USOO593.9633A
`5,939,633
`Patent Number:
`11
`Aug. 17, 1999
`(45) Date of Patent:
`OTHER PUBLICATIONS
`Ahmad et al., “ATwo-Dimensional Micromachined Accel
`erometer, IEEE Transactions On Instrumentation and Mea
`Surement, vol. 46, No. 1, Feb. 1997.
`
`United States Patent (19)
`Judy
`54). APPARATUS AND METHOD FOR MULTI-
`AXIS CAPACTIVE SENSING
`75 Inventor: Michael W. Judy, North Andover
`Mass
`y,
`s
`73 Assignee: Analog Devices, Inc., Norwood, Mass.
`Primary Examiner–Hezron Williams
`21 Appl. No.: 08/878,192
`ASSistant E.
`Helen C. Kwok
`EXfile-CIC . SWO)
`SSIS
`Attorney, Agent, or Firm-Hale and Dorr LLP
`22 Filed:
`Jun. 18, 1997
`5 7
`(51) Int. Cl." ................................................... G01P 15/125
`ABSTRACT
`57
`52 U.S. Cl. ..................................... 73/514.32; 73/514.18;
`A device for detecting with differential capacitors accelera
`73/862.626
`9.
`p
`58) Field of Seth 17.50402.504 o3. 3. tions in more than one orientation through time-division
`504 15 s62 61 s62 626. 361.2so 283.3
`multiplexing. A micromachined mass is movable along or
`• -u- as
`• / us
`•
`a s/s
`s
`about any axis in response to a force. The mass forms the
`References Cited
`common electrode of a set of differential capacitors, wherein
`the other electrodes of each differential capacitor are fixed.
`U.S. PATENT DOCUMENTS
`With each differential capacitor, one fixed electrode is set to
`one Voltage and the other fixed electrode is set to a Second
`4,932,259 6/1990 Ueno ....................................... 3.10/329
`Voltage. The mass is connected to the input of an amplifier
`4,987,779
`1/1991 McBrien ..........
`. 73/514.18
`5,134,881
`8/1992 Henrion et al...
`73/514.18
`and to a Switch for connecting the mass to a fixed Voltage.
`5,345,824 9/1994 Sherman et al. .......................... 73/517
`The output of the amplifier is coupled to a demodulator for
`5,440,939 8/1995 Barny et al. .....
`73/514.18
`each orientation. A timing circuit activates one demodulator
`5,441,300 8/1995 Yokota et al. ...
`... 280/735
`at a time. By toggling the Voltages on the fixed electrodes of
`5,487.305
`1/1996 Ristic et al. ..
`73/514.32
`5,511,420
`4/1996 Zhao et al.....
`... 73/514.32
`the differential capacitor corresponding to the active
`5,748,004 5/1998 Kelly et al. m 73/514.32
`demodulator, the movement of the mass in the orientation
`corresponding to the active demodulator can be determined.
`FOREIGN PATENT DOCUMENTS
`19537 546
`27 Claims, 4 Drawing Sheets
`A1 4/1997 Germany.
`
`56)
`
`
`
`VHIGH
`VLow
`
`VHIGH VLow
`
`APPLE 1136
`Apple v. Logan Tree
`IPR2022-00037
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`1
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`U.S. Patent
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`Aug. 17, 1999
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`Aug. 17, 1999
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`Sheet 1 of 4
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`Sheet 1 of 4
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`5,939,633
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`5,939,633
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`U.S. Patent
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`2
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`U.S. Patent
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`Aug. 17, 1999
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`Sheet 2 of 4
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`5,939,633
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`TOH_LNOO HE
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`U.S. Patent
`U.S. Patent
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`Aug. 17, 1999
`Aug. 17, 1999
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`Sheet 3 of 4
`Sheet 3 of 4
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`5,939,633
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`g
`FIG.3
`
`1PERIOD
`
`XpRIVE
`
`YDRIVE
`
`RESET
`
`SWwx1
`
`SWX2
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`SWY'1
`
`SWY2
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`4
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`U.S. Patent
`U.S. Patent
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`Aug. 17, 1999
`Aug. 17, 1999
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`Sheet 4 of 4
`Sheet 4 of 4
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`5,939,633
`5,939,633
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`FIG.4
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`
`
`1PERIOD—————>
`
`XDRIVE
`
`YDRIVE
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`RESET
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`SWXx1
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`SWX2
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`SWwyY1
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`SWY2
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`5
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`toggled between the two Voltage levels. That is, first the
`APPARATUS AND METHOD FOR MULTI
`higher Voltage is applied to one of the fixed electrodes and
`AXIS CAPACTIVE SENSING
`the lower Voltage is applied to the other fixed electrode, and
`then the Voltages are Switched. The resulting Voltage on the
`FIELD OF THE INVENTION
`Structure for each Setting is amplified and applied to a
`This invention relates to the field of capacitive Sensing
`demodulator. The difference between the two voltages is
`circuits and, more particularly, to capacitive circuits for
`used to determine the displacement along the axis and
`Sensing in more than one orientation.
`therefore the acceleration.
`Measurements are taken for each axis. In a preferred
`BACKGROUND OF THE INVENTION
`embodiment, a separate demodulator is used for the mea
`Individual Sensors are frequently used to measure a force
`Surements along each axis. Preferably, a reset Voltage,
`or an acceleration in each of Several axes of interest. For
`midway between the Voltages applied to the fixed electrodes,
`example, to measure acceleration along the X- and Y-axes,
`is applied to the Structure before measurements are taken for
`two Sensors are used. One has its Sensitive axis along the
`a different axis. The reset Voltage forces the Structure to that
`X-axis and the other has its Sensitive axis along the Y-axis.
`middle Voltage. A timing circuit is used to determine when
`These Sensors can be formed from micromachined Silicon
`to toggle the Voltages on the fixed electrodes, when to apply
`Structures. In Such Sensors, a movable mass is positioned
`the reset Voltage, and which demodulator registers the
`between two plates So that one capacitor is formed by one
`Voltage on the Structure. Many different timing Sequences
`plate and the mass and a Second capacitor is formed by the
`are possible.
`Second plate and the mass.
`The application of a force along a Sensitive axis causes the
`BRIEF DESCRIPTION OF THE DRAWINGS
`mass to move relative to the plates, causing a change in the
`FIG. 1 is a partial block, partial Schematic diagram of a
`capacitances in the two capacitors of the differential capaci
`first embodiment of the present invention.
`tor. This causes a Signal to appear on the mass that reflects
`FIG. 2 is a partial block, partial Schematic diagram of a
`the amount of acceleration. An accelerometer based on this
`Second embodiment of the present invention.
`principle and a process for fabricating Such an accelerometer
`FIG. 3 is a timing diagram illustrating one possible timing
`are described in commonly assigned U.S. Pat. Nos. 5,345,
`sequence for the circuit shown FIG. 1.
`824 and 5,314,572, which are incorporated herein by refer
`FIG. 4 is a timing diagram illustrating a Second possible
`CCC.
`timing Sequence for the circuit shown in FIG. 1.
`As the mass in Such an accelerometer (or other sensor)
`forms a Single electric node, and is used to output a signal
`DETAILED DESCRIPTION OF PREFERRED
`corresponding to the acceleration or force, it has proven
`EMBODIMENTS
`difficult to measure acceleration or a force in more than one
`With reference to FIG. 1, micromachined mass 20 is
`axis using one movable mass.
`Suspended by four Suspension arms 21 and four anchorS 22
`The present invention overcomes this and other problems
`above substrate 24. Mass 20 is movable along both the X
`as will be shown in the remainder of the specification
`and Y-axes relative to Substrate 24. It is not necessary that
`referring to the attached drawings.
`mass 20 respond equally to forces of the same magnitude
`applied along different axes. Alternatively, mass 20 also
`SUMMARY OF THE INVENTION
`could be movable along the Z-axis.
`A circuit measures the acceleration of or force applied to
`Fingers 26 extend from opposite Sides of mass 20, along
`a device in more than one orientation through the use of
`the Y-axis. Fingers 28 extend from the other two sides of
`time-division multiplexing. This circuit is particularly Suited
`mass 20, along the X-axis. Mass 20, including fingers 26 and
`for applications in which full-scale accelerations on the
`28 and Suspension arms 21, is formed from polysilicon and
`order of 5-10 g’s are to be measured. In a preferred
`constitutes a Single electric node. AnchorS 22 also are
`embodiment, fingers on a micromachined Structure Serve as
`formed from polysilicon. Preferably, Suspension arms 21 are
`the common electrode for two differential capacitors. The
`Serpentine-shaped.
`Structure (and its fingers) is movable while the other elec
`Polysilicon fingers 34 extend parallel to and to one side of
`trodes of the two differential capacitors are fixed. The
`each finger 26. Polysilicon fingers 36 extend parallel to and
`differential capacitors are positioned along different axes. AS
`to the other side of each finger 26. Thus, movement of mass
`a result, if a force is applied to the device, the Structure is
`20 to the right along the X-axis brings each finger 26 closer
`displaced a distance proportional to the magnitude of the
`to its corresponding finger 36 and further from its corre
`force along each axis. The displacement along a given axis
`sponding finger 34. Fingers 34 are all electrically connected,
`increases the capacitance of one of the capacitors and
`and fingers 36 are all electrically connected.
`decreases the capacitance of the other capacitor of the
`Similarly, electrically connected polysilicon fingers 38
`differential capacitor positioned along that axis.
`extend parallel to and to one side of each finger 28, and
`The two fixed electrodes of each differential capacitor are
`electrically connected polysilicon fingers 40 extend on the
`at different Voltages. In a preferred embodiment, each
`other side of each finger 28.
`capacitor has the same capacitance when no force is applied
`Fingers 34, 36, 38, and 40 are stationary relative to
`to the device, the fixed electrodes are at Ground (0 volts) and
`Vcc (5 volts), and the fingers of the structure are equidistant
`Substrate 24. Although, for clarity, only three sets of fingers
`are shown on each Side of mass 20, in a preferred embodi
`between the fixed electrodes when no force is applied to the
`ment twenty or more Sets of fingers are employed on each
`device.
`The displacement of the Structure in response to a force
`side of mass 20.
`changes the Voltage of the Structure. To determine the
`Fingers 26, 34, and 36 form the electrodes of differential
`capacitor 44, which consists of capacitor 46 (formed from
`displacement along an axis, the fixed electrodes of the
`fingers 26 and 34) and capacitor 48 (formed from fingers 26
`differential capacitor Sensitive to forces on that axis are
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`and 36), so that fingers 26 on mass 20 form the common
`At the beginning of a cycle, Xdrive is high and Ydrive is
`electrode of differential capacitor 44. Similarly, fingers 28,
`high. Shortly after Xdrive becomes low, a reset pulse is
`38, and 40 form differential capacitor 50, which consists of
`applied to reset Switch 74, Setting the Voltage on mass 20
`capacitor 52 (formed from fingers 28 and 38) and capacitor
`(and its fingers 26 and 28) to Vreset. By closing the first
`54 (formed from fingers 28 and 40).
`switch of X-axis demodulator 78 (SWx1) for a period before
`Line 70 connects one of the anchors 22 (as well as fingers
`Xdrive goes high and the Second Switch of X-axis demodu
`26 and 28, and mass 20) to the input to amplifier 72. Line 70
`lator 78 (SWX2) for a period after Xdrive goes high, a
`also connects to Switch 74. Preferably, Switch 74 is a
`voltage is obtained at the output of X-axis demodulator 78
`transistor. When Switch 74 is closed, reset voltage 76 is
`that is proportional to the displacement of mass 20 along the
`coupled to line 70 and mass 20 through switch 74.
`X-axis.
`The output of amplifier 72 connects to the input of X-axis
`While Xdrive is high and Ydrive is low, the first Switch of
`demodulator 78 and to the input of Y-axis demodulator 80.
`Y-axis demodulator 80 (SWy1) is closed for a period before
`X-axis demodulator 78 also has an offset input 82. The
`Ydrive goes high. The second Switch of Y-axis demodulator
`output of X-axis demodulator 78 is fed to adjustable buffer
`80 (SWy2) is closed for a period after Ydrive goes high, and
`amplifier86. Similarly, Y-axis demodulator 80 has an offset
`a voltage is obtained at the output of Y-axis demodulator 80
`input 84, and its output is fed to buffer amplifier88. Buffer
`that is proportional to the displacement of mass 20 along the
`amplifier88 is adjustable independent of buffer amplifier86.
`The adjustments of buffer amplifiers 86 and 88 can be used
`Y-axis.
`Many other timing Sequences can be used, as long as
`to account for sensitivity differences between the X- and
`Xdrive switches to the opposite state and Ydrive remains in
`Y-axes.
`the same state between the period in which SWx1 is toggled
`Offset inputs 82 and 84 are used to set the d.c. offset of the
`and the period in which SWX2 is toggled; and as long as
`demodulators and for offset correction of the Sensor.
`Ydrive Switches to the opposite state and Xdrive remains in
`Although a single amplifier 72 is shown, a separate amplifier
`the same state between the period in which SWy1 is toggled
`can be used before each demodulator.
`and the period in which SWy2 is toggled. Preferably, a reset
`Demodulators 78 and 80 are discrete time demodulators.
`pulse is applied to reset Switch 74 after both the X-axis and
`As shown with respect to demodulator 78, the demodulator
`the Y-axis have been sampled. The reset pulse can be after
`can include first capacitor 62, Second capacitor 64, first
`any integer number of X-axis and Y-axis Sampling pairs, or
`demodulator Switch 66, and second demodulator Switch 68.
`after each X-axis and Y-axis Sample.
`The output of amplifier 72 connects to one electrode of first
`capacitor 62. The other electrode of first capacitor 62
`In a second embodiment, selectors 92 and 94 each can
`provide either Vreset on both outputs or Vhigh on one output
`connects to one port of first demodulator Switch 66 and one
`and Vlow on the other output. A timing Sequence for this
`port of second demodulator Switch 68. The second port of
`Second embodiment is shown in FIG. 4. In this Sequence,
`first demodulator Switch 66 connects to reset voltage 76. The
`second port of second demodulator Switch 68 connects to
`Xdrive and Ydrive are each at Vreset for one half of each
`cycle, then at Vhigh for a quarter cycle, and then at Vlow for
`one electrode of Second capacitor 64 and to buffer amplifier
`a quarter cycle. Ydrive is 180 degrees out of phase from
`86. The other port of second capacitor 64 connects to
`ground. Alternatively, other discrete time demodulators may
`Xdrive.
`At the beginning of a cycle, with Xdrive high and Ydrive
`be used.
`at Vreset, a reset pulse is applied to reset Switch 74. By
`Timer 90 receives clock signals from clock 96, and
`closing SWX1 for a period before drive goes low and SWX2
`provides timing Signals to reset Switch 74, and to control
`for a period after Xdrive goes low, a Voltage is obtained at
`X-axis demodulator 78, Y-axis demodulator 80, and selec
`the output of X-axis demodulator 78 that is proportional to
`tors 92 and 94.
`the displacement of mass 20 along the X-axis.
`Fingers 34 and 36 are connected to first and second
`Halfway through the cycle, after Xdrive is at Vreset and
`outputs of selector 92 and fingers 38 and 40 are connected
`Ydrive is high, another reset pulse is applied to reset Switch
`to first and second outputs of selector 94.
`74. By closing SWy1 for a period before Yarive goes low
`In one embodiment, selectors 92 and 94 each provide a
`and SWy2 for a period after Yarive goes low, a voltage is
`high voltage (Vhigh) on one of the two outputs and a low
`obtained at the output of Y-axis demodulator 80 that is
`voltage (Vlow) on the other output. Timer 90 is used to
`proportional to the displacement of mass 20 along the
`Select the Voltage (Vhigh or Vlow) to appear on each of the
`outputs. Preferably, Vhigh is Vcc (e.g., 5 volts) and Vlow is
`Y-axis.
`With either embodiment, the cycling must be sufficiently
`Ground (0 volts). Reset voltage 76 (Vreset) is midway
`faster than the highest frequency to which the mass
`between Vhigh and Vlow.
`responds, So that the mass does not move before the mea
`Mass 20 will be displaced relative to fingers 34 and 36 if
`Surements are completed. In a preferred embodiment, the
`a force has been applied to the device that has a component
`mass generally is not responsive to frequencies greater than
`along the X-axis, and will be displaced relative to fingers 38
`10 kilohertz and the cycling is performed at 100 kilohertz or
`and 40 if the force has a component along the Y-axis. This
`displacement of mass 20 will alter the voltage on mass 20,
`OC. Preferably, all of the components shown in FIG. 1 are
`due to the affects of differential capacitors 44 and 50.
`located on a single integrated circuit. However, any group of
`A timing Sequence used to determine the displacements of
`the components, such as demodulators 78 and 80 and timer
`mass 20 along the X- and Y-axes for this first embodiment
`90, could be located on one or more separate chips.
`is shown in FIG. 3. Xdrive is the signal (Vhigh or Vlow) that
`An alternative Structure for the multi-axis Sensing appa
`appears at the first output of Selector 92, it being understood
`ratus of the present invention is shown in FIG. 2. In this
`that if Vhigh appears at the first output then Vlow appears
`at the Second output (and Vice versa), and Ydrive is the
`embodiment, the device is Sensitive to rotation about the
`Signal (Vhigh or Vlow) that appears at the first output of
`Z-axis or acceleration along the Z-axis.
`Polysilicon mass 120 is suspended above substrate 124 by
`selector 94. Xdrive and Ydrive are square waves, with
`four Serpentine-shaped Suspension arms 121 and four
`Ydrive 90 degrees out of phase from Xdrive.
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`electrically connected to the third electrode of the
`anchorS 122, So that it is movable along and rotatable about
`Second differential capacitor;
`the Z-axis. Fingers 126 extend from opposite Sides of mass
`an amplifier having an input electrically coupled to the
`120 along the Y-axis. Alternatively, additional fingers 126
`third electrode of the first differential capacitor and the
`could extend along the X-axis.
`third electrode of the Second differential capacitor;
`Polysilicon fingers 134 extend parallel to each finger 126.
`a first demodulator having an input electrically coupled to
`On one side of mass 120, fingers 134 are to the left of each
`an output of the amplifier;
`finger 126. On the opposite side of mass 120, fingers 134 are
`a Second demodulator having an input electrically coupled
`to the right of each finger 126. Polysilicon fingers 136
`to the output of the amplifier; and
`extend parallel to and to the other side of each finger 126
`a control timer electrically coupled to the first demodu
`from the corresponding finger 134. Thus, clockwise rotation
`lator and the Second demodulator.
`of mass 120 about the Z-axis brings each finger 126 of mass
`2. The device according to claim 1, further comprising a
`120 closer to an adjacent finger 136 and further from an
`first Selector having a first output coupled to the first fixed
`adjacent finger 134. Fingers 134 are all electrically
`electrode of the first differential capacitor and a Second
`connected, and fingerS 136 are all electrically connected.
`output coupled to the Second fixed electrode of the first
`Polysilicon plate 138 extends parallel to and above (along
`differential capacitor, and a Second Selector having a first
`the Z-axis) mass 120. Polysilicon plate 140 (shown with
`output coupled to the first fixed electrode of the Second
`dashed lines within cutaway area 139) extends on the
`differential capacitor and a Second output coupled to the
`Substrate below mass 120.
`Second fixed electrode of the Second differential capacitor;
`and wherein the control timer is electrically coupled to a
`Fingers 134 and 136, and plates 138 and 140 are all
`Stationary relative to Substrate 124. Although, for clarity,
`control input of the first Selector and a control input of the
`only three Sets offingers are shown on the Sides of mass 120,
`Second Selector.
`3. The device according to claim 2, further comprising a
`in a preferred embodiment twenty or more Sets of fingers
`reset Switch having a first port electrically coupled to the
`would be employed on each Side.
`third electrode of the first differential capacitor and the third
`Fingers 126, 134, and 136 form the electrodes of differ
`electrode of the Second differential capacitor, and a control
`ential capacitor 144, so that fingers 126 on mass 120 form
`input electrically coupled to the control timer.
`the common electrode of differential capacitor 144.
`4. The device according to claim 3, wherein the reset
`Similarly, mass 120, plate 138, and plate 140 form the
`Switch includes a Second port and further comprising means
`electrodes of differential capacitor 150, in which mass 120
`for coupling the Second port of the reset Switch to a fixed
`again is the common electrode.
`Voltage level.
`The other components (shown by line 170, amplifier 172,
`5. The device according to claim 3, wherein the first
`switch 174, reset voltage 176, demodulators 178 and 180,
`differential capacitor is responsive to a force in a first
`offset inputs 182 and 184, buffer amplifiers 186 and 188,
`orientation and the Second differential capacitor is respon
`timer 190, selectors 192 and 194, and clock 196) operate like
`Sive to a force in a Second orientation.
`the corresponding components discussed above with respect
`6. The device according to claim 5, wherein the first
`to FIG.1. However, the output of demodulator 178 measures
`orientation is along a first axis of the movable mass.
`the displacement (and the acceleration) along the Z-axis, and
`7. The device according to claim 6, wherein the second
`the output of demodulator 180 measures the rotation about
`orientation is about one of the first axis or a Second axis of
`the Z-axis.
`the movable mass.
`Through the appropriate placement of the fixed
`8. The device according to claim 5, wherein the second
`electrodes, the apparatus described above can detect and
`orientation is along a Second axis of the movable mass.
`measure forces along and/or about any axis or combination
`9. The device according to claim 5, wherein the first
`of axes. Moreover, although specific Sequences have been
`orientation is about a first axis of the movable mass.
`described, the Voltages on the fixed electrodes can be
`10. The device according to claim 9, wherein the second
`Switched, the reset can be actuated, and the Voltage on the
`orientation is about a Second axis of the movable mass.
`mass can be received by the demodulators in any manner
`11. The device according to claim 3, wherein the Sensor,
`that permits two measurements for each axis of interest.
`the amplifier, the reset Switch, the first demodulator, the
`More generally, the circuit described above can be used with
`Second demodulator, the control timer, the first Selector, and
`other Sets of two or more differential capacitors, where the
`the Second Selector, are on a single integrated circuit.
`common electrodes of each differential capacitor are elec
`12. The device according to claim 3, wherein the first
`trically coupled.
`demodulator further has an offset input.
`While there have been shown and described examples of
`13. The device according to claim 12, wherein the second
`the present invention, it will be readily apparent to those
`demodulator further has an offset input.
`skilled in the art that various changes and modifications may
`14. The device according to claim 3, wherein the first
`be made therein without departing from the Scope of the
`demodulator is a discrete time demodulator.
`invention as defined by the appended claims. Accordingly,
`15. The device according to claim 14, wherein the second
`the invention is limited only by the following claims and
`demodulator is a discrete time demodulator.
`equivalents thereto.
`16. The device according to claim 3, further comprising a
`first buffer amplifier having an input electrically coupled to
`I claim:
`1. A Sensing device comprising:
`an output of the first demodulator.
`17. The device according to claim 16, further comprising
`a micromachined Sensor having a movable mass, first and
`a Second buffer amplifier having an input electrically
`Second fixed electrodes of a first differential capacitor,
`coupled to an output of the Second demodulator.
`and first and Second fixed electrodes of a Second
`18. The device according to claim 16, wherein the first
`differential capacitor, wherein the mass includes a third
`buffer amplifier is adjustable.
`electrode of the first differential capacitor and a third
`19. A circuit for measuring change in capacitance com
`electrode of the Second differential capacitor, and the
`prising:
`third electrode of the first differential capacitor is
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`a first differential capacitor having a first electrode, a
`a movable mass having a first and a Second electrode,
`Second electrode, and a third electrode, with the first
`a first Set of fixed electrodes arranged relative to the first
`electrode forming a first capacitor of the first differen
`movable mass electrode to form a first differential
`tial capacitor with the Second electrode, and the first
`capacitor;
`electrode forming a Second capacitor of the first dif
`a Second Set of fixed electrodes arranged relative to the
`ferential capacitor with the third electrode,
`Second movable mass electrode to form a Second
`a Second differential capacitor having a first electrode, a
`differential capacitor;
`Second electrode, and a third electrode, with the first
`a Sensing circuit coupled to the movable mass to detect a
`electrode forming a first capacitor of the Second dif
`change in capacitance of the first differential capacitor
`ferential capacitor with the Second electrode, and the
`during a first time period and a change in capacitance
`first electrode forming a Second capacitor of the Second
`of the Second differential capacitance during a Second
`differential capacitor with the third electrode, and
`time period; and
`wherein the first electrode of the first differential
`capacitor is coupled to the first electrode of the Second
`wherein the first and Second differential capacitors are
`differential capacitor;
`arranged relative to one another to respond to different
`force components.
`an amplifier having an input coupled to the first electrode
`of the first differential capacitor and the first electrode
`23. The device of claim 22 wherein the first and second
`time periods are non-overlapping.
`of the Second differential capacitor,
`24. The device of claim 23 wherein the sensing circuit
`a first demodulator having an input coupled to an output
`detects a first force component during the first time period
`of the amplifier;
`and a Second force component during a Second time period.
`a Second demodulator having an input coupled to the
`25. A Sensing device comprising:
`output of the amplifier; and
`a first capacitive Structure arranged to change capacitive
`a control timer coupled to the first demodulator and the
`values in response to a first force component;
`Second demodulator.
`a Second capacitive structure arranged to change capaci
`20. The circuit according to claim 19, further comprising
`tive values in response to a Second force component;
`a reset Switch having a first port coupled to the first electrode
`an activation and detection circuit for charging the first
`of the first differential capacitor and to the first electrode of
`capacitive Structure and detecting a capacitance value
`the Second differential capacitor, and a control input elec
`trically coupled to the control timer.
`in a first time period and for charging the Second
`capacitive Structure and detecting a capacitance value
`21. The circuit according to claim 20, further comprising
`a first Selector having a first output coupled to the Second
`in a Second time period that does not overlap with the
`first period.
`electrode of the first differential capacitor, a second output
`coupled to the third electrode of the first differential
`26. The device of claim 25 wherein the first and second
`capacitor, and a control input coupled to the control timer;
`capacitive Structures share a common electrode.
`and a Second Selector having a first output coupled to the
`27. The device of claim 26 wherein the activation and
`Second electrode of the Second differential capacitor, a
`detection circuit is coupled to the common electrode and
`activates that electrode to attain a predetermined Voltage at
`Second output coupled to the third electrode of the Second
`differential capacitor, and a control input coupled to the
`a time in between the first and Second time periods.
`control timer.
`22. A Sensing device comprising:
`
`25
`
`k
`
`k
`
`k
`
`k
`
`k
`
`1O
`
`15
`
`35
`
`40
`
`9
`
`