Integrated reference circuits for low-power capacitive sensor interfaces

This thesis consists of nine publications and an overview of the research topic, which also summarizes the work. The research described in this thesis concentrates on the design of low-power sensor interfaces for capacitive 3-axis micro-accelerometers. The primary goal throughout the thesis is to optimize power dissipation. Because the author made the main contribution to the design of the reference and power management circuits required, the overview part is dominated by the following research topics: current, voltage, and temperature references, frequency references, and voltage regulators. After an introduction to capacitive micro-accelerometers, the work describes the typical integrated readout electronics of a capacitive sensor on the functional level. The readout electronics can be divided into four different functional parts, namely the sensor readout itself, signal post-processing, references, and power management. Before the focus is shifted to the references and further to power management, different ways to realize the sensor readout are briefly discussed. Both current and voltage references are required in most analog and mixed-signal systems. A bandgap voltage reference, which inherently uses at least one current reference, is practical for the generation of an accurate reference voltage. Very similar circuit techniques can be exploited when implementing a temperature reference, the need for which in the sensor readout may be justified by the temperature compensation, for example. The work introduces non-linear frequency references, namely ring and relaxation oscillators, which are very suitable for the generation of the relatively low-frequency clock signals typically needed in the sensor interfaces. Such oscillators suffer from poor jitter and phase noise performance, the quantities of which also deserve discussion in this thesis. Finally, the regulation of the supply voltage using linear regulators is considered. In addition to extending the battery life by providing a low quiescent current, the regulator must be able to supply very low load currents and operate without off-chip capacitors

Low-power front-ends for capacitive three-axis accelerometers

This thesis consists of six publications and an overview of the research topic. The overview concentrates on background information of the capacitive accelerometers and front-ends. The publications focus on two low-power front-ends that were implemented for capacitive three-axis accelerometers and their operation as a part of an interface. The switched-capacitor front-ends that were implemented are based on the charge-balancing structures, namely a self-balancing bridge and a ΔΣ front-end, which convert the capacitive acceleration information to analog and digital signals, respectively. Both structures operate mechanically in open-loop configuration and are capable of reducing the effects of the electrostatic forces and displacement-to-capacitance conversion. According to the performance comparison presented in this thesis, both interfaces, which were implemented around the front-ends, exhibit competitive performance when compared to the commercial products of the day

A low-power delta-sigma modulator ADC for sensor system applications

This paper discusses a third-order tri-level quantizer delta-sigma modulator analog-digital converter (ADC) for cascaded integrators with distributed feedback (CIFB) and cascaded integrators with distributed feedforward (CIFF) structure for sensor system applications. The signal transfer function (STF) and noise transfer function (NTF) discussed for poles and zeroes. Oversampling ratio (OSR) and different quantizer level presented for the modulator structure to trade-off the targeted bandwidth and complexity of increased quantizer level. NTF zero optimization technique also implemented to further reduce inband quantization noise by shaping at high frequency, which is later filtered by digital low-pass filter. Mismatch simulation results also performed for quantizer levels considering the performance degradation of the modulator. Operational amplifier (op-amp) for the front-end integrator optimized for minimum power consumption by considering low finite DC-gain, limited slew-rate, minimum required gain-bandwidth product (GBW). The proposed model simulations provided and discussed. Non-ideal effect for the proposed complete modulator CIFF structure for switched-capacitor circuit level implementation performed. The non-ideal parameters like thermal noise, sampling jitter, white noise, and switch nonlinearity also discussed. Modeling simulation results for CIFF structure with trilevel quantizer, shows that proposed modulator structure can achieve signal-to-noise ratio (SNR) of 133dB for sensor system bandwidth of 10kHz with OSR = 128

System design of a low-power three-axis underdamped MEMS accelerometer with simultaneous electrostatic damping control

Recently, consumer electronics industry has known a spectacular growth that would have not been possible without pushing the integration barrier further and further. Micro Electro Mechanical Systems (MEMS) inertial sensors (e.g. accelerometers, gyroscopes) provide high performance, low power, low die cost solutions and are, nowadays, embedded in most consumer applications. In addition, the sensors fusion has become a new trend and combo sensors are gaining growing popularity since the co-integration of a three-axis MEMS accelerometer and a three-axis MEMS gyroscope provides complete navigation information. The resulting device is an Inertial measurement unit (IMU) able to sense multiple Degrees of Freedom (DoF). Nevertheless, the performances of the accelerometers and the gyroscopes are conditioned by the MEMS cavity pressure: the accelerometer is usually a damped system functioning under an atmospheric pressure while the gyroscope is a highly resonant system. Thus, to conceive a combo sensor, aunique low cavity pressure is required. The integration of both transducers within the same low pressure cavity necessitates a method to control and reduce the ringing phenomena by increasing the damping factor of the MEMS accelerometer. Consequently, the aim of the thesis is the design of an analog front-end interface able to sense and control an underdamped three-axis MEMSaccelerometer. This work proposes a novel closed-loop accelerometer interface achieving low power consumption The design challenge consists in finding a trade-off between the sampling frequency, the settling time and the circuit complexity since the sensor excitation plates are multiplexed between the measurement and the damping phases. In this context, a patenteddamping sequence (simultaneous damping) has been conceived to improve the damping efficiency over the state of the art approach performances (successive damping). To investigate the feasibility of the novel electrostatic damping control architecture, several mathematical models have been developed and the settling time method is used to assess the damping efficiency. Moreover, a new method that uses the multirate signal processing theory and allows the system stability study has been developed. This very method is used to conclude on the loop stability for a certain sampling frequency and loop gain value. Next, a 0.18μm CMOS implementation of the entire accelerometer signal chain is designed and validated

A Low-Power Interface for Capacitive Sensors With PWM Output and Intrinsic Low Pass Characteristic

A compact, low power interface for capacitive sensors, is described. The output signal is a pulse width modulated (PWM) signal, where the pulse duration is linearly proportional to the sensor differential capacitance. The original conversion approach consists in stimulating the sensor capacitor with a triangular-like voltage waveform in order to obtain a square-like current waveform, which is subsequently demodulated and integrated over a clock period. The charge obtained in this way is then converted into the output pulse duration by an approach that includes an intrinsic tunable low pass function. The main non idealities are thoroughly investigated in order to provide useful design indications and evaluate the actual potentialities of the proposed circuit. The theoretical predictions are compared with experimental results obtained with a prototype, designed and fabricated using 0.32 mu M CMOS devices from the BCD6s process of STMicroelectroncs. The prototype occupies a total area of 1025 x 515 mm(2) and is marked by a power consuption of 84 mu W. The input capacitance range is 0-256 fF, with a resolution of 0.8 fF and a temperature sensitivity of 300 ppm/degrees C

MICRO-ELECTRO-MECHANICAL SYSTEM OSCILLATING ACCELERAMETERS WITH CMOS READOUT CIRCUITS

Ph.DDOCTOR OF PHILOSOPH

Sub-Femto-Farad Resolution Electronic Interfaces for Integrated Capacitive Sensors: A Review

Capacitance detection is a universal transduction mechanism used in a wide variety of sensors and applications. It requires an electronic front-end converting the capacitance variation into another more convenient physical variable, ultimately determining the performance of the whole sensor. In this paper we present a comprehensive review of the different signal conditioning front-end topologies targeted in particular at sub-femtofarad resolution. Main design equations and analysis of the limits due to noise are reported in order to provide the designer with guidelines for choosing the most suitable topology according to the main design specifications, namely energy consumption, area occupation, measuring time and resolution. A data-driven comparison of the different solutions in literature is also carried out revealing that resolution, measuring time, area occupation and energy/conversion lower than 100 aF, 1 ms 0.1 mm2, and 100 pJ/conv. can be obtained by capacitance to digital topologies, which therefore allow to get the best compromise among all design specifications

Design of electronic systems for automotive sensor conditioning

This thesis deals with the development of sensor systems for automotive, mainly targeting the exploitation of the new generation of Micro Electro-Mechanical Sensors (MEMS), which achieve a dramatic reduction of area and power consumption but at the same time require more complexity in the sensor conditioning interface. Several issues concerning the development of automotive ASICs are presented, together with an overview of automotive electronics market and its main sensor applications. The state of the art for sensor interfaces design (the generic sensor interface concept), consists in sharing the same electronics among similar sensor applications, thus saving cost and time-to-market but also implementing a sub-optimal system with area and power overheads. A Platform Based Design methodology is proposed to overcome the limitations of generic sensor interfaces, by keeping the platform generality at the highest design layers and pursuing the maximum optimization and performances in the platform customization for a specific sensor. A complete design flow is presented (up to the ASIC implementation for gyro sensor conditioning), together with examples regarding IP development for reuse and low power optimization of third party designs. A further evolution of Platform Based Design has been achieved by means of implementation into silicon of the ISIF (Intelligent Sensor InterFace) platform. ISIF is a highly programmable mixed-signal chip which allows a substantial reduction of design space exploration time, as it can implement in a short time a wide class of sensor conditioning architectures. Thus it lets the designers evaluate directly on silicon the impact of different architectural choices, as well as perform feasibility studies, sensor evaluations and accurate estimation of the resulting dedicated ASIC performances. Several case studies regarding fast prototyping possibilities with ISIF are presented: a magneto-resistive position sensor, a biosensor (which produces pA currents in presence of surface chemical reactions) and two capacitive inertial sensors, a gyro and a low-g YZ accelerometer. The accelerometer interface has also been implemented in miniboards of about 3 cm2 (with ISIF and sensor dies bonded together) and a series of automatic trimming and characterization procedures have been developed in order to evaluate sensor and interface behaviour over the automotive temperature range, providing a valuable feedback for the implementation of a dedicated accelerometer interface