104 • 2011 IEEE International Solid-State Circuits Conference ISSCC 2011 / SESSION 6 / SENSORS & ENERGY HARVESTING / 6.1 6.1 A Low-Power 3-Axis Digital-Output MEMS Gyroscope with Single Drive and Multiplexed Angular Rate Readout Luciano Prandi 1 , Carlo Caminada 1 , Luca Coronato 1 , Gabriele Cazzaniga 1 , Fabio Biganzoli 1 , Riccardo Antonello 2 , Roberto Oboe 2 1 STMicroelectronics, Cornaredo, Italy, 2 University of Padova, Vicenza, Italy Motivated by the increasing demand of integrated inertial-sensing solutions for motion processing and dead-reckoning navigation in handheld devices and low- cost GPS navigators, this paper reports the details of a 3-axis silicon MEMS vibratory gyroscope that fulfills the pressing market requirements for low power consumption, small size and low cost. Thanks to a compact mechanical design that combines a triple tuning-fork structure within a single vibrating element, our solution achieves satisfactory performance in terms of thermal stability, cross- axis error, and acoustic noise immunity by using a small die size. Furthermore, the presence of a single primary vibration mode for the excitation of the 3 tun- ing-forks, together with the possibility of sensing the pickoff modes in a multi- plexing fashion, allows to design a small-area, low-power ASIC. The mechanical structure design is explained with the aid of Fig. 6.1.1. The structure comprises 4 suspended plates coupled to each other by means of 4 folded springs connected to their outer corners, and elastically connected to a central cross-shaped hinge by an additional set of central coupling springs. The primary mode of vibration (driving mode) consists of an in-plane inward/out- ward radial motion of the plates: on the whole, the structure cyclically expands and contracts, similarly to a “beating heart” (hence, the name). Primary actua- tion is provided with a set of comb-finger electrodes placed on a pair of oppo- site plates; the mechanical motion is then propagated to the second pair by means of the coupling folded springs at the corners. The secondary modes of vibration (sensing modes) consist of an in-plane, opposite-phase motion of the second pair of plates (yaw mode), and two out-of-plane, opposite-phase motions of both pairs (pitch & roll modes). The yaw mode is sensed by a set of parallel-plate electrodes located on the second pair of plates, whereas the pitch and roll motions are detected by sensing the capacitive variations between each plate and an electrode placed underneath; additional comb-finger electrodes are reserved for sensing the vibrating motion of the driving mode.The mechanical coupling between the 2 proof masses of each sensing pair allows to read the secondary vibrating motions in differential mode, thus improving rejection of external linear accelerations and vibrations; moreover, each secondary mode has a single resonant sensing frequency, instead of 2 independent frequencies that require accurate matching to avoid performance degradations, such as in designs with uncoupled proof masses. The overall mechanical structure has fre- quency-unmatched primary and secondary modes, with a nominal primary res- onant frequency of 20kHz. This design choice, combined with a high Q-factor, guarantees a satisfactory level of acoustic noise isolation. The Coriolis force exciting a secondary mode is proportional to the velocity of the driving mode and the input angular rate, and directed orthogonally to both the driving axis and sensor rotation axis. The angular rate measurement is obtained from the sensed Coriolis acceleration by demodulation, once the driv- ing mode is oscillated at constant amplitude [1]. The primary mode is excited to oscillate at resonance by closing a feedback loop around the micro-resonator made up of the resonating masses and the drive- readout/drive-forcing comb-finger electrodes. In the feedback path, the capaci- tive unbalancing generated by the oscillating motion of the primary mode is transduced into a voltage signal by a differential charge amplifier (CA); then, a band-pass (BP) switched-capacitor (SC) amplifier removes the residual offset and provides the necessary phase adjustment to have a total loop phase shift of 360° at the resonant frequency, which is required for enforcing a sustained oscil- lation in the electromechanical loop. The BP amplifier output is interpolated by a 2 nd -order continuous-time (CT) low-pass (LP) Chebyshev filter and amplified by a variable-gain amplifier (VGA). The VGA gain is automatically tuned by an outer automatic gain control (AGC) loop to regulate and verify the amplitude of the sustained oscillation at the CT-LP filter output to a constant set-point value. Finally, the VGA output, boosted by a charge-pump, is fed back to the comb- drive actuating electrodes. All internal timings are generated by a PLL synchro- nized with the CT-LP filter output. A single, time-division-multiplexed open-loop readout interface is used to retrieve the angular rate measurements out of the Coriolis accelerations along the 3 sensing axes. A differential CA front-end converts the capacitive unbalanc- ing induced by the Coriolis movement into a voltage signal, which is then syn- chronously AM-demodulated using a carrier in-phase with the velocity of the pri- mary mode motion. A 12b SAR ADC performs internal A/D conversion at a rate of 6.06kHz/axis; a 100/200/400/800Hz output data rate (ODR) is selected by changing the decimation factor of the output sinc-decimators. The final output wordlength is equal to 16b. The compensation of the quadrature error is per- formed at the CA input with a purely passive structure consiting of a dynamical- ly reconfigurable bank of calibrated capacitors [2]. The micro-mechanical element is fabricated with the STMicroelectronics (STM) proprietary thick-film epitaxial polysilicon surface-micromachining process (ThELMA) on a 3.2×3.2mm 2 die; the ASIC is implemented on a 2.5×2.5mm 2 die with a 0.13μm min channel length CMOS process. The MEMS and ASIC dies are stack-assembled in a single 4×4×1.1mm 3 plastic LGA package. Thanks to the STM mechanical structure design, involving only attachment points to the sub- strate (instead of multiple attachments to the cap and the substrate), the sensor output exhibits excellent immunity to external mechanical stresses applied to the package. The characterization results of 33 different samples are reported in Figs. 6.1.3–6.1.5. The overall performances are excellent: the average noise density level is <0.03dps/√Hz (with BW=40Hz and ODR=200Hz – note: dps = degree per second), and the zero-rate output (ZRO) and sensor scale factor (So) are very stable over temperature – with FS=2000dps, the ZRO temperature sensitivity is less than ±0.04dps/°C, while the scale-factor change over the temperature range of -40 to +85°C is within ±2% of the factory-trimmed value (So=70mdps/LSB with FS=2000dps). The design robustness is certified by the tight statistical dis- tributions of the ZRO and So temperature sensitivities. Thanks to the highly sym- metric mechanical design, the cross-axis sensitivities, measured as a percentage of the nominal selected full scale, are always below ±2% and mainly due to mounting tolerances during packaging. High immunity to acoustic noise is evi- dent from the sensor output response to an acoustical stimulus (i.e. sine noise at 90 dBSPL with frequency sweeping in the range 500Hz to 25kHz with steps of 5Hz) reported in Fig. 6.1.6. The plots show the average values of the pitch/roll/yaw angular rate outputs for each frequency of the sinusoidal acousti- cal stimulus (tests are performed with FS=2000dps, ODR=200Hz and output BW=50Hz). Regarding power consumption, with a supply voltage in the range of 2.16 to 3.6V, the current absorbtion is 6.1mA during normal operation, 1.5mA in sleep mode (sensing electronics switched off, but with the driving microresonator still operative to reduce the power-on time), and 5μA in power-down mode. References: [1] J. Geen, et al., “Single-Chip Surface Micromachined Integrated Gyroscope with 50°/h Allan Deviation,” IEEE J. Solid-State Circuits, pp. 1860-1866, Dec. 2002. [2] R. Antonello, R. Oboe, L. Prandi, F. Biganzoli, and C. Caminada, “Open loop Compensation of the Quadrature Error in MEMS Vibrating Gyroscopes,” Annual Conference of IEEE Industrial Electronics Society (IECON), Nov. 2009. 978-1-61284-302-5/11/$26.00 ©2011 IEEE
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104 • 2011 IEEE International Solid-State Circuits Conference
ISSCC 2011 / SESSION 6 / SENSORS & ENERGY HARVESTING / 6.1
6.1 A Low-Power 3-Axis Digital-Output MEMS Gyroscope with Single Drive and Multiplexed Angular Rate Readout
Luciano Prandi1, Carlo Caminada1, Luca Coronato1, Gabriele Cazzaniga1,Fabio Biganzoli1, Riccardo Antonello2, Roberto Oboe2
1STMicroelectronics, Cornaredo, Italy, 2University of Padova, Vicenza, Italy
Motivated by the increasing demand of integrated inertial-sensing solutions formotion processing and dead-reckoning navigation in handheld devices and low-cost GPS navigators, this paper reports the details of a 3-axis silicon MEMSvibratory gyroscope that fulfills the pressing market requirements for low powerconsumption, small size and low cost. Thanks to a compact mechanical designthat combines a triple tuning-fork structure within a single vibrating element, oursolution achieves satisfactory performance in terms of thermal stability, cross-axis error, and acoustic noise immunity by using a small die size. Furthermore,the presence of a single primary vibration mode for the excitation of the 3 tun-ing-forks, together with the possibility of sensing the pickoff modes in a multi-plexing fashion, allows to design a small-area, low-power ASIC.
The mechanical structure design is explained with the aid of Fig. 6.1.1. Thestructure comprises 4 suspended plates coupled to each other by means of 4folded springs connected to their outer corners, and elastically connected to acentral cross-shaped hinge by an additional set of central coupling springs. Theprimary mode of vibration (driving mode) consists of an in-plane inward/out-ward radial motion of the plates: on the whole, the structure cyclically expandsand contracts, similarly to a “beating heart” (hence, the name). Primary actua-tion is provided with a set of comb-finger electrodes placed on a pair of oppo-site plates; the mechanical motion is then propagated to the second pair bymeans of the coupling folded springs at the corners. The secondary modes ofvibration (sensing modes) consist of an in-plane, opposite-phase motion of thesecond pair of plates (yaw mode), and two out-of-plane, opposite-phasemotions of both pairs (pitch & roll modes). The yaw mode is sensed by a set ofparallel-plate electrodes located on the second pair of plates, whereas the pitchand roll motions are detected by sensing the capacitive variations between eachplate and an electrode placed underneath; additional comb-finger electrodes arereserved for sensing the vibrating motion of the driving mode.The mechanicalcoupling between the 2 proof masses of each sensing pair allows to read thesecondary vibrating motions in differential mode, thus improving rejection ofexternal linear accelerations and vibrations; moreover, each secondary mode hasa single resonant sensing frequency, instead of 2 independent frequencies thatrequire accurate matching to avoid performance degradations, such as indesigns with uncoupled proof masses. The overall mechanical structure has fre-quency-unmatched primary and secondary modes, with a nominal primary res-onant frequency of 20kHz. This design choice, combined with a high Q-factor,guarantees a satisfactory level of acoustic noise isolation.
The Coriolis force exciting a secondary mode is proportional to the velocity ofthe driving mode and the input angular rate, and directed orthogonally to boththe driving axis and sensor rotation axis. The angular rate measurement isobtained from the sensed Coriolis acceleration by demodulation, once the driv-ing mode is oscillated at constant amplitude [1].
The primary mode is excited to oscillate at resonance by closing a feedback looparound the micro-resonator made up of the resonating masses and the drive-readout/drive-forcing comb-finger electrodes. In the feedback path, the capaci-tive unbalancing generated by the oscillating motion of the primary mode istransduced into a voltage signal by a differential charge amplifier (CA); then, aband-pass (BP) switched-capacitor (SC) amplifier removes the residual offsetand provides the necessary phase adjustment to have a total loop phase shift of360° at the resonant frequency, which is required for enforcing a sustained oscil-lation in the electromechanical loop. The BP amplifier output is interpolated by a
2nd-order continuous-time (CT) low-pass (LP) Chebyshev filter and amplified bya variable-gain amplifier (VGA). The VGA gain is automatically tuned by an outerautomatic gain control (AGC) loop to regulate and verify the amplitude of thesustained oscillation at the CT-LP filter output to a constant set-point value.Finally, the VGA output, boosted by a charge-pump, is fed back to the comb-drive actuating electrodes. All internal timings are generated by a PLL synchro-nized with the CT-LP filter output.
A single, time-division-multiplexed open-loop readout interface is used toretrieve the angular rate measurements out of the Coriolis accelerations alongthe 3 sensing axes. A differential CA front-end converts the capacitive unbalanc-ing induced by the Coriolis movement into a voltage signal, which is then syn-chronously AM-demodulated using a carrier in-phase with the velocity of the pri-mary mode motion. A 12b SAR ADC performs internal A/D conversion at a rateof 6.06kHz/axis; a 100/200/400/800Hz output data rate (ODR) is selected bychanging the decimation factor of the output sinc-decimators. The final outputwordlength is equal to 16b. The compensation of the quadrature error is per-formed at the CA input with a purely passive structure consiting of a dynamical-ly reconfigurable bank of calibrated capacitors [2].
The micro-mechanical element is fabricated with the STMicroelectronics (STM)proprietary thick-film epitaxial polysilicon surface-micromachining process(ThELMA) on a 3.2×3.2mm2 die; the ASIC is implemented on a 2.5×2.5mm2 diewith a 0.13μm min channel length CMOS process. The MEMS and ASIC dies arestack-assembled in a single 4×4×1.1mm3 plastic LGA package. Thanks to theSTM mechanical structure design, involving only attachment points to the sub-strate (instead of multiple attachments to the cap and the substrate), the sensoroutput exhibits excellent immunity to external mechanical stresses applied to thepackage.
The characterization results of 33 different samples are reported in Figs.6.1.3–6.1.5. The overall performances are excellent: the average noise densitylevel is <0.03dps/√Hz (with BW=40Hz and ODR=200Hz – note: dps = degree persecond), and the zero-rate output (ZRO) and sensor scale factor (So) are verystable over temperature – with FS=2000dps, the ZRO temperature sensitivity isless than ±0.04dps/°C, while the scale-factor change over the temperature rangeof -40 to +85°C is within ±2% of the factory-trimmed value (So=70mdps/LSBwith FS=2000dps). The design robustness is certified by the tight statistical dis-tributions of the ZRO and So temperature sensitivities. Thanks to the highly sym-metric mechanical design, the cross-axis sensitivities, measured as a percentageof the nominal selected full scale, are always below ±2% and mainly due tomounting tolerances during packaging. High immunity to acoustic noise is evi-dent from the sensor output response to an acoustical stimulus (i.e. sine noiseat 90 dBSPL with frequency sweeping in the range 500Hz to 25kHz with steps of5Hz) reported in Fig. 6.1.6. The plots show the average values of thepitch/roll/yaw angular rate outputs for each frequency of the sinusoidal acousti-cal stimulus (tests are performed with FS=2000dps, ODR=200Hz and outputBW=50Hz).
Regarding power consumption, with a supply voltage in the range of 2.16 to3.6V, the current absorbtion is 6.1mA during normal operation, 1.5mA in sleepmode (sensing electronics switched off, but with the driving microresonator stilloperative to reduce the power-on time), and 5μA in power-down mode.
References:[1] J. Geen, et al., “Single-Chip Surface Micromachined Integrated Gyroscopewith 50°/h Allan Deviation,” IEEE J. Solid-State Circuits, pp. 1860-1866, Dec.2002.[2] R. Antonello, R. Oboe, L. Prandi, F. Biganzoli, and C. Caminada, “Open loopCompensation of the Quadrature Error in MEMS Vibrating Gyroscopes,” AnnualConference of IEEE Industrial Electronics Society (IECON), Nov. 2009.
978-1-61284-302-5/11/$26.00 ©2011 IEEE
105DIGEST OF TECHNICAL PAPERS •
ISSCC 2011 / February 21, 2011 / 1:30 PM
Figure 6.1.1: Micrograph of the MEMS gyroscope (actual die size = 3.2×3.2mm2). Figure 6.1.2: System architecture block diagram.
Figure 6.1.3: ZRO stability over temperature (top row); statistical distributionof the ZRO temperature sensitivity over a set of 33 samples (bottom row).Tests are performed with FS=2000dps.
Figure 6.1.5: Statistical distributions of the cross-axis sensitivities over a setof 33 samples. Tests are performed with FS=2000dps.
Figure 6.1.6: Sensor output response to an acoustical stimulus (sine noise at90 dBSPL with frequency sweeping in the range 500Hz to 25kHz with steps of5Hz).
Figure 6.1.4: Scale factor (So) variation over temperature (top row); statisti-cal distribution of the scale factor temperature sensitivity over a set of 33 sam-ples (bottom row). Tests are performed with FS=2000dps.
6
• 2011 IEEE International Solid-State Circuits Conference 978-1-61284-302-5/11/$26.00 ©2011 IEEE
ISSCC 2011 PAPER CONTINUATIONS
Figure 6.1.7: ASIC die micrograph (actual die size = 2.5×2.5mm2).
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