WO2007132422A1 - Capacitive mems sensor device - Google Patents
Capacitive mems sensor device Download PDFInfo
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- WO2007132422A1 WO2007132422A1 PCT/IB2007/051820 IB2007051820W WO2007132422A1 WO 2007132422 A1 WO2007132422 A1 WO 2007132422A1 IB 2007051820 W IB2007051820 W IB 2007051820W WO 2007132422 A1 WO2007132422 A1 WO 2007132422A1
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- WIPO (PCT)
- Prior art keywords
- plate
- signal
- bias voltage
- sensing element
- sensor device
- Prior art date
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
- G01D5/14—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
- G01D5/24—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance
- G01D5/241—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance by relative movement of capacitor electrodes
- G01D5/2417—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance by relative movement of capacitor electrodes by varying separation
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D3/00—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups
- G01D3/08—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups with provision for safeguarding the apparatus, e.g. against abnormal operation, against breakdown
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
- G01D5/14—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
- G01D5/24—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying capacitance
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
- H04R3/00—Circuits for transducers, loudspeakers or microphones
- H04R3/007—Protection circuits for transducers
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
- H04R19/00—Electrostatic transducers
- H04R19/005—Electrostatic transducers using semiconductor materials
Definitions
- the present invention relates to a capacitive MEMS sensor device for sensing a mechanical quantity, to an electronic circuit for such a capacitive MEMS sensor device and to a method for operating such a capacitive MEMS sensor device.
- MEMS Micro Electro Mechanical System
- actuators are a fast-growing market.
- the present invention relates to capacitive MEMS sensors, e.g. microphones, accelerometers and pressure sensors.
- MEMS sensor comprises a capacitor which includes two parallel plates.
- One rigid plate is, for instance, fixed onto a frame, the other plate (e.g. a membrane) moves or flexes under the influence of the mechanical quantity to be measured, e.g. acoustic (air) pressure or acceleration.
- a change in the distance between the plates results in a change of the capacitance, thus an electrical signal that can be measured and processed.
- the senor will have to handle mechanical excitations over a wide amplitude range.
- the sound pressure may vary between 34 dB and 120 dB sound pressure level (SPL) or even higher.
- SPL sound pressure level
- the mechanical properties of the membrane can be dimensioned to maximize the sensitivity while avoiding mechanical contact with the fixed plate.
- the transient sound pressure exceeds the maximum level, like inside a train when it enters or leaves a tunnel. In this case, the distance between the plates may become very small or even zero.
- the electrical charge that is applied to the plates of the sensor causes an attracting electrical force between the plates and may lead to membrane sticking. This disables the sensor until the charge is sufficiently reduced, e.g. after power down.
- the membrane is often covered with an electrically insulating anti-sticking coating. The coating obstructs a discharge current between the plates and thereby a fast self-recovery from sticking.
- US 2006/0008097 Al discloses a condensor microphone having a transducer element comprising a diaphragm having an electrically conductive portion, a back-plate having an electrically conductive portion, a DC bias voltage element operatively coupled to the diaphragm and the back-plate, a collapse detection element adapted to determine a physical parameter value related to a separation between the diaphragm and the back-plate, and a collapse control element adapted to control the DC bias voltage element based on the determined physical parameter value.
- BIOS terminal is explicitly defined by the voltage source and the series impedance.
- the IN terminal does not have an explicitly defined DC level.
- ESD Electrostatic Discharge
- an ESD (Electrostatic Discharge) protection diode attached to the bondpad of the "IN" terminal takes care of this function, as it is commonly used for almost all IC pins to form a discharge path to ground. This has two undesirable side effects: a) Closing the switch does not completely discharge the microphone. A residual voltage equal to the threshold voltage of the protection diode will remain present, which has an adverse effect on membrane recovery. b) Closing the switch also causes a large transient signal on the amplifier input. As the transient will exceed the input range by a factor, the amplifier will likely clip.
- US 2006/0008097 Al does not present a complete and/or optimum solution for the problem of a (nearly) sticking membrane. It is an object of the present invention to provide a capacitive MEMS sensor device which enables fast recovery from (near-)sticking after a mechanical overload situation. It is further object of the present invention to provide a corresponding electronic circuit for such a capacitive MEMS sensor device and a corresponding method for operating such a capacitive MEMS sensor device. The object is achieved according to the present invention by a capacitive
- MEMS sensor device as claimed in claim 1, comprising:
- a capacitive MEMS sensing element having two plates arranged in parallel at a distance that varies in response to said mechanical quantity and for converting the sensed mechanical quantity into an electrical quantity
- a first bias voltage unit for supplying a first bias voltage to a first plate of said MEMS sensing element
- a second bias voltage unit for supplying a second bias voltage to the second plate of said MEMS sensing element
- a signal processing unit for processing said electrical quantity into an output signal
- a comparator unit for comparing said output signal to a reference signal for detection of an overload condition of said MEMS sensing element and for outputting a comparator signal
- a control unit for controlling the discharge of said MEMS sensing element in case of an overload condition signalled by said comparator signal by connecting, in case of an overload condition, said first plate to a first discharge terminal during a first time interval and said second plate to a second discharge terminal during a second time interval.
- a corresponding electronic circuit for such a capacitive MEMS sensor device is defined in claim 11
- a corresponding method for operating such a capacitive MEMS sensor device is defined in claim 12.
- Preferred embodiments of the invention are defined in the dependent claims.
- the present invention is based on the idea to apply a dual-switch principle for separately controlling the discharge of the two plates of the sensing element in case of an overload condition (it shall be noted that the expression "overload condition” shall also included near-overload conditions).
- This dual-switch principle ensures the fastest possible recovery from a mechanical (near-)overload.
- the voltage (DC) level at the input of the signal processing unit remains equal to the nominal level of the first bias voltage applied to the first plate.
- this has the effect of audio muting, i.e. the transient effects are limited to the effect of reducing the sensor's sensitivity to zero. Additionally, the pop noise is minimized.
- the solution of the present invention overcomes both problems.
- a well-defined bias voltage is received on both sensor terminals. Since the sensing element will, in case of a (near-)overload condition, be short-circuited, no residual voltage will be left. This yields the fastest membrane recovery. Further, since the first bias voltage is short-circuited, the DC level at the signal processing unit will not change so that the pop noise is minimized.
- the first bias voltage unit that is adapted for providing a first bias voltage to the first plate of the sensing element is a low- impedance voltage unit
- the second bias voltage unit that is adapted to provide a second voltage to the second plate of the sensing element
- the recharge time of the sensing element in case of a (near- overload condition is thus almost entirely determined by the high impedance of the second voltage unit.
- the shortest possible recharge time ensures the shortest recovery time after which the sensor can resume normal operation.
- the connection between said first voltage source and that first impedance element is provided as said first and second discharge terminal, i.e. there is only one common discharge terminal used.
- the supply voltage of said first voltage source is larger than the supply voltage of said second voltage source.
- the first and second time intervals have substantially the same start time, but the first time interval is shorter than the second time interval.
- both plates of the sensing element are connected to their respective discharge terminals (or to a common discharge terminal, as used in a preferred embodiment) so that the voltage applied over both plates is decreased to zero while the voltage level at the second plate remains unaltered.
- the first plate is no longer connected to the discharge terminal so that the respective bias voltage of the sensing element (i.e. the first bias voltage applied to the first plate) will be restored.
- the second plate is still connected to the second voltage source for some time (until the end of the second time interval) ensuring that the recharge time is reduced by a factor depending on the impedance values of the first and second voltage units.
- the second bias voltage is again supplied to the second plate of the sensing element, and the electronic circuit returns from a voltage source mode (having a low impedance) into a charge pump mode (having a high impedance) again.
- the MEMS sensor device is then again ready for accurate conversion of a mechanical input signal into an electrical output signal.
- the discharge time is only limited by the residual resistance of the switches and the wires.
- the discharge time is preferably much smaller than the mechanical recovery time of the membrane.
- the first time interval is preferably slightly longer than the mechanical recovery time.
- the second time interval is preferably long enough to recharge the sensor to e.g. 90% of the final value.
- control unit comprises a first pulse generator having a first pulse time corresponding to said first time interval and a second pulse generator having a second pulse time corresponding to said second time interval.
- Said pulse generators control, for instance, first and second switches as defined in claim 8 for connecting the respective plates of the sensing element to their respective discharge terminal.
- the output signal of the MEMS sensor according to the present invention can be analog or digital.
- An embodiment for obtaining an analog output signal is defined in claim 9, an embodiment for obtaining a digital output signal is defined in claim 10.
- the comparator is implemented as a simple counter for counting a predetermined number of subsequent bits having a predetermined bit value, e.g. for detecting a series of subsequent "l"s or "0"s depending on the polarity of the counter.
- the MEMS sensor can be applied for different purposes.
- the MEMS sensor can be a MEMS microphone (e.g. for use in a cell phone), an accelerometer and a pressure sensor.
- Fig. 1 shows the principle of a capacitive MEMS sensor
- Fig. 2 shows a simple bias circuit for a MEMS sensor
- Fig. 3 shows a circuit diagram of a practical realisation of a bias circuit for a
- Fig. 4 shows a circuit diagram of a capacitive MEMS sensor device according to the present invention
- Fig. 5 shows a circuit diagram of an analog implementation of a MEMS sensor device according to the present invention
- Fig. 6 shows a circuit diagram of a digital implementation of a MEMS sensor device according to the present invention
- Fig. 7 shows signal diagrams of signals in the analog implementation
- Fig. 8 shows signal diagrams of signals in the digital implementation.
- Fig. 9. shows a practical solution of an RC filter for use in combination with a vo ltage multiplier
- a MEMS microphone is generally mentioned as an exemplary embodiment of the present invention.
- the invention can generally be applied to all capacitive MEMS sensors and is not limited to a MEMS microphone.
- Fig. 1 shows the principle of a capacitive MEMS sensor 10 for converting a mechanical quantity P, e.g. an incident acoustic pressure, into a change of capacitance by variation of the distance between the capacitor plates 11, 14.
- the capacitor comprises two parallel plates, in particular one rigid plate 11 which is fixed onto a first frame 12 and connected to an electrical contact 13 and another plate 14 (in particular a membrane) which is connected to a second frame 15 (which can form one rigid unit with the first frame 12, but is electrically isolated therefrom) and connected to an electrical contact 16.
- the latter plate 14 moves or flexes under the influence of the mechanical quantity P to be measured, e.g. acoustic (air) pressure or acceleration.
- a change in the distance do between the plates 11 and 14 results in a change of the capacitance, i.e. in a measurable electrical signal.
- A is the plate area and do is the default distance between the plates 11 , 14.
- a mechanical excitation changes the distance from do to do+ ⁇ d.
- the voltage Vp across the plates 11, 14 changes according to
- the bias charge is statically present inside the membrane.
- Vbias bias voltage
- Fig. 2 illustrates the principle of a constant-charge bias circuit for a MEMS sensor.
- the bias resistor Rb ias may be implemented as a low- duty cycle switching circuit or as long-channel MOS transistors in weak inversion.
- the circuit behaves as a constant-charge bias.
- a practical value for this corner frequency is the lower end of the audio spectrum, e.g. 50 Hz.
- a high- impedance bias generator V2 including a voltage source VS2 and a bias impedance Rbias2 defines the DC level Vbias2 of this plate, while a second, low-impedance bias generator Vl including a voltage source VSl (e.g.
- a voltage multiplier for deriving the bias voltage Vbiasi from a supply voltage V SU ppiy
- a bias impedance Rbiasi defines the (high) DC level Vbiasi of the other plate.
- Fig. 3 illustrates a practical realization of the sensor biasing (where preferably Rbias2 » Rbiasi).
- a likely circuit for the latter generator is a Dickson voltage multiplier.
- the sensor output signal of the MEMS sensor 10 is processed by a signal processing unit 20 which particularly includes a preamplifier to obtain the output signal V OUT of the sensor arrangement.
- a first embodiment of a MEMS sensor arrangemement according to the present invention based on the dual-switch recovery principle is shown in Fig. 4.
- the MEMS sensor device further comprises a comparator 21 and a control unit 22 for controlling the discharge of said MEMS sensor 10 in case of an overload condition.
- Said control unit 22 comprises a first and a second (preferably one-shot) pulse generator 23, 24 for generating pulse signal V T1 and V T2 , respectively, having predetermined pulse times Tl and T2, respectively.
- the first plate is the membrane to which the first bias voltage Vbiasi is applied.
- the recovery circuit shown in Fig. 4 for dealing with an overload comprises a feedback loop with basically the following components: a) The comparator 21 compares the output signal V OUT of the signal- processing unit 20 to a predetermined reference level V re f. In case of a (near-)overload condition, the output signal V OUT becomes active.
- the reference level V re f is related to Vbiasi and Vbias2-
- the bias voltages are, for instance, related to a bandgap reference voltage (e.g.
- the sensor sensitivity is equal to Vbiasi - Vbias2- Mechanical dimensions, stiffness, compliance etc. determine which output voltage (V max ) corresponds with an overload condition.
- V max the reference voltage
- V re f is a fraction of Vbiasi - Vbias2- Vref is set equal to Vbias2 + V max .
- Vc of the comparator 21 controls the mono-stable pulse generator 23 having a pulse time Tl. This generator 23 controls a switch Sl.
- the output signal Vc of the comparator 21 also controls the mono-stable pulse generator 24 having a pulse time T2.
- T2 is preferably longer than Tl.
- This second generator 24 controls the switch S2.
- the switch S2 By keeping the switch S2 still closed, the recharge time will be reduced by a factor of Rbias2/(Rbiasi+Rbias2). After the interval T2, also the switch S2 is released so that the bias circuit returns from voltage source (low impedance) mode into a charge pump (high impedance) mode again. The sensor 10 is now ready again for accurate conversion of the input signal.
- the one-shot pulse Tl is mainly necessary to enable the membrane to return to its normal position.
- the recovery time depends on the mechanical characteristics of the device. Recovery cannot be measured electronically because closing Sl and S2 directly restores the electrical signal at the output terminal V OUT , which resets the comparator 21.
- the dual-switch principle ensures the fastest possible recovery from a mechanical (near-)overload.
- the DC level at the input of the signal processing unit 20 remains equal to the nominal level Vbias2-
- this has the effect of audio muting, in other words, the transient effects are limited to the effect of reducing the sensor's sensitivity to zero. Additionally, the pop noise is minimized.
- the two mono-stable pulse generators When viewed in the frequency domain, the two mono-stable pulse generators act as low-pass filters in the loop to ensure stable operation.
- the MEMS sensor and the electronic circuit are preferably packed together in a Multi-Chip Module (MCM) or integrated on one chip.
- MCM Multi-Chip Module
- the output signal V OUT of the sensor device can either be analog or digital.
- the electronic circuit - as shown in the circuit diagram of Fig. 5 - contains at least two functional blocks, the bias voltage units Vl, V2 and the signal processing unit 20 including a pre-amp lifter 25.
- the comparator unit 21 comprises an analog comparator.
- Its reference level is related to the bias voltage Vbiasi over the sensor 10, which in turn is usually derived from a bandgap voltage reference unit VR providing the reference voltage V re f.
- the implementation shown in Fig. 5 is, for instance, preferably used for a MEMS microphone.
- the electronic circuit - as shown in the circuit diagram of Fig. 6 - comprises as third functional block an Analog-to-Digital Converter (ADC) 26.
- ADC Analog-to-Digital Converter
- the pre-amplif ⁇ er 25 and ADC 26 are interwoven so that only the digital output can be accessed to recognize an overload condition. This is not a limitation because a digital comparator function is preferable over an analog one for reasons of current consumption (which is, for instance, important for mobile applications), ease of design, chip area (cost) and accuracy.
- the common ADC type is a 1-bit Sigma-Delta ADC. Its output signal is a pulse-density modulated bitstream.
- the stream will consist of a series of subsequent ' 1 's or 'O's, depending on the polarity.
- the digital comparator 21 ' can be a simple counter. Its output V'c becomes active in case a pre-defined number of subsequent ' 1 's occurs.
- a specific advantage of this digital comparator 21 ' is that the reference level V re f is simply a programmable number.
- FIG. 7 shows timing diagrams of the signal appearing in the analog implementation as depicted in Fig. 5.
- Fig. 7a shows the distance d between the plates of the MEMS sensor 10.
- a sinusoidal input signal of normal amplitude is applied. The distance varies around do with an amplitude smaller than d max .
- a mechanical overload signal occurs which reduces the distance to do-d max .
- Fig. 7b shows the output voltage V OUT of the signal processor. The overload condition is detected when V OUT exceeds V re f.
- Fig. 7c shows the comparator output signal Vc. At detection of the overload condition, the comparator output becomes active.
- Figs. 7d and 7e show the output signals V T1 and V T2 of the pulse generators 23 and 24. It can be seen that the detection of the overload condition causes the output of pulse generators 23 and 24 to become active simultaneously, which closes the switches Sl and S2.
- Fig. 7a As can be seen in Fig. 7a, during interval Tl, the membrane returns to its normal position. Here it acts as a purely mechanical spring-mass system. No electrical force is present.
- the pulse generator 23 After interval Tl, the pulse generator 23 returns to inactive and the switch Sl is reopened, as can be seen from Fig. 7d.
- Fig. 7f illustrates, the sensor is rapidly recharged over Rbiasi, which is low-ohmic. The voltage reaches e.g. 90% of the final value VbI 3 Si-VbI 3 S 2 , and the sensor device is ready again for signal conversion.
- the pulse generator 24 After interval T2, the pulse generator 24 returns to inactive and the switch S2 is re-opened, (see Fig. 7e.) As Fig. 7f illustrates, the sensor recharges slowly to its final value over Rb 13 Si and RbI 382 , where RbI 382 is high-ohmic hence dominant.
- Fig. 8 shows timing diagrams of the input signal V OUT and the output signal Vc of the comparator (counter) 21 ' in the digital implementation as depicted in Fig. 6.
- Fig. 8a shows the output signal V OUT of the Sigma-Delta ADC 26 (depictive, not on an accurate time schale) in case of a distance signal d as shown in Fig. 7a.
- the ADC produces a stream of ' 1 's and 'O's with on average equal occurrence.
- interval i2 the sensor moves towards overload.
- the fraction of ' 1 's in the ADC stream rises from 50% to almost 100%.
- Interval i3 represents the overload situation, where the stream only contains ' 1 's.
- the Sigma-Delta ADC 26 is probably not be able to reach a 0% or 100% modulation level, i. e. an output consisting of exclusively 'O's or Ts, but just a lower modulation level, e.g. 5% - 95%.
- the output of the comparator (counter) 21 ' has to become active, when a predefined ratio of Ts and 'O's (e.g. 10:1) is exceeded.
- a predefined ratio of Ts and 'O's e.g. 10:1
- the ratio between the output of a first counter for the Ts and a second counter for the 'O's is evaluated.
- the preset number of subsequent Ts once the output of a single counter becomes active can be lowered compared to the counter for the 100% modulation level so that the discharge procedure is initiated after the same time of overload condition. That means that defining the preset number of subsequent Ts depends on the tolerated time duration of overload condition, the output frequency of the Sigma-Delta ADC 26 and the modulation level which is reached at overload.
- a Sigma-Delta ADC 26 Another limitation for the application of a Sigma-Delta ADC 26 in a practical design is the signal headroom in the Sigma-Delta ADC 26.
- the total gain of pre-amplifier 25 and the Sigma-Delta ADC 26 is dimensioned such that the maximum ADC modulation level coincides with the maximum input signal swing.
- the signal-to-noise ratio of the Sigma-Delta ADC 26 can be a limiting factor in the signal path, meaning that the Sigma-Delta ADC 26 can be the weakest part in the chain with respect to the SNR. So, even signals in the normal operation mode can cause a maximum signal at the output of the Sigma-Delta ADC 26, i.e.
- the Sigma-Delta ADC 26 can start to clip even in the normal operation mode. However, this means there is no headroom left for larger signal excursions which could be used to identify an upcoming overload condition.
- the overload detection has to be performed by an (additional) analog comparator 21 as depicted in Fig. 5.
- a low- impedance bias generator Vi including a voltage source VSl (e.g. including a voltage multiplier for deriving the bias voltage Vbiasi from a supply voltage Vsupply) and a bias impedance Rbiasi was used to define the (high) DC level Vbiasi of the other plate.
- a common way to implement said voltage multiplier is a circuit consisting of switches and capacitors.
- the unloaded output voltage becomes the sum of the individual capacitor voltages.
- a resistive load causes a ripple on the output voltage at the switch frequency of the voltage multiplier.
- the ripple's frequency is well above the audio band (20 kHz), it may still harm the audio performance because aliasing in the Sigma-Delta ADC 26 can fold the ripple frequency back into the audio band.
- an RC filter is used to suppress the ripple at the bias voltage when a load current is drawn from the voltage multiplier.
- Fig. 9. shows a practical solution which saves chip area in comparison to a traditional fully-integrated anti-aliasing filter.
- the resistor Rbiasi is implemented as a diode, e.g. by a PN-junction or a long-channel MOS transistor TRl whose gate is connected to the drain and which then acts as a diode.
- the capacitor is implemented as gate of a MOS transistor TR3.
- a typical capacitor value is one order larger than the sensor's capacitance.
- the large decoupling capacitor provides a low impedance level for audio frequencies so effectively the upper sensor node acts as a virtual ground.
- the diode (respectively the transistor TRl acting as a diode) acts as a low-ohmic resistor and enables fast charging when the bias voltage across the sensor is lower than the voltage multiplier output (directly after power-on or after recovering from an overload).
- the diode has to supply only the (very low) leakage current of the sensor in a steady state. Hence, the voltage across the diode is low why the diode acts as a very high impedance.
- the RC time constant of the filter becomes sufficiently long to suppress the ripple to a level where aliasing products remain below the noise floor.
- the DC impedance level at the upper node of the sensor is very high.
- the large decoupling capacitor provides a low AC impedance level for audio frequencies so effectively the upper node acts as a virtual ground.
- the resistor Rbias2 and voltage source Vbias2 are also implemented as a diode or a long-channel MOS transistor TR2 whose gate is connected to the drain and which then acts as a diode. Similarly, it will also conduct a very small leakage current in the steady state. For a PN diode this results in a forward bias of a few hundred mV. For the long-channel MOS transistor TR2 it will also result a Vds of a few hundred mV, which is a convenient DC level for the pre-amplifier 25.
- Fields of application of the present invention are all capacitive MEMS sensors, e.g. MEMS microphones, accelerometers and pressure sensors.
- MEMS sensors e.g. MEMS microphones, accelerometers and pressure sensors.
- the invention can be used as a microphone for a cell phone.
- a magnetic compass, GPS and MEMS accelerometer sensors shall be combined for accurate positioning in portable electronics like cell phones.
- the present invention provides a novel feedback loop for the electronics in capacitive MEMS sensor devices. Its purpose is to let the sensor and its signal- processing electronics recover quickly from an overload caused by a mechanical over- excitation. This invention strongly reduces the duration of the resulting error signal, which is a benefit to the quality of the application. A fast recovery is obtained by an improved bias circuit, no audible side effects are obtained (in case of use as a microphone).
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- Acoustics & Sound (AREA)
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- Electrostatic, Electromagnetic, Magneto- Strictive, And Variable-Resistance Transducers (AREA)
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Abstract
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Priority Applications (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2009510598A JP2009537817A (en) | 2006-05-17 | 2007-05-14 | Capacitance MEMS sensor device |
EP07735891.9A EP2021739B1 (en) | 2006-05-17 | 2007-05-14 | Capacitive mems sensor device |
US12/301,163 US8134375B2 (en) | 2006-05-17 | 2007-05-14 | Capacitive MEMS sensor device |
CN2007800176524A CN101443633B (en) | 2006-05-17 | 2007-05-14 | Capacitive MEMS sensor device |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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EP06114074 | 2006-05-17 | ||
EP06114074.5 | 2006-05-17 |
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WO2007132422A1 true WO2007132422A1 (en) | 2007-11-22 |
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PCT/IB2007/051820 WO2007132422A1 (en) | 2006-05-17 | 2007-05-14 | Capacitive mems sensor device |
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US (1) | US8134375B2 (en) |
EP (1) | EP2021739B1 (en) |
JP (1) | JP2009537817A (en) |
CN (1) | CN101443633B (en) |
WO (1) | WO2007132422A1 (en) |
Cited By (8)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2459864A (en) * | 2008-05-07 | 2009-11-11 | Wolfson Microelectronics Plc | Filtered bias voltage for a MEMS capacitive transducer circuit |
WO2010103452A1 (en) | 2009-03-11 | 2010-09-16 | Nxp B.V. | Low-frequency filter comprising maxwell-wagner stack |
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Also Published As
Publication number | Publication date |
---|---|
US20100013501A1 (en) | 2010-01-21 |
CN101443633A (en) | 2009-05-27 |
CN101443633B (en) | 2011-03-16 |
EP2021739A1 (en) | 2009-02-11 |
EP2021739B1 (en) | 2017-10-11 |
JP2009537817A (en) | 2009-10-29 |
US8134375B2 (en) | 2012-03-13 |
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