Sebastian Nessler

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.

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.

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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.

Noise Floor Over a 40 Hz Band

This paper presents the implementation of a self-test for a gyroscope readout based on electro-mechanical ΔΣ modulation. Commonly, sensor element and readout ASIC are fabricated on separate wafers. Therefore, the ability to characterize sensor element and ASIC before packaging is desirable in order to reduce unnecessary expense. For the proposed self-test, a charge integrator with collocated detection and feedback is configured to generate an additional feedback path, which is used to operate a purely electrical ΔΣ modulator on wafer level. The self-test utilizing this modulator is performed in two steps. First, an automatic tuning loop is proposed to configure the collocated feedback and to verify its functionality. In a second step, an offset is added to the settled value of the tuning loop to generate a negative feedback path. The resulting purely electrical ΔΣ modulator structure is utilized to validate the functionality of every single circuit component of the later operated electro-mechanical ΔΣ modulator. Furthermore, it allows the characterization of the bandpass loop filter by observing the ΔΣ modulator output power spectral density.

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).