection circuit  14 ,  FIG. 2( b )  shows the case where noise  1  at twice the frequency of difference output V 12  is input to synchronous detection circuit  14 , and  FIG. 2( c )  shows the case where noise  2  at three times the frequency of difference output V 12  is input to synchronous detection circuit  14 . 
     As shown in  FIG. 2( a ) , difference output V 12  is detected with the detection clock CL to produce detection output V 13  whose AC component is then cut off by LPF  15 , producing the angular velocity detection signal V 14 , which is a DC voltage having a certain value. 
     As shown in  FIG. 2( b ) , when noise  1  at twice the frequency of the difference output V 12  is input to synchronous detection circuit  14 , noise  1  is detected by the detection clock CL, but since synchronous detection output V 13  in this case has an upper-lower symmetrical waveform, the output that LPF  15  produces by cutting off the AC becomes zero, hence no ill effect on the angular velocity detection signal V 14 . On the other hand, as shown in  FIG. 2( c ) , when noise  2  at three times the frequency of difference output V 12  is input to synchronous detection circuit  14  and detected by detection clock CL, resulting synchronous detection output V 13  has an upper-lower asymmetrical waveform; as a result, even if the AC is cut off by the LPF  15 , the DC component, and hence noise, remains in angular velocity detection signal V 14 . While  FIG. 2( c )  has been described for the case where noise at three times the difference output V 12  is input, the same problem occurs when harmonic noise at an odd multiple of the frequency of the difference output V 12  is input. 
     That is, there has been the problem that when harmonic noise superimposed on the detection signal is input to the synchronous detection circuit, the angular velocity detection signal is affected by the noise. 
     Patent document: Japanese Unexamined Patent Publication No. 2007-57340 (FIG. 9) 
     SUMMARY 
     It is an object of the present invention to provide a physical quantity sensor aimed at solving the above problem. 
     It is another object of the present invention to provide a physical quantity sensor capable of producing a highly accurate physical quantity detection signal. 
     It is also an object of the present invention to provide a physical quantity sensor capable of producing a highly accurate angular velocity detection signal. 
     The physical quantity sensor having an oscillator for converting an externally applied physical quantity into an electrical signal, an oscillation circuit for causing the oscillator to oscillate, and a detector circuit for detecting a to-be-detected signal output from the oscillator by using a detection signal output from the oscillation circuit, includes a delta-sigma modulator, preceding the detector circuit, for delta-sigma modulating either one of the detection signal output from the oscillation circuit and the to-be-detected signal output from the oscillator, and for outputting a modulated signal, a variable voltage source capable of varying an output voltage, and a control unit for controlling the output voltage of the variable voltage source, and wherein the delta-sigma modulator performs the delta-sigma modulation by using a feedback signal created based on the output voltage. 
     A gyro sensor includes an oscillator, an oscillation circuit for causing the oscillator to oscillate, a delta-sigma modulator for delta-sigma modulating either one of a detection signal output from the oscillation circuit and a to-be-detected signal output from the oscillator, a detector circuit for detecting, based on the output signal of the delta-sigma modulator, the other one of the detection signal output from the oscillation circuit and the to-be-detected signal output from the oscillator, and a low-pass filter for removing an AC component from an output signal of the detector circuit. 
     Preferably, in the gyro sensor, the detector circuit further includes a first switching circuit for outputting, based on the output signal of the delta-sigma modulator, either one of the to-be-detected signal supplied from the oscillator and an inverted version of the to-be-detected signal. 
     Preferably, the gyro sensor further includes a constant voltage source, and a second switching circuit for outputting, based on the output signal of the delta-sigma modulator, either one of a voltage signal supplied from the constant voltage source and an inverted version of the voltage signal, and wherein the delta-sigma modulator uses the output signal of the second switching circuit as a feedback signal. 
     According to the physical quantity sensor, since the reference voltage is variable, it is possible to provide the physical quantity sensor with a variable gain amplification function (sensitivity compensation function). Furthermore, since there is no need to provide a dedicated variable gain amplifier circuit, it also becomes possible to prevent problems such as the generation of noise and an increase in the amount of circuitry associated with the provision of a dedicated circuit. 
     According to the physical quantity sensor, since the ambient temperature of the oscillator is detected, and the reference voltage is varied based on the result of the detection, it is possible to provide the physical quantity sensor with a temperature compensation function. 
     According to the gyro sensor, since the modulated signal produced by delta-sigma modulating either one of the detection signal output from the oscillation circuit and the to-be-detected signal output from the oscillator is used to detect the other one of the detection signal output from the oscillation circuit and the to-be-detected signal output from the oscillator, it is possible to obtain a highly accurate angular velocity detection signal without being affected by harmonics superimposed on the detection signal. 
     Furthermore, according to the gyro sensor, by generating the feedback signal using the constant voltage source, an angular velocity detection signal unaffected by variations in supply voltage can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing one example of a prior art gyro sensor. 
         FIG. 2  is a diagram showing signal examples in a synchronous detection circuit  14  in the gyro sensor shown in  FIG. 1 . 
         FIG. 3  is a diagram showing a gyro sensor  100  as an example of a physical quantity sensor. 
         FIG. 4  is a diagram showing how various electrodes are connected to a crystal oscillator. 
         FIG. 5  is a diagram showing signal examples in the gyro sensor  100 . 
         FIG. 6  is a diagram showing an example of an acceleration sensor device that can be applied to the physical quantity sensor. 
         FIG. 7  is a diagram showing how various electrodes are connected to the device  120  shown in  FIG. 6 . 
         FIG. 8  is a diagram showing a gyro sensor  200  as another example of the physical quantity sensor. 
         FIG. 9  is a diagram showing a gyro sensor  201  as still another example of the physical quantity sensor. 
         FIG. 10  is a diagram showing a gyro sensor  202  as yet another example of the physical quantity sensor. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A physical quantity sensor will be described below with reference to the drawings. It will, however, be noted that the technical scope of the present invention is not limited to the specific embodiments described herein but extends to the inventions described in the appended claims and their equivalents. 
       FIG. 3  is a diagram showing a gyro sensor  100  as an example of the physical quantity sensor. 
     The gyro sensor  100  includes an oscillation circuit  30  and a detection circuit  50 , and the detection circuit  50  is constructed to output an angular velocity detection signal V 28 . 
     The oscillation circuit  30  includes a crystal oscillator  20 , an I/V conversion circuit  37 , an LPF  38 , an automatic gain control circuit (AGC)  39 , a variable gain amplifier (VGA)  40 , and a phase circuit  45 . 
       FIG. 4  is a diagram showing how various electrodes are connected to the crystal oscillator. 
     The crystal oscillator  20  is a three-tined oscillator having three tines, i.e., two driving tines  20   a  and  20   b  and one detection tine  20   c . Driving electrodes  23  and  24  are formed in pairs on each of the driving tines  20   a  and  20   b . When an AC drive voltage Vout is applied to the driving electrode  23 , the crystal oscillator  20  is set into vibration, and an AC output current Iout is output from the driving electrode  24 . The structure of the crystal oscillator  20  is not limited to the three-tined oscillator of the type shown in  FIG. 4 , but other suitable types of three-tined oscillator or two-tined tuning-fork oscillators or the like may be used. Further, the material for the oscillator is not limited to a crystal, but an piezoelectric material such as PZT may be used. 
     The driving electrode  23  includes driving electrodes  23   a  and  23   b  formed on two opposite sides of the driving tine  20   a  and driving electrodes  23   c  and  23   d  formed on two opposite sides of the driving tine  20   b . Likewise, the driving electrode  24  includes driving electrodes  24   a  and  24   b  formed on the other two opposite sides of the driving tine  20   a  and driving electrodes  24   c  and  24   d  formed on the other two opposite sides of the driving tine  20   b . The driving electrodes  23   a ,  23   b ,  23   c , and  23   d  are electrically coupled together and connected as the driving electrode  23  to the outside, while the driving electrodes  24   a ,  24   b ,  24   c , and  24   d  are electrically coupled together and connected as the driving electrode  24  to the outside. 
     Detection electrodes  25  and  26  are formed in pairs on the detection tine  20   c . The detection electrode  25  includes detection electrodes  25   a  and  25   b  formed on designated portions on opposite sides of the detection tine  20   c . Likewise, the detection electrode  26  includes detection electrodes  26   a  and  26   b  formed on other designated portions on the opposite sides of the detection tine  20   c . The detection electrodes  25   a  and  25   b  are electrically coupled together and connected as the detection electrode  25  to the outside, while the detection electrodes  26   a  and  26   b  are electrically coupled together and connected as the detection electrode  26  to the outside. 
     The I/V conversion circuit  37  in the oscillation circuit  30  takes as input the output current Iout flowing out of the driving electrode  24  of the crystal oscillator  20 , and outputs an AC signal V 1 . The LPF  38  takes the AC signal V 1  as input and outputs a filter output signal V 2 . The AGC  39  takes the AC signal VT as input, compares it with a predetermined reference voltage, and outputs a control voltage V 5 . The VGA  40  takes the filter output signal V 2  as input, and outputs, in response to the control voltage V 5 , the drive voltage Vout which is applied to the driving electrode  23  of the crystal oscillator  20 . 
     The phase circuit  45  outputs a detection signal V 9  after adjusting the phase of the AC signal V 1  so that the phase difference between the phase of the detection signal V 9  and the phase of the currents I 1  and I 2  as the signals to be detected becomes 0°. Alternatively, the phase circuit  45  may be configured to output the detection signal V 9  based on the drive voltage Vout. 
     With the above configuration of the oscillation circuit  30 , the crystal oscillator  20  is driven by the drive voltage Vout and continues to self-oscillate. As the crystal oscillator  20  continues to oscillate, the driving tines  20   a  and  20   d  vibrate in direction X (see  FIG. 4 ), and the detection tine  20   c  vibrates in the same direction X synchronously with the driving tines  20   a  and  20   d.    
     The detection circuit  50  comprises I/V conversion circuits  51  and  52 , a differential amplifier  53 , a detector circuit  60 , a second buffer  65  and a second inverting amplifier  66  both connected to a reference power supply (not shown), a second switch  67  which operates to select one or the other of the outputs of the second buffer  65  and second inverting amplifier  66 , a switch control unit  68 , a sigma-delta modulator  70 , and an LPF  80 . 
     The detector circuit  60  includes a first buffer  61  and a first inverting amplifier  62  both connected to the differential amplifier  53 , and a first switch  63  which operates to select one or the other of the outputs of the first buffer  61  and first inverting amplifier  62 . The delta-sigma modulator  70  includes an adder  71 , a loop filter  72 , an A/D converter  73 , and a D/A converter  74 . The A/D converter may be a single-bit converter, in which case the D/A converter  74  may be omitted.  FIG. 5  shows waveforms when the A/D converter is a single-bit converter. 
     The crystal oscillator  20  continues to oscillate with a constant amplitude under the control of the oscillation circuit  30 ; if, at this time, the crystal oscillator  20  is rotated with an angular velocity w, a Coriolis force F proportional to the angular velocity a) acts in direction Z at right angles to the direction of vibration (direction X) of the driving tines  20   a  and  20   b  of the crystal oscillator  20  (see  FIG. 4 ). The Coriolis force F is expressed as F=2·m·ω·V, where m represents the equivalent mass of the driving tines  20   a  and  20   b  or the detection tine  20   c , and V represents the velocity oscillating at the drive frequency f 0  (Hz). Due to the stress induced by the Coriolis force F, the crystal oscillator  20  is set into vibration at a frequency equal to the drive frequency, as a result of which electrical charges due to the piezoelectric effect are set up on the detection electrodes  25  and  26  formed on the detection tine. 
     These electrical charges cause the detection currents I 1  and I 2 , very small currents of opposite phases, to flow in the detection electrodes  25  and  26 , respectively. The I/V conversion circuits  51  and  52  in the detection circuit  50  convert the detection currents I 1  and I 2  into detection voltages V 10  and V 11 , respectively, and the differential amplifier  53  amplifies the difference between the detection voltages V 10  and V 11  to provide a difference output V 12 . 
     The first buffer  61  in the detector circuit  60  takes the difference output V 12  as input and produces an output V 20  which is the same as the difference output; on the other hand, the first inverting amplifier  62  inverts the difference output V 12  to produce an inverted output V 21 . Similarly, the second buffer  65  takes as input a reference voltage signal (Vstd) from the reference power supply and produces an output signal which is the same as the reference voltage signal; on the other hand, the second inverting amplifier  66  produces an output signal by inverting the reference voltage signal. 
     The output selected by the second switch  67  is supplied to the D/A converter  74  and converted into an analog signal. V 23  which is applied to the adder  71 . The adder  71  subtracts the analog signal V 23  from the detection signal V 9  output from the phase circuit  45  in the oscillation circuit  30 , and outputs the result as a subtraction output signal V 24 . The loop filter  72  takes the subtraction output signal V 24  as input, integrates this signal, and outputs the result as a filter output signal V 25 . The A/D converter  73  converts the filter output signal V 25  into a digital signal V 26  for output. 
     When the digital signal V 26  is high, the switch control unit  68  controls the first switch  63  to select the output V 20  of the first buffer  61  and controls the second switch  67  to select the output of the second buffer  65 . When the digital signal V 26  is low, the switch control unit  68  controls the first switch  63  to select the output V 21  of the first inverting amplifier  62  and controls the second switch  67  to select the output of the second inverting amplifier  66 . 
       FIG. 5  is a diagram showing signal examples in the gyro sensor  100 . 
     In  FIG. 5( a ) , the output V 20  of the first buffer  61  is indicated by a solid line, while the output V 21  of the first inverting amplifier  62  is indicated by a dashed line. The voltage waveform of the output V 20  is the same as that of the difference output V 12  of the differential amplifier  53 , and corresponds to the signal detected based on the signals output from the detection tine  20   c  of the crystal oscillator  20 . 
     The output of the A/D converter  73  in the delta-sigma modulator  70  is applied as a control signal to the second switch  67  which is thus operated to switch at a rate sufficiently higher than the detection signal V 9 , and its output V 22  is converted by the D/A converter into an analog signal to produce the D/A converter output signal V 23 . The adder  71  compares the detection signal V 9  with the output V 23  of the D/A converter  74 , the difference is integrated by the loop filter  72 , and the result is fed back to the A/D converter  73 . 
     In this way, the delta-sigma modulator  70  creates the delta-sigma modulated digital signal. V 26  (see  FIG. 5( b ) ) from the detection signal V 9  output from the phase circuit  45  in the oscillation circuit  30 . The voltage value when the digital signal V 26  is high is approximately equal to the voltage Vstd of the reference power supply. The switch control unit  68  controls the switching operation of the first switch  63  based on the digital signal V 26  output from the A/D converter  73 ; this is equivalent to synchronously detecting the difference signal V 12 , i.e., the signal to be detected, by using the digital signal V 26 . The LPF  80  removes the AC component from the output signal V 27  (see  FIG. 5( c )  of the first switch  63  and outputs the angular velocity detection signal V 28  (see  FIG. 5( d ) ) which is a DC voltage proportional to the angular velocity. 
     The digital signal V 26  output from the A/D converter  73  is produced by converting the detection signal V 9  into digital form, and does not contain any particular frequency component other than the detection signal V 9  and the sampling frequency of the A/D converter  73 . Accordingly, even when an odd-order harmonic of V 9  is superimposed on the detection signal, as previously described with reference to  FIG. 2( c ) , it will, have very little effect on the angular velocity detection signal V 28 . 
     From the circuit configuration of the detection circuit  50 , it is considered that the following equation holds.
 
(( LV 9− LV 22· DA )· LF+E )· LVstd=LV 22
 
hence
 
 LV 22= LV 9· LF·Vstd /(1 +DA·LF·Vstd )+ E·Vstd /(1 +DA·LF·Vstd )
 
IF represents the transfer function of the loop filter  72 , DA the transfer function of the D/A converter  74 , e the quantization noise in the A/D converter  73 , and E the result of the Laplace transform of the quantization noise e. Further, LV 27  represents the result of the Laplace transform of the output signal V 27  of the first switch  63 , LV 22  the result of the Laplace transform of the output signal V 22  of the second switch  67 , and LV 9  the result of the Laplace transform of the detection signal V 9 .
 
     In the above equation, if DA·LF·Vstd&gt;&gt;1, then LV 22 ≈LV 9 /DA; assuming that DA≈1, the following equation holds.
 
 LV 22 ≈LV 9  (1)
 
     Further, based on the similarity between the detector circuit  60  and the circuit comprising the second buffer  65  and second inverting amplifier  66  connected to the reference power supply and the second switch  67 , it is apparent that the relationship between LV 12  and LV 27  is the same as the relationship between Vstd and LV 22 ; therefore, the relation defined by the following equation (2) holds.
 
 LV 27= LV 12· LV 22 /Vstd   (2)
 
     From the equations (1) and (2), the relation defined by the following equation (3) holds.
 
 LV 27= LV 9· LV 12 /Vstd   (3)
 
     Thus, it can be understood that the output signal V 27  of the first switch  63  in the detection circuit  50  is proportional to the product of the detection signal V 9  and the difference signal V 12  to be detected. 
     In the above gyro sensor  100 , since the synchronous detection is performed by using the digital signal that is converted from the detection signal V 9  by the delta-sigma modulator  70 , errors can be prevented from occurring in the angular velocity detection signal V 28  due to harmonics induced by such factors as periodic external mechanical vibrations, etc. Furthermore, since the detection is performed by using the switching of the first switch  63 , there is also offered the advantage that the entire gyro sensor  100  can be implemented in CMOS. 
     It should also be noted that the gyro sensor  100  uses the delta-sigma modulator  70 ; in the case of delta-sigma modulation, by suitably setting the loop filter  72 , quantization noise can be shifted toward higher frequencies, reducing the noise at lower frequencies (noise shaving). This offers the advantage that the quantization noise of the A/D converter  73 , superimposed on the detected low-frequency components important to the gyro sensor, can be reduced. 
       FIG. 6  is a diagram showing an example of an acceleration sensor device that can be applied to the physical quantity sensor. 
     The device  120  shown in  FIG. 6  comprises a first tuning-fork crystal oscillator  121 , a second tuning-fork crystal oscillator  122 , and a base joint  123 . The first tuning-fork crystal oscillator  121  on the driving side include a first driving tine  121   a  and a second driving tine  121   b , and the second tuning-fork crystal oscillator  122  on the detection side include a first detection tine  122   a  and a second detection tine  122   b.    
     When an AC voltage is applied across the driving electrodes of the first tuning-fork crystal oscillator  121 , the first driving tine  121   a  and the second driving tine  121   b  are caused to vibrate in such a manner as to twist about the Y′ axis in opposite phase to each other and continue to vibrate in this fashion. In this condition, when acceleration occurs in the direction of ±Z axis symmetrical about the XY plane, vibrations of another mode are generated in the first driving tine  121   a  and the second driving tine  121   b  due to Coriolis forces. The generated vibrations are propagated via the base joint  123  to the second tuning-fork crystal oscillator  122  on the detection side. The propagated vibrations cause the first detection tine  122   a  and second detection tine  122   b  of the second tuning-fork crystal oscillator  122  to vibrate in such a manner as to twist about the Y′ axis in opposite phase to each other. An acceleration signal proportional to the acceleration can be obtained by detecting the AC signal generated by the vibrations. 
       FIG. 7  is a diagram showing how various electrodes are connected to the device  120  shown in  FIG. 6 . 
     The first driving tine  121   a  of the first tuning-fork crystal oscillator  121  is provided with an outside driving electrode  124   a , a middle driving electrode  124   b , and an inside driving electrode  124   c , formed on the upper face as viewed from the direction of its Z′ axis, and an outside driving electrode  124   d , a middle driving electrode  124   e , and an inside driving electrode  124   f , formed on the lower face as viewed from the direction of its Z′ axis. Likewise, the second driving tine  121   b  of the first tuning-fork crystal oscillator  121  is provided with an outside driving electrode  125   c , a middle driving electrode  125   b , and an inside driving electrode  125   a , formed on the upper face as viewed from the direction of its Z′ axis, and an outside driving electrode  125   f , a middle driving electrode  125   e , and an inside driving electrode  125   d , formed on the lower face as viewed from the direction of its Z′ axis. 
     The electrodes  124   a ,  124   c ,  124   e ,  125   b ,  125   d , and  125   f  are electrically coupled together and connected as the driving electrode  23  to the outside. On the other hand, the electrodes  124   b ,  124   d ,  124   f ,  125   a ,  125   c , and  125   e  are electrically coupled together and connected as the driving electrode  24  to the outside. 
     The first detection tine  122   a  of the second tuning-fork crystal oscillator  122  is provided with an electrode  126   a  formed on the upper face as viewed from the direction of its Z′ axis, an electrode  126   c  on the lower face, and electrodes  126  and  126   d  formed on both side faces. Likewise, the second detection tine  122   b  of the second tuning-fork crystal oscillator  122  is provided with an electrode  127   a  formed on the upper face as viewed from the direction of its Z′ axis, an electrode  127   c  on the lower face, and electrodes  127   b  and  127   d  formed on both side faces. 
     The electrodes  126   b ,  126   d ,  127   a , and  127   c  are electrically coupled together and connected as the detection electrode  25  to the outside. On the other hand, the electrodes  126   a ,  126   c ,  127   b , and  127   d  are electrically coupled together and connected as the detection electrode  26  to the outside. 
     By applying a prescribed AC voltage Vout across the driving electrodes  23  and  24  shown in  FIG. 7  from the oscillation circuit  30  shown in  FIG. 3 , the first driving tine  121   a  and the second driving tine  121   b  can be made to continue to vibrate in such a manner as to twist about the Y′ axis in opposite phase to each other. In this case, when the voltage (corresponding to V 9 ) output from the phase circuit  45  in the oscillation circuit  30  and the currents (corresponding to I 1  and I 2 ) output from the detection electrodes  25  and  26  connected to the second tuning-fork crystal oscillator  122  are applied to the detection circuit  50 , the signal proportional to the acceleration exerted on the device  120  can be obtained from the output V 28  of the detection circuit  50 . In this way, the configuration of the physical quantity detection sensor according to the present invention applied to the gyro sensor  100  shown in  FIGS. 3 to 5  can also be applied to the acceleration sensor. The device  120  shown as a device constituting the acceleration sensor in  FIGS. 6 and 7  is only one example, and is not limited to any particular example. 
       FIG. 8  is a diagram showing a gyro sensor  200  as another example of the physical quantity sensor. 
     In the gyro sensor  200  shown in  FIG. 8 , the same component elements as those in  FIG. 3  are designated by the same reference numerals, and such component elements will be not further described herein. The gyro sensor  200  shown in  FIG. 8  differs from the gyro sensor  100  shown in  FIG. 3  by the inclusion of a detection circuit  210  which contains a power supply circuit  220  capable of outputting a variable voltage, not a constant voltage, instead of the reference voltage signal (Vstd) in the detection circuit  50  of the gyro sensor  100 . 
     The power supply circuit  220  includes a digital-analog converter (DAC)  90  connected to a reference power supply, a control circuit  91  which outputs a setting signal for setting the output of the DAC  90 , and a memory  92  which stores a plurality of setting data. 
     In the gyro sensor  100  shown in  FIG. 3 , from the fact that the output signal V 27  whose AC component has been removed by the LPF  80  provides the angular velocity detection signal V 28  and from the earlier given equation (3), the relationships among the angular velocity detection signal V 28 , the voltage V 10  corresponding to the signal I 1  to be detected, the voltage V 11  corresponding to the signal I 2  to be detected, the detection signal V 9 , and the reference voltage signal (Vstd) can be expressed as shown by the following equation (4),
 
 V 28=( V 11− V 10) V 9/ Vstd   (4)
 
     From the equation (4), it is seen that by using the output voltage V 30  of the DAC  90  instead of the reference voltage signal (Vstd), and by varying the value of the output voltage V 30 , the gain of the angular velocity detection signal V 28  can be adjusted in the gyro sensor  200 . More specifically, as the output voltage V 30  is increased, the value of the angular velocity detection signal V 28  decreases, and as the output voltage V 30  is reduced, the value of the angular velocity detection signal V 28  increases. 
     The value of the output angular velocity detection signal V 28  can vary due to differences in characteristics between each individual crystal oscillator  20 . To address this, the output range is designed for the output voltage V 30  so as to be able to compensate for the individual differences expected to exist in the characteristics of the crystal oscillator  20 . Then, when the characteristics of the crystal oscillator  20  mounted in the gyro sensor  100  are identified, the memory  92  is updated and the output voltage V 30  is set so as to compensate for the individual differences expected to exist in the characteristics of the crystal oscillator. The value to be written to the memory  92  may be determined from the characteristics of the crystal oscillator  20  itself or from the result of the measurement of the angular velocity detection signal V 28 . In any case, the control circuit  91  controls the DAC  90  to output the output voltage V 30  best suited to the type of the crystal oscillator  20  mounted in the gyro sensor  100 . 
     With the provision of the above power supply circuit  220 , it becomes possible to output the angular velocity detection signal V 28  that is substantially unaffected by the individual differences existing in the characteristics of the crystal oscillator  20 . That is, the power supply circuit  220  adds a variable gain amplification function (sensitivity compensation function) to the detection circuit  210 , but since there is no need to provide a dedicated variable gain amplifier circuit, there is the further advantage of being able to prevent problems such as the generation of noise and an increase in the amount of circuitry associated with the provision of a dedicated circuit. 
       FIG. 9  is a diagram showing a gyro sensor  201  as still another example of the physical quantity sensor. 
     In the gyro sensor  201  shown in  FIG. 9 , the same component elements as those in  FIG. 3  are designated by the same reference numerals, and such component elements will be not further described herein. The gyro sensor  201  shown in  FIG. 9  differs from the gyro sensor  100  shown in  FIG. 3  by the inclusion of a detection circuit  211  which contains a power supply circuit  221  capable of outputting a variable voltage, not a constant voltage, instead of the reference voltage signal (Vstd) in the detection circuit  50  of the gyro sensor  100 . 
     The power supply circuit  221  includes a digital-analog converter (DAC)  90  connected to a reference power supply, a control circuit  91  which outputs a setting signal for setting the output of the DAC  90 , and a temperature sensor  93  which detects the ambient temperature of the crystal oscillator  20  and outputs a temperature signal proportional to the detected temperature. 
     Since the characteristics of the crystal oscillator  20  are temperature dependent, when the ambient temperature changes, the value of the output angular velocity detection signal V 28  changes correspondingly. In view of this, the control circuit  91  varies the output of the DAC  90  in accordance with the output from the temperature sensor  93 . 
     With the provision of the above power supply circuit  221 , it becomes possible to output the angular velocity detection signal V 28  that is substantially unaffected by the temperature characteristics of the crystal oscillator  20 . That is, the power supply circuit  221  adds a temperature compensation function to the detection circuit  211 . 
       FIG. 10  is a diagram showing a gyro sensor  202  as yet another example of the physical quantity sensor. 
     In the gyro sensor  202  shown in  FIG. 10 , the same component elements as those in  FIG. 3  are designated by the same reference numerals, and such component elements will be not further described herein. The gyro sensor  202  shown in  FIG. 10  differs from the gyro sensor  100  shown in  FIG. 3  by the inclusion of a detection circuit  212  which contains a power supply circuit  222  capable of outputting a variable voltage, not a constant voltage, instead of the reference voltage signal (Vstd) in the detection circuit  50  of the gyro sensor  100 . 
     The power supply circuit  222  includes a digital-analog converter (DAC)  90  connected to a reference power supply, a control circuit  91  which outputs a setting signal for setting the output of the DAC  90 , a memory  92  which stores a plurality of setting data, and a temperature sensor  93  which detects the ambient temperature of the gyro sensor  202  and outputs a temperature signal corresponding to the detected temperature. 
     In the power supply circuit  222 , in order to compensate for the temperature dependence of the crystal oscillator  20  as well as the individual differences in the characteristics thereof, the memory  92  stores data of the characteristics of the crystal oscillator, and the temperature sensor  93  is provided for compensating for the temperature characteristics of the crystal oscillator. Accordingly, the power supply circuit  222  outputs the output voltage V 30  that has been corrected by the temperature signal from the temperature sensor  93  on the basis of the voltage corresponding to the characteristics of the crystal oscillator  20  stored as data in the memory  92 . Differences in temperature characteristics between each individual crystal oscillator may also exist. In view of this, the output voltage  30  corrected by the temperature signal from the temperature sensor  93  may be further corrected by the control circuit  91  so as to compensate for the individual differences existing in the temperature characteristics of the crystal oscillator. 
     With the provision of the above power supply circuit  222 , it becomes possible to output the angular velocity detection signal V 28  that is substantially unaffected not only by the individual differences existing in the characteristics of the crystal oscillator  20  but also by the temperature characteristics thereof. That is, the power supply circuit  222  serves to add a variable gain amplification function (sensitivity compensation function) and a temperature compensation function to the detection circuit  212 . 
     The gyro sensors  100 ,  200 ,  201 , and  202  described above have been configured to detect the difference signal V 12  by using the digital signal that is converted from the detection signal V 9  by the delta-sigma modulator  70 . Alternatively, the gyro sensors may be configured to detect the detection signal V 9  by using a digital signal that is converted from the difference signal V 12  by the delta-sigma modulator  70 . 
     Further, the gyro sensors  100 ,  200 ,  201 , and  202  described above may be configured to detect the difference signal V 12  by using a digital signal that is converted from an inverted version of the detection signal V 9  by the delta-sigma modulator  70 , or may be configured to detect the detection signal V 9  by using a digital signal that is converted from an inverted version of the difference signal V 12  by the delta-sigma modulator  70 . 
     The physical quantity sensor described above can be applied advantageously to a sensor, such as a gyro sensor or an acceleration sensor, that measures a physical quantity by using a crystal oscillator.