Mikko Saukoski

Zero-Rate Output and Quadrature Compensation in Vibratory MEMS Gyroscopes

In this paper, issues related to the zero-rate output (ZRO) of a vibratory microgyroscope are studied. Different sources of the ZRO are discussed and how the effect of each source can be minimized and their stability improved is described. The effects of synchronous demodulation and electrostatic quadrature compensation performed with a dc voltage on the final ZRO are analyzed. Ways to implement the control loop for electrostatic quadrature compensation performed with a dc voltage are described, concentrating on a case where the compensation voltage is generated with a digital-to-analog converter and the controller is digital. In particular, extending the resolution with SigmaDelta techniques is studied. The experimental work shows the feasibility of the implemented quadrature compensation loop and analyzes the ZRO sources of one particular gyroscope implementation. How to perform the ZRO measurements in such a way that the various sources can be distinguished from each other is also described.

A Micropower

In this paper, a micropower interface IC for a capacitive 3-axis micro-accelerometer is presented. The IC is implemented in a 0.25-mum CMOS process. The fully-integrated sensor interface is based on a DeltaSigma sensor front-end that operates mechanically in an open-loop configuration and converts the acceleration signals directly into the digital domain, thus avoiding the use of separate analog-to-digital converters. A detailed analysis with transfer functions is presented for the front-end circuit. Furthermore, the interface IC includes a decimator, a frequency reference, a clock generator for the front-end, a voltage and current reference, the required reference buffers, and low-dropout regulators (LDOs) needed for system-on-chip power management. The interface IC provides operating modes with 12-bit resolution for 1 and 25 Hz signal bandwidths. The former is optimized for very low power dissipation at the cost of reduced bandwidth, and is intended for example for activity monitoring in otherwise powered-off devices. The chip, with a 1.73 mm2 active area, draws typically 21.2 muA in the 1 Hz mode, and 97.6 muA in the 25 Hz mode, from a 1.2-2.75 V supply. In the 1 and 25 Hz modes with a plusmn 4-g capacitive 3-axis accelerometer, the measured noise floors in the x-, y-, and z-directions are 1080, 1100 and 930 mug/radic{Hz}, and 360, 320 and 275 mug/radic{Hz} , respectively. The implemented prototype achieves competitive figures of merit (FOMs) compared to the other published or commercially available, low-g, low-power accelerometers.

\Delta\Sigma

In this paper, a micropower interface IC for a capacitive 3-axis micro-accelerometer implemented in a 0.13- BiCMOS process is presented. The sensor interface consists of a front-end that converts the acceleration signal to voltage, two algorithmic ADCs, two frequency references, and a voltage, current, and temperature reference circuit. Die area and power dissipation are reduced by using time-multiplexed sampling and varying duty cycles down to 0.3%. The chip with a 0.51 active area draws 62 from a 1.8 V supply while sampling temperature at 100 Hz, and four proof masses, each at 1.04 kHz. With a 4-g capacitive 3-axis accelerometer, the measured noise floors in the x-, y-, and z-directions are 482 , 639 , and 662 , respectively.

-Based Interface ASIC for a Capacitive 3-Axis Micro-Accelerometer

This paper presents a charge sensitive amplifier (CSA) for readout of micromechanical capacitive sensors. The transfer function and noise performance together with noise optimisation are studied from a theoretical point of view. The feedback resistors of the CSA are implemented as long-channel MOS transistors to set the -3 dB corner frequency well below 1 kHz. The common-mode feedback is designed to keep the input common-mode level constant to achieve an accurate gain from the change in capacitance to output signal. A CSA was designed and implemented for the readout of a bulk micromachined gyroscope with a resonance frequency of 8-10 kHz and signal bandwidth of 100 Hz. A measured resolution of 2.47-3.00 aF (rms) in capacitance change integrated over the signal band is achieved. The CSA tolerates leakage currents of over 5 nA at its input.

A Micropower Interface ASIC for a Capacitive 3-Axis Micro-Accelerometer

In this paper, measurement results for a micropower 2 MHz CMOS frequency reference circuit fabricated with a 0.13 mum CMOS process are presented. This frequency reference circuit, based on source-coupled CMOS multivibrator, provides the clock signal for a read-out circuit of a capacitive sensor. In addition to low power consumption, good frequency stability is required. Supply independent biasing and symmetrical loads are used to optimize the frequency stability. The typical power consumption is 3.0 muW at room temperature with 1.8 V supply voltage. When properly calibrated, the frequency stays within plusmn2.5% of the nominal oscillation frequency in the operating voltage range of 1.8-2.5 V (with plusmn10% variation) over a temperature range from -35 to +85 degC. The measured phase noise and jitter agree well with the simulations

Fully integrated charge sensitive amplifier for readout of micromechanical capacitive sensors

An ASIC is implemented for readout and control of a bulk micromachined capacitive gyroscope. Both the system architecture and details on the circuitry of different subsystems are presented. Together with digital signal processing on an FPGA chip, a complete working prototype of a microelectromechanical gyroscope is realized and measured. This plusmn100 deg/s gyroscope achieves 0.053 deg/s/yradic(Hz) spot noise and 49.9 dB signal-to-noise ratio over the 36 Hz bandwidth, with plusmn4 % offset stability over the temperature range from -40 to +85 degC. The work shows that a gyroscope can be realized with the chosen architecture, and that good performance can be achieved

A 3 /spl mu/W, 2 MHz CMOS frequency reference for capacitive sensor applications

This paper describes a ratio-independent algorithmic analog-digital (A/D) converter architecture that is insensitive to capacitance ratio, amplifier offset voltage, amplifier input parasitics, and flicker noise. It requires only one differential amplifier, a dynamic latch, six capacitors, 36 switches, and some digital logic. The prototype 12-bit, 40-kS/s A/D converter (ADC) with an active die area of 0.041 mm2 is implemented in a 0.13-mum CMOS. The power dissipation is minimized using a dynamically biased operational amplifier. With a 68.4-muW power dissipation, the ADC achieves 80.2-dB spurious-free dynamic range and 63.3-dB signal-to-noise and distortion ratio.

Integrated Readout and Control Electronics for a Microelectromechanical Angular Velocity Sensor

In a vibratory microgyroscope, the synchronous demodulation of the secondary (sense) resonator output is a critical part of the signal processing. This is because different nonidealities can corrupt the resulting signals, including the angular velocity information. The effect of these nonidealities is revealed in the demodulation process. In this paper, issues related to synchronous demodulation are discussed. A detailed analysis of the effects caused by various nonidealities is provided. The nonidealities considered are as follows: delay in the displacement-to-voltage conversion, differences in the transfer functions of the two sidebands in the force-to-displacement conversion in the secondary resonator, nonlinearities in the displacement-to-voltage conversion, and mechanical-thermal noise. In addition to the analysis of the nonidealities, a general insight into the demodulation process, which is necessary for a successful system-level design, is provided.

A 12-Bit Ratio-Independent Algorithmic A/D Converter for a Capacitive Sensor Interface

Micromechanical structures with capacitive readout provide a feasible alternative for feedback by utilizing electrostatic forces without any requirement for additional electrodes. For this purpose high voltages are often required. This paper presents circuit structures for on-chip high voltage generation. A high voltage amplifier with digitally controllable output or a high voltage DAC is implemented and, as an example application, the DAC is used for quadrature compensation of a microelectro-mechanical gyroscope. The implemented high voltage generator achieves output voltages between zero and 27 V in 29 mV steps. The charge pump output voltage is limited in all cases to prevent reliability problems

Effects of Synchronous Demodulation in Vibratory MEMS Gyroscopes: A Theoretical Study

In this paper, the system-level implementation of readout and control electronics for a microelectromechanical angular velocity sensor will be presented. The sensor element for which the electronics is designed is a bulk micromachined silicon device with electrostatic detection and excitation. The electronics is divided into two parts, analog and digital. Analog electronics is implemented with a 0.7 mum CMOS process that offers high voltage devices and precision analog components. Digital signal processing is implemented with a field-programmable gate array chip. One of the most important system-level nonidealities, the zero-rate output, is analysed, paying attention to its effects on the electronics design. The implemented plusmn100deg/s angular velocity sensor achieves 0.053deg/s/radicHz measured spot noise and 49.9 dB signal-to-noise ratio over the 36 Hz bandwidth, with plusmn4% zero-rate output stability over the temperature range from -40 to +85 degC