An auto-balancing capacitance-to-pulse-width converter for capacitive sensors

A novel auto-balancing capacitance-to-pulse- width converter (CPC) that uses sinusoidal excitation, and operates in a closed-loop configuration, is presented in this paper. Unlike most of the existing CPCs, the proposed interface circuit is compatible with both single-element and differential capacitive sensors. In addition, it provides a pulse-width modulated (PWM) signal which can easily be digitized using a counter. From this PWM signal, a ratio output is derived when a single-element sensor is interfaced, and a ratiometric output is obtained for a differential sensor.The authors would like to thank the Department of Science and Technology (DST), Govt. of India, for its financial assistance (Grant Number SERB/F/4573/2016-17) in carrying out the research activities presented in this paper.Postprint (published version

Control Circuitry for Self-Repairable MEMS Accelerometers

A BISR (Built-in Self-Repairable) MEMS comb accelerometer with modularized design has been previously reported. In this paper, the differential capacitance sensing circuitry for MEMS comb accelerometer is discussed. The BISR control circuitry based on CMOS transmission gates (TGs) is proposed. Each BISR module is connected to the capacitance sensing circuitry through a transmission gate. By turning on or off a transmission gate, the corresponding module can be either connected to or isolated from the capacitance sensing circuitry. In this way, the faulty module can be easily replaced with a good redundant module for self-repair. The parasitic model for the BISR control circuitry is also analyzed. The analysis results show that the parasitic capacitance will not affect the proper operation of the BISR control circuitry. Furthermore, the signal strength will not be degraded due to the insertion of analog multiplexers. The control circuitry can effectively isolate the faulty module of the BISR MEMS comb accelerometer. Both BISR and non-BISR MEMS accelerometer designs are suggested and their performances are also extracted for comparison

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

Measurement of three independent components in impedance sensors using a single square wave

Two-wire impedance-based sensors involving electrolytes add the impedance of the electrodes to the electrical impedance of the medium to measure. An equivalent circuit for the measured impedance is a resistance in series with the parallel combination of another resistance and a capacitance. If the two electrodes are modelled by equal impedances, the equivalent circuit for the complete set up consists of three impedance components, which can be determined from three independent measurements. This paper describes a novel method to obtain those three components using a single square wave voltage (period 2T) instead of several sine waves and provides the equations to calculate their value from the three current intensity amplitudes measured at T/8, 3T/8 and 5T/8. Other measurement times would need different equations, but the same procedure applies. Anyway, the proposed method keeps the advantages of synchronous detection and relies on analytical solutions instead of the customary curve fitting procedures. Computer simulation and experimental results obtained by measuring the conductivity of known electrolyte samples validate the proposed methodPeer Reviewe

Micropower front-end interface for differential-capacitive sensor systems

Accepted versio

Advances in ultra-low contact force nanometric surface metrology

This dissertation describes the theoretical design, practical construction and experimental use of a novel profiler intended to bridge the gap between atomic force microscopes (AFMs) and conventional stylus instruments. More specifically, it may be regarded as a hybrid instrument, combining the long-range of stylus instruments with the low contact force, high-speed operation of the AFM. The heart of the new instrument is a miniature capacitance-based force probe, constructed of glass and ceramic materials chosen primarily for thermal stability. This force probe can sense forces encompassing the range from atomic force levels (10¯7 N) to stylus instrument levels (10-⁴ N). Probes used in subsequent studies range from ISO standard spherical diamond styli (radii 2, 5 and 10 ᶙm) to 20 nm radius Berkovich diamond tips. A custom designed low excitation voltage, high frequency capacitance gage, used to monitor the sub-nanometer displacements of the force probe is presented. To measure surface profiles, the force probe is mounted on a PZT actuator and, much like an AFM, follows a contour of constant force under servo control. The specimen traverses underneath the force probe on an ultra-precision kinematic slideway using a flat glass datum surface and polymeric dry bearings. A novel, inexpensive laser interferometer used to monitor specimen position and control data acquisition of the profiler is described. In this manner, profiler repeatability is enhanced to the nanometer level in two axes. Profiler performance is tested for repeatability, noise force servo bandwidth and temperature stability. A force servo response bandwidth of 300 Hz was ascertained. This compares favorably with the sub- ten Hz responses of stylus instruments. A series of experiments designed to validate the high-speed performance of the profiler are presented. This high speed operation is some 10 to 100 times faster than conventional stylus instruments. Dynamic, non-linear interactions between the stylus tip and specimen are first derived and then examined experimentally. These dynamic interactions may eventually make it possible to measure specimen internal damping and interface stiffness or mechanical properties at the point contact level

Asic gas sensors based on ratiometric principles

The wide-scale usage of VOCs in industrial processes requires monitoring the concentrations of these vapours to keep a safe operating environment. Most combustible hydrocarbons can be ignited as a gas-air mixture in the range of 0.5% to 15% by volume. This has led to the development of several portable air quality monitoring instruments. However, the high costs and lack of durability of these instruments has remained an issue to be addressed. This PhD thesis reports on the development and characterization of a novel low cost smart gas sensor technology adaptable for use in a portable instrument. The smart gas sensor devices have been developed to target four different VOCs in air. The smart gas sensor device combines a smart ASIC (SRL 194 designed at SRL, Warwick University) fabricated in standard 0.7 μm CMOS technology and two alkyl-dithiol based self-assembled gold nanoparticle chemoresistive sensors (fabricated at Sony Deutschland GmbH) in a ratiometric array to offer a robust system which can address the common mode variations found in polymer based gas sensor systems. The ratiometric ASIC sensor array architecture allows for the reduction of the baseline value’s dependence on environmental variations and the elimination of baseline drift due to long term application of DC voltage. Three ratiometric array arrangements - mono-type uni-variate with only one chemosensor per device, mono-type bi-variate with two chemosensors of the same film material per device and duo-type with a polar and a non-polar chemosensor per device and their variations were characterized in an automated FIA test station against exposure to methanol, ethanol, propan-1-ol, and toluene at 30°C and 0-5% rh. It was determined that the devices’ response output to VOC analytes was entirely dependent on the variation of the resistance ratio of the chemoresistive sensors in the ratiometric sensor array. The effects of variations of the temperature and rh on the smart sensor output were calibrated. The mono-type devices gave a high magnitude response to the vapours whereas the duo-type arrangement offered a high degree of discrimination between the test analytes with little post-processing steps. Three different alkyl-dithiol chemoresistive sensor films on gold electrodes were successfully used as the VOC vapour sensitive elements in each arrangement. The effects of using a silicone sealant gel as a partitioning layer were characterized and it was observed that at vapour concentrations less than 3000 ppm the silicone encapsulated chemosensor devices reported a larger response to the VOC analytes as compared to those without the silicone. The test devices reported promising response repeatability and reproducibility with excellent return to baseline properties, a negligible hysteresis and an error margin of under 10%. Ideal operating temperature was determined to be 40°C at which rh variations were found to be minimal. The test devices were found to be robust with little variation in the quality of the device output over the course of 18 months. The novel research demonstrated that it is possible to get high level of diversification between analytes from a low cost and robust gas sensor system for monitoring VOCs. The work carried out here has opened the opportunity to develop highly integrated programmable hand-held gas sensor and e-nose systems for environmental monitoring use in health and safety applications

Carbon-Based Nanomaterials for (Bio)Sensors Development

Carbon-based nanomaterials have been increasingly used in sensors and biosensors design due to their advantageous intrinsic properties, which include, but are not limited to, high electrical and thermal conductivity, chemical stability, optical properties, large specific surface, biocompatibility, and easy functionalization. The most commonly applied carbonaceous nanomaterials are carbon nanotubes (single- or multi-walled nanotubes) and graphene, but promising data have been also reported for (bio)sensors based on carbon quantum dots and nanocomposites, among others. The incorporation of carbon-based nanomaterials, independent of the detection scheme and developed platform type (optical, chemical, and biological, etc.), has a major beneficial effect on the (bio)sensor sensitivity, specificity, and overall performance. As a consequence, carbon-based nanomaterials have been promoting a revolution in the field of (bio)sensors with the development of increasingly sensitive devices. This Special Issue presents original research data and review articles that focus on (experimental or theoretical) advances, challenges, and outlooks concerning the preparation, characterization, and application of carbon-based nanomaterials for (bio)sensor development