Drive circuit for controlling a resonant type mems oscillator

Abstract

Embodiments of the present disclosure relate to a drive circuit for controlling a resonant type MEMS oscillator. A drive circuit for controlling a MEMS oscillator, the drive circuit comprising a digital conversion stage to acquire a differential sense signal indicative of a displacement of a movable mass of the MEMS oscillator and to convert the differential sense signal of an analog type into a digital differential signal of a digital type. A processing circuitry is configured to generate a digital control signal of the digital type from a comparison between the digital differential signal and a differential reference signal indicative of a target amplitude of an oscillation of the movable mass causing a resonance of the MEMS oscillator. An analog conversion stage comprising a sigma-delta DAC and configured to convert the digital control signal into a PDM control signal of the analog type. A filter stage of a low-pass type generates a control signal for controlling an amplitude of the oscillation of the movable mass via filtering of the PDM control signal.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a driving circuit for controlling a MEMS oscillator of the resonant type. In particular, the present disclosure relates to a driving circuit comprising an analog conversion stage comprising a digital-to-analog converter (DAC) of the sigma-delta type, to a MEMS oscillator (in particular a MEMS gyroscope) comprising the driving circuit, to an apparatus comprising the MEMS oscillator, and to a method for controlling the MEMS oscillator. BACKGROUND

[0002] As known, gyroscopes of the MEMS ("Micro-Electro-Mechanical System") type are devices of the resonant type. Therefore, the use of a gyroscope requires controlling its main resonator so as to ensure its correct oscillation. This usually occurs via a closed-loop electronic control which senses the displacement of the oscillating proof mass of the gyroscope and, according to these sensed displacements, controls the proof mass in such a way that the oscillation is ensured at the desired amplitude. In particular, the oscillation is maintained via compliance with the well-known Barkhausen condition, which determines that the closed loop at the resonant frequency needs or should have a unitary gain and needs or should have a total phase shift equal to 2π.

[0003] The known solutions allow to control the gyroscope in closed loop via a pure analog type control. In particular, this analog control is implemented via an analog driving circuit which usually comprises a capacitance-to-voltage (C2V) converter which senses the displacement of the proof mass of the gyroscope by means of a capacitive effect and generates a corresponding sensed signal indicative of such displacement. A phase-locked loop (PLL) is used to receive the sensed signal and calculate a clock signal of the oscillation, so as to use this clock signal to command the proof mass, thus imposing the condition on the total phase shift of the oscillation. Moreover, an AGC ("Automatic Gain Control") block in parallel with the PLL is used to receive the sensed signal and thus command the amplitude of the oscillation, so that it remains equal to the desired amplitude.

[0004] However, the known AGC block extracts from the sensed signal information on the amplitude of the oscillation of the proof mass at the demodulation frequency f dem equal to the oscillation frequency f dr of the proof mass (usually approximately equal to the intrinsic resonant frequency of the gyroscope). In other words, the AGC block operates a demodulation of the synchronous type and which utilizes a signal with a sinusoidal or square waveform having f dem = f dr . Moreover, this AGC block needs or should have a filter block of the active type to insert the singularity required for a stable feedback loop and to filter the high-order harmonics generated by the demodulation (for example, at 2f dr), in order to prevent higher and spurious vibration modes in the oscillation of the proof mass induced by the high harmonics of the output signal. The filtering blocks of the active type induce further power consumption in their operation and complicate the structure of the analog driving circuit.

[0005] Therefore, the known analog driving circuit cannot be used for controlling low-power gyroscopes (for example, operating at a power lower than about 1 mW) due to the high power consumption caused by filtering the high harmonics of the signal generated as output and to the fact that the AGC block comprises a closed-loop amplifier with high consumption.

[0006] Other known solutions allow controlling the closed-loop gyroscope via a digital type of control.

[0007] This digital control is implemented via a digital driving circuit comprising a PLL and an AGC block, both implemented digitally via a FPGA ("Field Programmable Gate Array"). Similarly to what previously described, the AGC block extracts from the sensing signal and from the clock signal generated by the PLL the information about the amplitude of the oscillation of the proof mass at f dem = f dr In addition, a fourth-order Sigma-Delta type digital-to-analog converter (DAC) is used for generating a PDM ("Pulse-Density Modulation") signal based on the signal generated by the AGC block, which is used for directly controlling the oscillation of the proof mass.

[0008] However, the high analog-to-digital conversion frequency of the ADC block of the AGC block, used for ensuring the correct sampling of the sensing signal and the final correct operation of the AGC block and of the digital PLL, causes a high power consumption. In addition, the fourth-order Sigma-Delta DAC increases the complexity and the cost of the digital control and the output PDM signal has a high frequency noise (given by the quantization noise of the Sigma-Delta DAC), so that the output PDM signal generates in the oscillation of the proof mass higher vibration modes that are harmful to the operation of the gyroscope. SUMMARY

[0009] In various embodiments, the present disclosure provides a driving circuit, a MEMS oscillator, an apparatus and a method for controlling a MEMS oscillator that overcome the drawbacks of the prior art.

[0010] In at least one embodiment, there is provided a drive circuit for controlling a MEMS oscillator, the drive circuit comprising a digital conversion stage configured to: acquire a differential sense signal indicative of a sensed displacement of a movable mass of the MEMS oscillator, and convert the differential sense signal of an analog type into a digital differential signal of a digital type. A processing circuitry is electrically coupled to the digital conversion stage and configured to generate a digital control signal of a digital type from a comparison between the digital differential signal and a differential reference signal indicative of a target amplitude of oscillations of the movable mass, the differential reference signal causing resonance of the MEMS oscillator at a resonance frequency of the MEMS oscillator. An analog conversion stage is electrically coupled to the processing circuitry and configured to convert the digital control signal into a PDM control signal of an analog type, the analog conversion stage comprising a sigma-delta type digital-to-analog converter (DAC). A low-pass type filtering stage is electrically coupled to the analog conversion stage and configured to generate a control signal for controlling an amplitude of oscillations of the movable mass via filtering of the PDM control signal.

[0011] In at least one embodiment, there is provided a MEMS oscillator comprising a substrate, a movable mass, and a drive circuit integrated in the substrate. The movable mass is elastically coupled to the substrate and controllable via a control signal to resonate with respect to the substrate.

[0012] In at least one embodiment, there is provided a method of controlling a MEMS oscillator, comprising: acquiring, via a digital conversion stage, a differential sense signal indicative of a sensed displacement of a movable mass of the MEMS oscillator; converting, via the digital conversion stage, the differential sense signal of an analog type into a digital differential signal of a digital type; generating, via a processing circuitry electrically coupled to the digital conversion stage, a digital control signal of a digital type from a comparison between the digital differential signal and a differential reference signal indicative of a target amplitude of oscillations of the movable mass, the differential reference signal causing resonance of the MEMS oscillator at a resonance frequency of the MEMS oscillator; converting, via an analog conversion stage electrically coupled to the processing circuitry, the digital control signal into a PDM control signal of an analog type, the analog conversion stage comprising a sigma-delta type digital-to-analog converter (DAC); generating, via a low-pass type filtering stage electrically coupled to the analog conversion stage, a control signal by filtering of the PDM control signal; and controlling, via the control signal, an amplitude of oscillations of the movable mass. BRIEF DESCRIPTION OF DRAWINGS

[0013] For a better understanding of the present disclosure, preferred embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings in which:

[0014] Figure 1a simplified and schematic representation of a MEMS oscillator, in particular a gyroscope;

[0015] Figure 2 is a block diagram of a drive circuit of a gyroscope according to an embodiment of the present disclosure Figure 1 is a block diagram of a drive circuit of a gyroscope according to an embodiment of the present disclosure

[0016] Figure 3 comprises three upper, middle and lower graphs respectively showing in Figure 1 is a block diagram of a drive circuit of a gyroscope according to an embodiment of the present disclosure Figure 2 is a block diagram of a drive circuit of a gyroscope according to an embodiment of the present disclosure

[0017] Figure 4 comprises three upper, middle and lower graphs respectively showing in Figure 1 is a block diagram of a drive circuit of a gyroscope according to an embodiment of the present disclosure Figure 3 is a block diagram of a drive circuit of a gyroscope according to an embodiment of the present disclosure

[0018] Elements common to the different embodiments of the present disclosure described below are denoted with the same reference numerals. DETAILED DESCRIPTION

[0019] Figure 1 a resonant type MEMS device 1, in particular a second order MEMS resonator (or oscillator), is schematically shown in a Cartesian (three-axial) reference frame of X, Y, Z axes.

[0020] In more detail, Figure 1 a possible embodiment of a MEMS gyroscope 1 of known type is shown by way of example only; in this case, the gyroscope 1 is exemplarily of the single-axis type, i.e. the gyroscope is able to sense an angular velocity acting along a single sensing axis, in this example around the Z axis, for example the angular velocity Ω Z .

[0021] The MEMS gyroscope 1 comprises a micromechanical structure having an excitation drive mass 2 having a main extension in an XY plane defined by the X and Y axes. The excitation mass 2 is coupled to a substrate S via an anchor 3 to which it is connected by an elastic anchoring element 4 configured to allow a displacement movement of the vibrating mass 2 along the X axis.

[0022] A drive electrode 5 and a first sense electrode 6 are coupled to the excitation mass 2 and comprise respective movable electrodes integrated with the excitation mass 2 and respective fixed electrodes fixed with respect to the substrate S. For the drive electrode 5 and the first sense electrode 6, the movable electrodes and the fixed electrodes are capacitively coupled to each other and in detail have an interdigitated structure.

[0023] In use, the drive electrodes 5 can be biased via drive (or excitation) signals D1 and D2 so as to generate the aforesaid drive movement of the proof mass 2 by effect of electrostatic coupling between the respective movable electrodes and the respective fixed electrodes; in particular, they allow generating a resonant motion of the proof mass 2 at an oscillation frequency f dr corresponding to or otherwise should correspond to the natural oscillation frequency of the micromechanical structure 1' (or the resonant frequency of the MEMS gyroscope 1) in order to make the MEMS gyroscope 1 resonate. Conversely, the first sensing electrodes 6 generate sensing signals I1 and I2, in particular capacitive variation signals, indicative of the extent of the movement of the proof mass 2 along the X axis, i.e. the oscillation amplitude of the proof mass 2. In detail, the sensing signals I1, I2 are of the differential type, i.e. they have variations opposite to each other in response to the movement of the proof mass 2. As shown in Figure 1 , the first sensing electrodes of the first series, indicated with 6', are in fact configured to generate a first capacitive variation due to the movement of the proof mass 2, and the first sensing electrodes of the second series, indicated with 6", are configured to generate a second capacitive variation opposite to the first capacitive variation due to the same movement of the proof mass 2.

[0024] The micromechanical structure 1' of the MEMS gyroscope 1 further comprises an inertial mass 8 elastically coupled to the proof mass 2 by means of an elastic coupling element 9 configured so that the inertial mass 8 constitutes an integral whole with the proof mass 2 during the oscillation motion of the proof mass 2 along the X axis and is free to move (sensing movement) along the Y axis also due to the Coriolis force generated in the presence of an angular velocity Ω Z around the Z axis. The proof mass 2 and the inertial mass 8 form the movable mass 7 of the MEMS gyroscope 1.

[0025] The second sensing electrodes 10 are coupled to the inertial mass 8 and comprise respective movable electrodes constituting an integral whole with the inertial mass 8 and respective fixed electrodes fixed with respect to the substrate S. The movable electrodes and the fixed electrodes of the second sensing electrodes 10 are capacitively coupled to each other and in detail have an interdigital structure. Therefore, the second sensing electrodes 10 generate a differential capacitive variation due to the sensing movement.

[0026] In detail, in use, the second sensing electrodes 10 allow generating reading signals V s1 and V s2 , in particular capacitive variation signals indicative of the extent of the sensing movement, i.e. the oscillation amplitude of the inertial mass 8 along the Y axis, which can be suitably processed in a known manner to determine the value of the angular velocity Ω z to be sensed.

[0027] Furthermore, the MEMS gyroscope 1 comprises a sensing or readout circuit 12 coupled to the second sensing electrodes 10 and configured to generate an output signal (an output voltage V s1 and V s2 ) as a function of the readout signals V out ; and a driving circuit 14, discussed better below, coupled to the driving electrodes 5 and to the first sensing electrodes 6 and configured to generate the driving signals D1 and D2 via feedback control based on the sensing signals I1 and I2 and on a desired amplitude of the oscillation of the proof mass 2, the value of which is determined in a known way during a design step so as to ensure a desired sensitivity of the MEMS gyroscope 1 at the sensing angular velocity z . In fact, the oscillation amplitude of the proof mass 2 needs to be controlled or should be otherwise carefully controlled, since its value affects the sensitivity of the MEMS gyroscope 1 at the sensing angular velocity z .

[0028] Furthermore, the proof mass 2 and the inertial mass 8 are biased in use at a constant voltage, in Figure 1 and in the subsequent figures denoted by V ROT .

[0029] In more detail, the driving circuit 14 is integrated in the MEMS gyroscope 1.

[0030] As exemplified in Figure 2 , the driving circuit 14 has a first input 14a and a second input 14b configured to receive the sensing signals I1 and I2, respectively, and a first output 14c and a second output 14d configured to provide the driving signals D1 and D2, respectively. The first input 14a is electrically connected to the first sensing electrodes 6’ of the first series, the second input 14b is electrically connected to the first sensing electrodes 6” of the second series, and the first and second outputs 14c and 14d are electrically connected to the driving electrodes 5.

[0031] The driving circuit 14 optionally comprises an input stage 15 coupled to the first and second inputs 14a and 14b and configured to generate as output a differential sensing signal V SD , in particular a differential voltage signal, as a function of the sensing signals I1 and I2. The input stage 15 is for example a capacitance-to-voltage (C2V) converter configured to generate the differential sensing signal V SD indicative of the displacement of the proof mass 2 along the X axis as a function of the capacitive variation signal received by the first sensing electrodes 6 as input. However, alternative embodiments can be provided for the input stage 15, which can for example comprise a transimpedance amplifier. In more detail, the differential sensing signal V SD is a sinusoidal signal at the oscillation frequency f dr of the movable mass 7.

[0032] The drive circuit 14 may also optionally include a comparator stage 16, which receives the differential sensing signal V. SD As input, and generate the oscillation frequency f dr The natural clock signal ck is used as the output (e.g., by sensing zero crossing); and PLL stage 17 of known type receives the natural clock signal ck as input and is synchronized with the oscillation frequency f. dr An appropriate number of derived clock signals ck are generated as output at an appropriate relevant frequency. These clock signals ck are used in the MEMS gyroscope 1 in a known manner (e.g., for operations performed by the same drive circuit 14 and sensing circuit 12).

[0033] The drive circuit 14 also includes a digital conversion stage 20, which is electrically coupled to the input stage 15 and the PLL stage 17 to receive the differential sensing signal V. SD And one of the derived clock signals ck is used as input, and according to the differential sensing signal V SD Generate a digital differential signal V of type digital. diff As output. In particular, the digital conversion stage 20 includes an analog-to-digital converter (ADC). Specifically, based on the received derived clock signal ck, the digital conversion stage 20 converts the differential sensing signal V in a predetermined manner. SD The sampled signal is converted into a digital signal to generate a digital differential signal V. diff More specifically, the digital conversion stage 20 performs differential sensing on the signal V. SD The value of interest (e.g., in the differential sensing signal V) SD The differential sensing signal is sampled at points (with known and constant phase shifts between each other) and the values ​​form the differential sensing signal V. SD A subset of the total value. Therefore, the digital differential signal V diff Indicator differential sensing signal V SD This value of interest. According to an exemplary aspect of this disclosure, due to the derived clock signal ck, by adjusting the differential sensing signal V... SD The peak value is sampled to obtain the digital differential signal V. diff However, sampling can occur in different ways and, for example, in differential sensing of the signal V. SD Execute at the trough of the wave.

[0034] The drive circuit 14 includes a processing circuit device 22 (which may be referred to herein as a processing block 22), which is electrically coupled to the digital conversion stage 20 and receives the digital differential signal V. diff Sum of differential reference signal V ref As input, and based on the digital differential signal V diff Generate digital control signal V ctrl,digas output. In particular, by comparing the digital differential signal V diff and the differential reference signal V ref to obtain a comparison signal and amplifying the comparison signal, a digital control signal V ctrl,dig is obtained. More in detail, the comparison signal is indicative of the difference between the digital differential signal V diff and the differential reference signal V ref and is also multiplied by a gain factor (k > 1 and, for example, k = 100). In other words, V ctrl,dig = k · (V diff - V ref ). The differential reference signal V ref is a target signal, which generates the bias of the drive electrode 5 so that the MEMS gyroscope 1 operates in resonance with a controlled oscillation amplitude (i.e., so that the movable mass 7 oscillates in resonance). Therefore, the digital control signal V ctrl,dig is indicative of the error in the oscillation amplitude between the oscillation of the movable mass 7 being measured and the target oscillation that makes the MEMS gyroscope resonate when the MEMS gyroscope 1 operates at the resonance frequency. For example, although this value is not limiting, since it depends on the process and design parameters and on the technology used, the differential reference signal V ref may be equal to about 1 V.

[0035] The drive circuit 14 comprises an analog conversion stage 24 electrically coupled to the processing block 22 and receiving as input the digital control signal V ctrl,dig to convert the digital control signal into an analog signal.

[0036] In particular, the analog conversion stage 24 comprises a digital-to-analog converter (DAC) of the sigma-delta type (hereinafter also referred to as sigma-delta DAC 24) that generates a PDM control signal V ctrl,dig from the digital control signal V ctrl,PDM . For example, the sigma-delta DAC 24 is a 1-bit DAC. In detail, the PDM control signal V ctrl,PDM is modulated via pulse density modulation (PDM) and varies over time between a first voltage (V0, for example equal to 0 V) and a second voltage (V DD > 0 V, for example equal to the voltage source of the MEMS gyroscope 1). According to an embodiment of the present disclosure, the sigma-delta DAC 24 is first order and is composed of an accumulator of the overflow type.

[0037] The drive circuit 14 comprises a filtering stage 26 of the passive type, electrically coupled to the sigma-delta DAC 24, receiving as input the PDM control signal V ctrl,PDM and filtering it, thus generating a control signal V ctrlas output. In particular, the filtering stage 26 comprises a low-pass filter configured to remove the high-frequency noise introduced by the sigma-delta DAC 24. The filtering stage 26 is configured to pass substantially only the DC component of the PDM control signal V ctrl,PDM and performs, in more detail, a low-pass filtering having a cut-off frequency comprised between a few hundred Hz and a few kHz (for example, between about 100 Hz and about 5 kHz) and, for example, equal to about 500 Hz.

[0038] Furthermore, the drive circuit 14 optionally comprises a buffer 28 (or voltage follower) having an input electrically coupled to the filtering stage 26 and a low-impedance output (i.e., having an impedance lower than the impedance of the input of the buffer 28). In detail, the buffer 28 receives the control signal V ctrl as input and transmits it to the output, wherein the output impedance of the buffer 28 is decoupled with respect to the input impedance of the buffer 28.

[0039] The drive circuit 14 further comprises a drive stage 30 of the known type, electrically coupled to the buffer 28 and to the PLL stage 17. The drive stage 30 has first and second outputs forming, respectively, the first output 14c and the second output 14d of the drive circuit 14. In particular, the drive stage 30 receives from the PLL stage 17 one of the derived clock signals ck appropriate, from the buffer 28 the control signal V ctrl and generates, according to the control signal V ctrl and in a known manner, the drive signals Dl and D2 as output. In particular, the drive stage 30 implements an H-bridge to control the drive electrodes 5 and, therefore, the movable mass 7.

[0040] The drive circuit 14 then implements a feedback control in order to force the values of the drive signals Dl and D2 to be such that the differential sensing signal V SD has the desired relationship with the differential reference signal V ref (in order to thus obtain the desired amplitude of the oscillation of the movable mass 7). In other words, the control signal V ctrl is a feedback with which the MEMS gyroscope 1 is controlled in such a way that the oscillation of the movable mass 7 which is measured adapts to the target oscillation (i.e., such that the digital differential signal V diff becomes equal to the differential reference signal V ref ).

[0041] Figure 3 comprise three graphs, upper, central and lower, which show, respectively, the PDM control signal V ctrl,PDM output from the analog conversion stage 24, the control signal V ctrl output from the filtering stage 26 and the resulting sensed displacement of the movable mass 7 (with the reference Xm (This refers to the position of the movable mass 7 relative to its rest position). Figure 3 In the diagram, these signals are shown as a function of time, and in a first time interval (e.g., included between the initial time T0 and the first time T1 > T0), the first time interval represents the initial transient of MEMS gyroscope 1: at the initial time T0, MEMS gyroscope 1 is operated (i.e., the movable mass 7 is controlled to switch from a stationary state to an oscillating state), and at the first time T1, MEMS gyroscope 1 has reached a stable oscillating state (i.e., the maximum amplitude of the oscillation of the movable mass 7 is substantially constant in time). As described above, between the initial time T0 and the intermediate time T... * (T0<T * Between <T1), the PDM control signal V ctrl,PDM It is constant and equal to the second voltage VDD, and in the intermediate time T * Subsequently, the PDM control signal is between the first voltage V0 and the second voltage V DD Oscillates between the initial time T0 and the intermediate time T. * Between, control signal V ctrl In the corresponding first value V ctrl,1 It is constant at time T and in the middle of time T * Then, the control signal decreases (e.g., exponentially) until it reaches its second value V. ctrl,2 (V ctrl,2 <V ctrl,1 It oscillates stably in the vicinity, such as Figure 3 The middle chart is best shown. Via control signal V ctrl The movable mass 7 is controlled to oscillate resonantly, and in particular, it exhibits displacement X with maximum amplitude. m The displacement starts from zero (at the initial time T0) and increases until it oscillates stably near the resonant amplitude (at the intermediate time T). * Then, and for example, at the first time T1). The resonance amplitude depends in a known manner on the design specifications of MEMS gyroscope 1.

[0042] Figure 4 The chart includes three sections: top, middle, and bottom, which respectively show the control signal V of the PDM when MEMS gyroscope 1 resonates. ctrl,PDM Control signal V ctrl The combined displacement X of the movable mass 7 m .exist Figure 4In this case, the signals are shown as a function of time and in a second time interval (for example, comprised between a second time T2 and a third time T3, with T* < T2 < T3 and, for example, T1 < T2 < T3), the second time interval shows a stable resonant state (also called "steady state") of the MEMS gyroscope 1. As mentioned above, the PDM control signal V ctrl,PDM oscillates between a first voltage V0 and a second voltage V DD , with a pulse density indicative of an oscillation amplitude error with which the displacement of the movable mass 7 is controlled. Moreover, the control signal V ctrl is stabilized in oscillation around the second value V ctrl,2 , for example, with respect to a variation of the second value V ctrl,2 which depends on the gain of the loop gain and which is typically comprised in a range of about ±1% of the second value V ctrl,2 , so that the displacement X m of the movable mass 7 has a substantially constant maximum amplitude (for example, depending on the gain of the loop gain and typically comprised in a range of about ±1% of the maximum amplitude).

[0043] Moreover, the MEMS gyroscope 1 can be comprised in a device (not shown and, for example, a mobile phone, a PC or a wearable apparatus, such as a smartwatch), so as to allow the device to sense the angular velocity Ω z experienced by it.

[0044] The advantages obtainable according to the present disclosure, in light of the features made according to the present disclosure, are evident.

[0045] In particular, the driving circuit 14 allows the MEMS gyroscope 1 to be controlled so as to ensure an oscillation in resonance thereof, and has a combination of control of the analog type and control of the digital type. In detail, the driving circuit 14 implements the control of the oscillation amplitude of the movable mass 7 in resonance via a digital conversion stage 20, a processing block 22, an analog conversion stage 24, a filtering stage 26 and a buffer 28.

[0046] The processing block 22 is constituted by a specific processing unit (for example, a dedicated integrated logic such as a CPU or a dedicated microcontroller) integrated in the MEMS gyroscope 1, rather than being implemented in an FPGA. This allows a reduction in the cost, complexity and consumption of the MEMS gyroscope 1.

[0047] Moreover, the use of a Sigma-Delta DAC as analog conversion stage 24 ensures a reduction in the power consumption with respect to known control circuits. In particular, the use of a first-order Sigma-Delta DAC minimizes the complexity of the driving circuit 14 and, therefore, its energy consumption.

[0048] The filter stage 26 reduces the high-frequency quantization noise generated by the PDM-type modulation of the analog conversion stage 24, thus preventing the generation of high and spurious vibration modes of the MEMS gyroscope 1. Furthermore, the filter stage 26 is passive (which reduces energy consumption compared to its use in known circuits with active filters).

[0049] Furthermore, the sampling performed by the digital conversion stage 20 (specifically, in the differential sensing signal V) SD (at the peak) at or below the oscillation frequency f dr This reduces the power consumption of the drive circuit 14 at the specified frequency.

[0050] Finally, it is clear that modifications and variations may be made to the present disclosure described and illustrated herein without departing from the scope of the disclosure as defined in the appended claims.

[0051] In particular, refer to Figure 1 The structure of the MEMS gyroscope 1 described is purely exemplary, and other known structures can be considered similarly. For example, the MEMS gyroscope can be of a multi-axis type. Furthermore, the MEMS gyroscope can be replaced by any second-order MEMS oscillator.

[0052] A drive circuit (14) for controlling a MEMS oscillator (1) can be summarized as including: a digital conversion stage (20) configured to: acquire a differential sensing signal (V0) indicating the sensed displacement of a movable mass (7) of the MEMS oscillator (1). SD ), and will use analog-type differential sensing signals (V SD Converted to a digital differential signal (V) diff ); processing block (22), electrically coupled to the digital conversion stage (20) and configured to: according to the digital differential signal (V diff The differential reference signal (V) indicating the target amplitude of the oscillation of the movable mass (7) is compared with the signal indicating the target amplitude of the oscillation of the movable mass (7). ref The comparison between the two signals generates a digital control signal (V). ctrl,dig The differential reference signal causes the MEMS oscillator (1) to resonate at its resonant frequency; the analog conversion stage (24), electrically coupled to the processing block (22), is configured to convert the digital control signal (V) into a digital signal. ctrl,dig ) is converted into an analog type PDM control signal (V ctrl,PDM The analog conversion stage (24) includes a ΣΔ type digital-to-analog converter (DAC); and a low-pass type filter stage (26), electrically coupled to the analog conversion stage (24) and configured to: control the PDM signal (V ctrl,PDM) is filtered to generate a control signal (V ctrl ) for controlling an oscillation amplitude of the movable mass (7).

[0053] The digital conversion stage (20) can be configured to sample the differential sense signal (V SD ) at a value of interest of the differential sense signal (V SD ).

[0054] The value of interest can be a peak value of the differential sense signal (V SD ).

[0055] The drive circuit (14) can further comprise a buffer (28) having an output and an input electrically coupled to the filter stage (26), wherein the buffer (28) can be configured to receive the control signal (V ctrl ) as an input and deliver the control signal (V ctrl ) as an output, and an output impedance of the buffer (28) can be smaller than an input impedance of the buffer (28).

[0056] The drive circuit (14) can further comprise a comparator stage (16) configured to receive the differential sense signal (V SD ) as an input and generate a natural clock signal (ck) having an oscillation frequency (f dr ) as an output; and a PLL stage (17) electrically coupled to the comparator stage (16) and configured to receive the natural clock signal (ck) as an input and generate one or more derived clock signals (ck) at a respective frequency related to the oscillation frequency (f dr ) as an output.

[0057] The drive circuit (14) can further comprise a drive stage (30) electrically coupled to the filter stage (26) and the PLL stage (17), the drive stage can be configured to receive one of the one or more derived clock signals (ck) from the PLL stage (17), can be configured to receive the control signal (V ctrl ), and can be configured to generate one or more drive signals (D1, D2) for controlling an oscillation of the movable mass (7) as a function of the control signal (V ctrl ).

[0058] The sigma-delta DAC can be first order.

[0059] The sigma-delta DAC can be a 1-bit DAC, and the PDM control signal (V ctrl,PDM ) can oscillate between a first voltage (V0) and a second voltage (V DD ) over time.

[0060] The digital control signal (Vctrl,dig ) can be a function of a gain factor and a difference between a digital differential signal (V diff ) and a differential reference signal (V ref ).

[0061] The filtering stage (26) can be of the passive type.

[0062] The filtering stage (26) can be configured to operate filtering at a cut-off frequency comprised between 100 Hz and 5 kHz.

[0063] The driving circuit (14) can further comprise an input stage (15), which can be electrically coupled to the digital conversion stage (20), can be configured to acquire one or more sense signals (II, I2) indicative of a respective variation of capacitance caused by a displacement of the movable mass (7) of the MEMS oscillator (1), and can be configured to generate, as an output, a differential sense signal (V SD ) as a function of the one or more sense signals (II, I2), the differential sense signal (V SD ) being a voltage signal.

[0064] A MEMS oscillator (1) can be summarized as comprising: a substrate (S); a driving circuit (14), integrated in the substrate (S); and the movable mass (7), elastically coupled to the substrate (S) and controllable, via the control signal (V ctrl ), to resonate oscillations with respect to the substrate (S).

[0065] The MEMS oscillator (1) can be a MEMS gyroscope.

[0066] An apparatus can be summarized as comprising a MEMS oscillator (1).

[0067] A method of controlling a MEMS oscillator (1) can be summarized as comprising the steps of: acquiring, via a digital conversion stage (20), a differential sensing signal (VSD) indicative of a sensed displacement of a movable mass (7) of the MEMS oscillator (1); converting, via the digital conversion stage (20), the differential sensing signal (VSD) of analog type into a digital differential signal (Vdiff) of digital type; generating, via a processing block (22) electrically coupled to the digital conversion stage (20), a digital control signal (Vctrl,dig) of digital type as a function of a comparison between the digital differential signal (Vdiff) and a differential reference signal (Vref) indicative of a target amplitude of oscillation of the movable mass (7), the differential reference signal causing resonance of the MEMS oscillator (1) at a resonance frequency of the MEMS oscillator (1); converting, via an analog conversion stage (24) electrically coupled to the processing block (22), the digital control signal (Vctrl,dig) into a PDM control signal (Vctrl,PDM) of analog type, the analog conversion stage (24) comprising a digital-to-analog converter DAC of Sigma Delta type; generating, via a filtering stage (26) of low-pass type electrically coupled to the analog conversion stage (24), a control signal (Vctrl) by filtering the PDM control signal (Vctrl,PDM); and controlling, via the control signal (Vctrl), an amplitude of oscillation of the movable mass (7). The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above Detailed Description. The terms used in the following claims should not be construed to limit the claims to the specific embodiments disclosed in the Specification and the claims are to include all possible embodiments and the full range of equivalents to which such claims are entitled. Accordingly, the claims are not limited to the disclosed embodiments.

Claims

1. A drive circuit for controlling a MEMS oscillator, comprising: a digital conversion stage configured to acquire a differential sense signal indicative of a sensed displacement of a movable mass of the MEMS oscillator, and to convert the differential sense signal of analog type into a digital differential signal of digital type; a processing circuitry electrically coupled to the digital conversion stage and configured to generate a digital control signal of digital type from a comparison between the digital differential signal and a differential reference signal indicative of a target amplitude of oscillation of the movable mass, the differential reference signal causing resonance of the MEMS oscillator at a resonance frequency of the MEMS oscillator; an analog conversion stage electrically coupled to the processing circuitry and configured to convert the digital control signal into a PDM control signal of analog type, the analog conversion stage comprising a sigma-delta type digital-to-analog converter, DAC; and a filter stage of low-pass type electrically coupled to the analog conversion stage and configured to generate a control signal for controlling an amplitude of oscillation of the movable mass by filtering the PDM control signal.

2. The drive circuit of claim 1, wherein, The digital conversion stage is configured to sample the differential sense signal at a value of interest of the differential sense signal.

3. The drive circuit of claim 2, wherein, The value of interest is a peak value of the differential sense signal.

4. The drive circuit of claim 1, further comprising a buffer having an output and an input electrically coupled to the filter stage, wherein the buffer is configured to receive the control signal as an input and to deliver the control signal as an output, and wherein an output impedance of the buffer is less than an input impedance of the buffer.

5. The drive circuit of claim 1, further comprising: a comparator stage configured to receive the differential sense signal as an input and to generate a natural clock signal having an oscillation frequency as an output; and a PLL stage electrically coupled to the comparator stage and configured to receive the natural clock signal as an input and to generate one or more derived clock signals at respective frequencies related to the oscillation frequency as outputs.

6. The drive circuit of claim 5, further comprising a drive stage electrically coupled to the filter stage and to the PLL stage, the drive stage being configured to: receive one of the one or more derived clock signals from the PLL stage, receive the control signal, and generate one or more drive signals for controlling oscillation of the movable mass from the control signal.

7. The drive circuit of claim 1, wherein, The sigma-delta DAC is first order.

8. The drive circuit of claim 1, wherein, The sigma-delta DAC is a 1-bit DAC and the PDM control signal oscillates between a first voltage and a second voltage over time.

9. The drive circuit of claim 1, wherein, The digital control signal is a function of a gain factor and a difference between the digital differential signal and the differential reference signal.

10. The drive circuit of claim 1, wherein, The filter stage is of passive type.

11. The drive circuit of claim 1, wherein, The filter stage is configured to operate filtering at a cutoff frequency between 100 Hz and 5 kHz.

12. The drive circuit of claim 1, further comprising an input stage electrically coupled to the digital conversion stage, the input stage configured to: obtain one or more sense signals indicative of respective capacitance variations caused by displacements of the movable mass of the MEMS oscillator, and generate, as an output, the differential sense signal as a function of the one or more sense signals, the differential sense signal being a voltage signal.

13. A MEMS oscillator, comprising: a substrate; a movable mass; and a drive circuit integrated in the substrate, the drive circuit comprising: a digital conversion stage configured to obtain a differential sense signal indicative of a sensed displacement of the movable mass, and to convert the differential sense signal of an analog type into a digital differential signal of a digital type; a processing circuitry electrically coupled to the digital conversion stage and configured to: generate a digital control signal of a digital type as a function of a comparison between the digital differential signal and a differential reference signal indicative of a target amplitude of oscillations of the movable mass, the differential reference signal causing resonance of the MEMS oscillator at a resonance frequency of the MEMS oscillator; an analog conversion stage electrically coupled to the processing circuitry and configured to: convert the digital control signal into a PDM control signal of an analog type, the analog conversion stage comprising a sigma-delta type digital-to-analog converter (DAC); and a filter stage of a low-pass type electrically coupled to the analog conversion stage and configured to generate a control signal for controlling an oscillation amplitude of the movable mass by filtering the PDM control signal, wherein the movable mass is elastically coupled to the substrate and is controllable via the control signal to resonantly oscillate relative to the substrate.

14. The MEMS oscillator of claim 13, wherein, The MEMS oscillator is a MEMS gyroscope.

15. The MEMS oscillator of claim 13, wherein, The digital conversion stage is configured to sample the differential sense signal at a value of interest of the differential sense signal.

16. The MEMS oscillator of claim 15, wherein, The value of interest is a peak value of the differential sense signal.

17. The MEMS oscillator of claim 13, further comprising a buffer having an output and an input electrically coupled to the filter stage, wherein the buffer is configured to receive a control signal as an input and to deliver the control signal as an output, and wherein an output impedance of the buffer is less than an input impedance of the buffer.

18. The MEMS oscillator of claim 13, further comprising: a comparator stage configured to receive the differential sense signal as an input and to generate a natural clock signal having an oscillation frequency as an output; and a PLL stage electrically coupled to the comparator stage and configured to receive the natural clock signal as an input and to generate one or more derived clock signals at respective frequencies related to the oscillation frequency as outputs.

19. A method of controlling a MEMS oscillator, comprising: obtaining, via a digital conversion stage, a differential sense signal indicative of a sensed displacement of a movable mass of the MEMS oscillator; convert, via a digital conversion stage, the differential sensing signal of analog type into a digital differential signal of digital type; generate, via processing circuitry electrically coupled to the digital conversion stage, a digital control signal of digital type from a comparison between the digital differential signal and a differential reference signal indicative of a target amplitude of oscillation of the movable mass, the differential reference signal causing resonance of the MEMS oscillator at a resonance frequency of the MEMS oscillator; convert, via an analog conversion stage electrically coupled to the processing circuitry, the digital control signal into a PDM control signal of analog type, the analog conversion stage comprising a digital-to-analog converter DAC of sigma-delta type; generate, via a filter stage of low-pass type electrically coupled to the analog conversion stage, a control signal by filtering the PDM control signal; and control, via the control signal, an amplitude of oscillation of the movable mass.

20. The method of claim 19, further comprising: sampling the differential sensing signal at a value of interest of the differential sensing signal.

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