Ruoyu Xu

Bio: Ruoyu Xu is an academic researcher from Hong Kong University of Science and Technology. The author has contributed to research in topics: Communication channel & CMOS. The author has an hindex of 10, co-authored 12 publications receiving 566 citations.

Electric-Field Intrabody Communication Channel Modeling With Finite-Element Method

Abstract: Electric-field intrabody communication (EF-IBC) is a promising new scheme for the data exchange among wearable biomedical sensors. It uses the body as the signal transmission media. Compared with existing body area network (BAN) schemes, EF-IBC can achieve higher data rate with less transmission power. Until now, the detailed EF-IBC channel mechanism is not well understood. In this work, finite-element method (FEM) is utilized for the first time to investigate the EF-IBC channel. A circuit-coupled FEM model is established for the EF-IBC channel. The FEM model is extensively verified by experimental measurements. The new physical model enables the revelation of characteristics and effects of different components in the EF-IBC channel. The FEM investigation finds that the capacitive return path is critical to the characteristics of the EF-IBC channel. Parameters of the capacitive return path are quantitatively measured. The investigation also finds that the body plays an important role to the return path capacitance. The forward body path can be well modeled by a cascade of π-shaped circuits. Based on the FEM model of the EF-IBC channel, a simplified circuit model is derived to provide an efficient tool for the transceiver design.

Digitally Calibrated 768-kS/s 10-b Minimum-Size SAR ADC Array With Dithering

Abstract: Array sensors require a high-performance analog-to-digital converter (ADC) array with small area and low power. Successive-approximation register (SAR) ADC has good potential for ADC array due to its simple analog circuits. However, SAR ADCs with 10-b resolution and higher normally need a large capacitor array due to the stringent matching requirement. The large capacitor array also limits the ADC dynamic performance. The capacitor mismatch has been compensated by analog calibration techniques. In this work, a novel digital calibration method is developed for SAR ADC based on dithering. With dithering, weights of most significant bit (MSB) capacitors can be measured accurately so that very small capacitors can be used in the SAR ADC due to the relaxed matching requirement. A modified bit-cycling procedure is developed to avoid the code gaps caused by capacitor dithering. This calibration technique requires no analog calibration overhead and simple digital decoders. The technique is implemented in an ADC array design including 256 SAR ADCs for a high-speed CMOS imaging sensor in a 0.18-μm CMOS process. The 10-b SAR ADC is designed with the minimum capacitor array size in the process. A single SAR ADC only occupies 15 μm × 710 μm. Sampling at 768 kS/s, peak DNL and peak INL of the original ADCs averaged across the array are 0.82 least significant bit (LSB) and 3.85 LSB, respectively. For a signal close to the Nyquist frequency, original ADCs have 7.96-b average ENOB. After calibration with dithering, ADCs have 0.55-LSB peak DNL and 0.77-LSB peak INL averaged across the array. The average ENOB improves to 9.83 b. Compared with the benchmark 10-b SAR ADCs, this design is the most area-efficient design. In this work, the calibration decoders are implemented off-chip. With a sample-and-hold amplifier, the calibration method can run in the background.

Equation Environment Coupling and Interference on the Electric-Field Intrabody Communication Channel

Abstract: Wearableand implantable medical sensors have been investigated continuously in recent years to provide better diagnostics and monitoring for personal health care. Much attention has been drawn to the establishment of the ubiquitous body area network (BAN) to reliably connect the body sensors and collect the sensor data in real time. Electric-held intrabody communication (EF-IBC) is a promising physical link technology for the body area network. Compared to existing wireless technologies, EF-IBC hts the body characteristics better and is able to achieve higher data rate with less transmission power. EF-IBC relies on the parasitic capacitive coupling between the transmitter and the receiver to close the signal circuit loop. With this parasitic coupling, EF-IBC links can be influenced by the environment. However until now, there is lack of systematic research on various environment coupling effects to the EF-IBC channel. In this paper, environment effects on the EF-IBC channel are comprehensively studied. The interference from the nearby EF-IBC channel is investigated for the first time to gain useful insights into the establishment of the BAN with EF-IBC. The FEM model is also established to explain the mechanism of the capacitive return path.

A 1500 fps Highly Sensitive 256 $\,\times\,$ 256 CMOS Imaging Sensor With In-Pixel Calibration

Abstract: High-speed CMOS imaging sensors (CIS) normally have low sensitivity because of the large integration capacitance. They also have high noise because pixel circuits cannot implement correlated double sampling (CDS) to remove the pixel reset noise. For applications, such as micro-computed tomography (micro-CT), this is a major limitation. In this work, we developed a technique to achieve high sensitivity and low noise for high-speed CIS. To maximize the sensitivity, we designed a new capacitive transimpedance amplifier (CTIA) pixel with a tiny metal-oxide-metal capacitor. The pixel circuit also implements CDS. As a result, the temporal noise is greatly reduced, and the sensitivity improves dramatically. To compensate the mismatch of small integration capacitors across the pixel array, an on-chip calibration scheme with in-pixel circuits is developed. Fully differential column circuits are designed to suppress the power supply injection in the large array of high-speed column circuits. A successive-approximation analog-to-digital (SAR ADC) is designed to achieve 10-bit resolution and to fit in the 15-μm column pitch. For testing modes, column circuits are configured into a two-step ADC to provide 13-bit dynamic range. The 256 × 256 CIS design is fabricated in a 0.18-μm CMOS process. The imager samples up to 1500 fps. The pixel integration capacitor is 0.7 fF, which enables 68.5 V/lux · s sensitivity under the white illumination. The CIS temporal noise is 13.6e-. This sensitivity and noise performances are much better than previous high-speed CIS benchmark designs. Running at 1500 fps, the CIS can capture recognizable images with illumination down to 1 lux. The on-chip calibration suppresses the fixed-pattern noise lower than 0.52%. The prototype chip consumes 390 mW of power.

A 12-bit 20 MS/s 56.3 mW Pipelined ADC With Interpolation-Based Nonlinear Calibration

Abstract: The linearity of a high-resolution pipelined analog- to-digital converter (ADC) is mainly limited by the capacitor mismatch and the finite operational amplifier (OPAMP) gain in the multiplying-digital-to-analog converter (MDAC). Therefore, high resolution pipelined ADCs usually require high-gain OPAMP and large capacitors, which causes large ADC power. In recent years, various nonlinear calibration techniques have been developed to compensate both linear and nonlinear error from MDCAs so that low-power MDACs with small capacitors and low-gain OPAMP can be used. Hence, the ADC power can be greatly reduced. This paper introduces a novel interpolation- based digital self-calibration architecture for pipelined ADC. Compared to previous techniques, the new architecture is free of adaptation. Hence, long convergence is not needed. The complexity of the digital processor is also considerably lower. The new architecture does not use backend ADC to measure MDACs. Hence, it is free of the accumulation of measurement error, which leads to more accurate calibration. A prototype ADC with the calibration architecture is fabricated in a 0.35 3.3 V CMOS process. The ADC samples at 20 MS/s. The calibration improves the ADC DNL and INL from 1.47 LSB and 7.85 LSB to 0.2 LSB and 0.27 LSB. For a 590 kHz sinusoidal signal, the calibration improves the ADC signal-to-noise-distortion ratio(SNDR) and spurious-free dynamic range (SFDR) from 41.3 dB and 52.1 dB to 72.5 dB and 84.4 dB respectively. The 11.8-ENOB 20 MS/s ADC consumes 56.3 mW power with 3.3 V supply. The 0.78 pJ/step figure-of-merit (FOM) is low for designs in 0.35 CMOS processes. At the Nyquist frequency, SNDR of the calibrated ADC drops 8 dB due to the slow settling of the first pipeline stage.

A Survey on Intrabody Communications for Body Area Network Applications

Abstract: The rapid increase in healthcare demand has seen novel developments in health monitoring technologies, such as the body area networks (BAN) paradigm. BAN technology envisions a network of continuously operating sensors, which measure critical physical and physiological parameters e.g., mobility, heart rate, and glucose levels. Wireless connectivity in BAN technology is key to its success as it grants portability and flexibility to the user. While radio frequency (RF) wireless technology has been successfully deployed in most BAN implementations, they consume a lot of battery power, are susceptible to electromagnetic interference and have security issues. Intrabody communication (IBC) is an alternative wireless communication technology which uses the human body as the signal propagation medium. IBC has characteristics that could naturally address the issues with RF for BAN technology. This survey examines the on-going research in this area and highlights IBC core fundamentals, current mathematical models of the human body, IBC transceiver designs, and the remaining research challenges to be addressed. IBC has exciting prospects for making BAN technologies more practical in the future.

The Signal Transmission Mechanism on the Surface of Human Body for Body Channel Communication

Abstract: The signal transmission mechanism on the surface of the human body is studied for the application to body channel communication (BCC). From Maxwell's equations, the complete equation of electrical field on the human body is developed to obtain a general BCC model. The mechanism of BCC consists of three parts according to the operating frequencies and channel distances: the quasi-static near-field coupling part, the reactive induction-field radiation part, and the surface wave far-field propagation part. The general BCC model by means of the near-field and far-field approximation is developed to be valid in the frequency range from 100 kHz to 100 MHz and distance up to 1.3 m based on the measurements of the body channel characteristics. Finally, path loss characteristics of BCC are formulated for the design of BCC systems and many potential applications.

Finding Common Ground: A Survey of Capacitive Sensing in Human-Computer Interaction

Abstract: For more than two decades, capacitive sensing has played a prominent role in human-computer interaction research. Capacitive sensing has become ubiquitous on mobile, wearable, and stationary devices - enabling fundamentally new interaction techniques on, above, and around them. The research community has also enabled human position estimation and whole-body gestural interaction in instrumented environments. However, the broad field of capacitive sensing research has become fragmented by different approaches and terminology used across the various domains. This paper strives to unify the field by advocating consistent terminology and proposing a new taxonomy to classify capacitive sensing approaches. Our extensive survey provides an analysis and review of past research and identifies challenges for future work. We aim to create a common understanding within the field of human-computer interaction, for researchers and practitioners alike, and to stimulate and facilitate future research in capacitive sensing.