Stefan Rombach

Drive and sense interface for gyroscopes based on bandpass sigma-delta modulators

This paper demonstrates a MEMS gyroscope system with extensive use of sigma-delta (ΣΔ) modulation in both, primary and secondary modes. The primary loop has a bandpass ΣΔ-digital-to-analog converter (DAC) driving the primary mass into resonance, which is implemented on a field programmable gate array (FPGA). With this strategy of shifting the primary oscillation control into the digital domain, the analog circuit complexity is enormously reduced. A continuous-time (CT) fourth-order micro-electro-mechanical ΣΔ Modulator (ΣΔM) incorporating the secondary resonator is used to convert the Coriolis rate signal into a bit stream. This ΣΔM is implemented on PCB performing an in-band noise (IBN) below −60 dBFS.

Q-enhancement of a low-power gm-C bandpass filter for closed-loop sensor readout applications

In this paper, a Q-enhancement technique for gm-C biquadratic bandpass filters is discussed. The frequencies of the parasitic pole-zero pair of a low-transconductance OTA are altered with an additional compensation capacitor, which cancels the nonidealities of the resonator. The presented technique is used to design a resonator which fulfills the requirements on a loop-filter for closed-loop delta-sigma sensor readout applications. The high stability of the Q-factor against transconductance and center frequency tuning, as well as against process variations is discussed and demonstrated with transistor-level simulations. The designed resonator exhibits a 3-sigma worst-case Q-factor larger than 3600 at the nominal center frequency of 25 kHz and a Q-factor larger than 1000 over the tuning range from 13 kHz to 32 kHz.

Modulated electro-mechanical continuous-time lowpass sigma-delta-modulator for micromachined gyroscopes

In this paper a novel system concept is presented for the implementation of lowpass sigma-delta (ΣΔ) modulators for micromachined gyroscopes. Within the electro-mechanical ΣΔ-modulator two modulation-stages are implemented, reducing the sampling frequency compared to bandpass- and conventional lowpass modulators by a factor of 4 and 100, respectively. By implementing two modulation-stages in the loop the frequency characteristic of the mechanical sensor is changed, thus it is possible to model the mechanical sensor as filter with lowpass-characteristic in the band of interest. Therefore the complete system functions with only one relevant frequency, namely the primary resonance frequency of the mechanical sensor. This concept enables the design of a continuous-time (CT) interface with an extremely low sampling frequency, hence the power dissipation is reduced enormously. A systematic design approach of this architecture is presented and simulation results are shown and discussed.

An Interface ASIC for MEMS Vibratory Gyroscopes With a Power of 1.6 mW, 92 dB DR and 0.007°/s/

This paper reports on the implementation of an interface ASIC for MEMS vibratory gyroscopes exhibiting a wide tuning range from 7 to 31 kHz with a power consumption of 1.6 mW and a dynamic range of 92 dB over a signal band of 40 Hz. The drive loop of the system employs a high voltage stage based on flying capacitors for the excitation of the drive mass allowing a startup time of 44 ms. For the detection of the Coriolis signal, a fourth-order closed-loop electromechanical sense loop deploying continuous-time (CT) circuit techniques and ΔΣ-modulation is implemented. The sense loop deploys a high-Q electronic resonator based on gm-C circuits and a collocated charge-integrator for signal readout and for generating feedback forces. The readout interface with an active circuit area of 2.42 mm2 is fabricated in a 0.35 μm CMOS technology with a HV-module. The ASIC achieves a noise floor of 0.007°/s/√Hz over 40 Hz while the full-scale range is higher than ±1400°/s and the bias stability measures 2.6°/h. The wide tuning capability of the ASIC is demonstrated on the basis of two tested MEMS gyroscopes for different applications with an equal ASIC performance.

\sqrt {\rm {Hz}}

This paper presents a phase-locked drive loop for gyroscopes with an amplitude regulation based on phase-shifting. Instead of regulating the AC or DC value of the driving stage in order to maintain a constant primary oscillation, this concept introduces a tunable phase delay into the driving loop. Compared to conventional amplitude regulation this concept is advantageous, since the high voltages of the driving stage can be set to a constant value and do not have to be adjusted. This reduces the complexity of the analog circuitry enormously and eases the use of higher driving voltages. Additionally this concept provides a sampling frequency for a sigma-delta modulator readout interface.

Noise Floor Over a 40 Hz Band

A front-end circuit for closed loop continuous-time delta-sigma (CT ΔΣ) micro-electro-mechanical gyroscope readout circuits is implemented. This work presents for the first time a CT collocated feedback, which simultaneously uses the detection capacitors of the sensor for the signal readout and the feedback. This is realized by the modulation of the input common mode of the readout amplifier and relies solely on CT techniques to achieve a low noise floor. Additionally, the concept decreases the number and the complexity of the signals in the high voltage (HV) domain. Therefore, the power and area demands of the developed HV interface for quadrature compensation and mode matching are reduced. The circuit is implemented in a 0.35 μm technology, requires an area of 0.65 mm2 and consumes 770 μW.

Phase-locked drive loop with amplitude regulation based on phase-shifting for gyroscopes

This work presents an integrated closed-loop driving circuit for previously reported PZT resonant micro-mirrors, which is based on embedded capacitive position sensors for minimizing the system footprint. Signals with a high SNR of 84 dB were measured, when the mechanical scan angle of the micro-mirror was 2◦, so that high controlling resolution of 14 bit for the complete motion range of the mirror is enabled. The total power consumption of the closed-loop system is only 0.86mW. Measurement results of the closed-loop driven micromirror system are presented, demonstrating its competitiveness due to the great reliability, high precision and low-power consumption. Additionally, the implementation and performance of a self-resonant loop is discussed. Finally, the fabrication, temperature dependency and performance of embedded capacitive position sensors for single and dual axis PZT resonant micro-mirrors is evaluated and presented.

A Continuous-Time Collocated Force-Feedback and Readout Front-End for MEM Gyroscopes

MEMS gyroscopes are used in closed-loop configuration (CL) to satisfy the demand for high-performance and stable inertial sensors [1]. Due to the higher complexity and power consumption compared to open-loop solutions, these systems have usually been unsuitable for mobile battery-driven devices, e.g., for indoor navigation. Recently, the utilization of CT-ΔΣM for the readout of gyroscopes has shown to be a promising approach for reduced power consumption in a CL system [2]. In general, an accurate matching of the electrical BPF [2] to the drive and sense resonance frequencies of the sensor [3] is a prerequisite for maximizing SNR. For systems with drive frequencies fd of some tens of kHz and with a typical angular rate bandwidth of BW=50Hz, the frequency matching needs to be as precise as BW/fd<0.5%. However, the frequency variation of CT BPFs in CT-ΔΣM over PVT is large compared to DT circuits. This paper presents a fully integrated frequency tuning circuit that is based on noise observation at the input of the electrical BPF in an electromechanical CT-ΔΣM. It works in the background during normal operation, achieving a precision better than 0.25% fd and featuring a considerably lower power of 27µW and lower area of 0.06mm2 than competing approaches (see Fig. 9.4.6).