This paper presents the design and implementation of a high-order /spl Sigma//spl Delta/ interface for micromachined inertial sensors, which employs an electronic filter in series with the mechanical sensor element to reject the excessive in-band quantization noise inherently present in state-of-the-art second-order solutions. A fourth-order prototype was fabricated in a standard 0.5-/spl mu/m CMOS process. The active circuit area measures 0.9 mm/sup 2/, and the interface consumes 13 mW from a 5-V supply and achieves resolution of 1/spl deg//s//spl radic/Hz with a gyroscope and 150/spl mu/g//spl radic/Hz with an accelerometer. Comparison between the measured and simulated behavior of the system shows that the contribution of the quantization error to the total noise is negligible.

A fourth-order /spl Sigma//spl Delta/ interface for micromachined inertial sensors

In this paper, we describe the system architecture and prototype measurements of a MEMS gyroscope system with a resolution of 0.025deg/s/ radic(Hz). The architecture makes extensive use of control loops, which are mostly in the digital domain. For the primary mode both the amplitude and the resonance frequency are tracked and controlled. The secondary mode readout is based on unconstrained SigmaDelta force-feedback, which does not require a compensation filter in the loop and thus allows more beneficial quantization noise shaping than prior designs of the same order. Due to the force-feedback, the gyroscope has ample dynamic range to correct the quadrature error in the digital domain. The largely digital setup also gives a lot of flexibility in characterization and testing, where system identification techniques have been used to characterize the sensors. This way, a parasitic direct electrical coupling between actuation and readout of the mass-spring systems was estimated and corrected in the digital domain. Special care is also given to the capacitive readout circuit, which operates in continuous time.

/pdf/a-closed-loop-digitally-controlled-mems-gyroscope-with-1t16dkc5r2.pdf

A Closed-Loop Digitally Controlled MEMS Gyroscope With Unconstrained Sigma-Delta Force-Feedback

Traditionally, most of the sensor interfaces must be tailored towards a specific application. This approach results in a high recurrent design cost and time to market. On the other hand, generic sensor interface design reduces the costs and offers a handy solution for multisensor applications. This paper presents a generic sensor interface chip (GSIC), which can read out a broad range of capacitive sensors. It contains capacitance-to-voltage converters, a switched-capacitor amplifier, an analog-to-digital converter, oscillators, clock generation circuits and a reference circuit. The system combines a very low-power design with a smart energy management, which adapts the current consumption according to the accuracy and speed requirements of the application. The GSIC is used in a pressure and an acceleration monitoring system. The pressure monitoring system achieves a current drain of 2.3 muA for a 10-Hz sample frequency and an 8-bit accuracy. In the acceleration monitoring system, we measured a current of 3.3 muA for a sample frequency of 10 Hz and an accuracy of 9 bits

Ultra-Low-Power Interface Chip for Autonomous Capacitive Sensor Systems

This review provides a comprehensive survey of noise research in MEMS. Some background on noise and on MEMS is provided. We review noise production mechanisms, and highlight work on the theory and modeling of noise in MEMS. Then noise measurements in the specific types of MEMS are reviewed. Inertial MEMS (accelerometers and angular rate sensors), pressure and acoustic sensors, optical MEMS, RF MEMS, surface acoustic wave devices, flow sensors, and chemical and biological MEMS, as well as data storage devices and magnetic MEMS, are reviewed. We indicate opportunities for additional experimental and computational research on noise in MEMS.

Noise in MEMS

In this paper, design, implementation and characterization of a 3-V switched-capacitor (SC) DeltaSigma CMOS interface circuit for the closed-loop operation of a lateral capacitive micro-gravity silicon-on-insulator (SOI) accelerometer is presented The interface circuit is based on a front-end programmable reference-capacitorless SC charge amplifier and a back-end second-order SC DeltaSigma modulator The accelerometer is fabricated through a dry-release high aspect-ratio reduced-gap process By incorporating the low-Q transfer function of the microaccelerometer in a feedback loop, the system's dynamic range is improved by 20 dB, leading to a measured resolution of 4 mug/radicHz and an output dynamic range of 95 dB at 20 Hz The bias instability is 2 to 8 mug for 12 hours The chip is fabricated in the 05-mum standard CMOS process with an area of 225 mm2 The integrated circuit (IC) consumes 45 mW of power

A 4.5-mW Closed-Loop $\Delta\Sigma$ Micro-Gravity CMOS SOI Accelerometer

This paper presents the design and implementation of a high-order interface for micromachined inertial sensors, which employs an electronic filter in series with the mechanical sensor element to reject the excessive in-band quantization noise inherently present in state-of-the-art second-order solutions. A fourth-order prototype was fabricated in a standard 0.5m CMOS process. The active circuit area measures 0.9 mm, and the interface consumes 13 mW from a 5-V supply and achieves resolution of 1 /s/ Hz with a gyroscope and 150 Hz with an accelerometer. Comparison between the measured and simulated behavior of the system shows that the contribution of the quantization error to the total noise is negligible.

17.6 A Fourth-Order Σ∆ Interface for Micromachined Inertial Sensors

This paper presents a closed-loop accelerometer interface based on a force-feedback ΔΣ loop. The interface reduces the offset, and its drift, arising from asymmetry in the parasitic capacitances of the bondwires connecting the CMOS interface IC and the MEMS sensor element. This is achieved by modulating the spring constant of the sensor electrostatically and continuously measuring and nulling the offset via an offset cancellation loop. In a bondwire deformation experiment, we show that the offset cancellation loop reduces an induced offset of 350 mg by 54 dB down to 0.7 mg. The interface was fabricated in a 0.18-μm CMOS technology. The active circuit area is 1.35 mm2 and the interface consumes 3.1 mW from a 3-V supply, while achieving a noise floor of 220 μg/√Hz over a 1-kHz bandwidth. When the offset cancellation loop is turned on the bandwidth available for the signal is 200 Hz.

A $\Delta \Sigma$ Interface for MEMS Accelerometers Using Electrostatic Spring Constant Modulation for Cancellation of Bondwire Capacitance Drift

Analysis of second-order electromechanical sigma-delta (/spl Sigma//spl Delta/) inertial sensors shows that in-band quantization error introduces a resolution penalty, which cannot be eliminated by oversampling. In addition, a tradeoff between resolution and phase compensation forces such systems to operate with reduced phase margin. This paper introduces high-order electromechanical /spl Sigma//spl Delta/ modulation as an approach, which eliminates the quantization noise overhead and allows for increased phase compensation without degrading the resolution. Quasi-linear analysis is used to evaluate the contribution of the individual noise sources to the output of the system and to examine the effect of noise interaction on the behavior of electromechanical /spl Sigma//spl Delta/ modulators.

High-order electromechanical /spl Sigma//spl Delta/ modulation in micromachined inertial sensors

An accelerometer for electronic stability control utilizes a two-mass mechanical sensor element to implement a fully-differential signal path, achieving robustness against electromagnetic interference (EMI) without the need for external shielding in the package. The EMI rejection is augmented further with a pseudo-random chopping scheme, which spreads the interference over a wide bandwidth, reducing its in-band portion to the level of the noise floor. The chopping function maintains zero-mean voltage waveforms across the sensor electrodes, which is also beneficial for the long-term offset stability of the device. A charge-balanced capacitance-to-voltage converter provides linear transduction for displacements of the proof-mass up to 70% of the gap and minimizes the residual electrostatic forces. A dual-axis design occupies 1.1 mm 2 in 0.18- μm CMOS and consumes 820 μA from an internally regulated 1.9-V supply. The system achieves 380 μg/ √Hz noise floor and 84-dB dynamic range. The offset variation in the automotive temperature range of -40 to +140°C has a 3 σ range of ±11 mg.

Vladimir Petkov

Papers

A fourth-order /spl Sigma//spl Delta/ interface for micromachined inertial sensors

17.6 A Fourth-Order Σ∆ Interface for Micromachined Inertial Sensors

A $\Delta \Sigma$ Interface for MEMS Accelerometers Using Electrostatic Spring Constant Modulation for Cancellation of Bondwire Capacitance Drift

High-order electromechanical /spl Sigma//spl Delta/ modulation in micromachined inertial sensors

A fully differential charge-balanced accelerometer for Electronic Stability Control