Low power biosensor and decimator design

2013 Summer.Includes bibliographical references.This paper examines the use of low power circuits applied to biosensors used to observe neurotransmission. The term "biosensors" in the broadest sense describes many devices which are used to measure a biological state e.g. neural signal acquisition. The methods for developing biosensors are just as diverse, but one common thread is that many biomedical devices are battery operated and require low power for mobility. As biosensors become more complex they also require more functions such as data storage, digital signal processing, RF transmission etc. The more functions a sensor needs, the tighter the constraint for power consumption on a battery operated device becomes. In order to solve this problem, biosensors are increasingly being designed for low power consumption while weighing tradeoffs for performance and noise. Designers accomplish this by lowering the supply voltage, which reduces the overall size, and thus the load, of the devices. The amount of individual components will also be reduced, allowing for a smaller, faster device. Biosensors are important because they grant the ability for scientists to better understand complex biological systems. While many other methods exist for observing biological systems, electrochemistry is a practical method for measuring redox reaction because it senses chemical reactions on the surface of an electrode. The reaction will create a current, which can be interpreted via electronics. With the use of electrochemistry, scientist can cheaply and practically observe changes occurring between cells. On the engineering side, modern silicon processes provide small, tightly packed microelectrodes for high spatial resolution. This allows scientists to detect minute changes over a small spatial range. With an array of electrodes on the scale of 1000s, electrochemistry can be used to record data from a sizable cellular sample. Such an array could be used to identify several biological functions such as communication between cells. By combining known electrochemistry methods with low power circuit designs, we can create a biosensor that can further advance the understanding of the operation of cells, such as neurotransmission. The goal of our project is to create a device that uses electrochemistry to detect a redox reaction between a chemical, such as nitric oxide, and an electrode. The device needs to be battery operated for mobility and it must contain all needed electronics on chip, including amplification, digital signal processing, data transmission etc. This requires a surface of electrodes on chip that can handle the environment needed for a living tissue such as: specific temperature, pH and humidity. In addition, it requires a chip that is low power and which produces little heat. This thesis describes two separate designs, both of which are part of a final biosensor design that will be used for the detection of nitric oxide. The first design is a biosensor microelectrode array. The array will be used along with electrochemistry to detect the release of nitric oxide from a living tissue sample. The electrodes are connected to a chain of electronics for on chip signal processing. The design runs at a voltage of 3V in a 0.6µm CMOS process. The final layout for the microelectrodes measured approximately 4.84mm2 with a total of 8,192 electrodes and consumed 0.310mW/channel. The second design is a low power decimator for a sigma-delta analog to digital converter designed for biomedical applications. The ADC will be used along with a chain of amplifying electronics to interpret the signals received from the microelectrode array. The design runs at a voltage of 0.9V in a 0.18µm CMOS process. Its final layout measured approximately 0.0158mm2 and consumed 3.3uW of power. The ADC and microelectrode array were designed and fabricated separately to ensure their validity as standalone designs

A Low Power Mid-Rail Dual Slope Analog-To-Digital Converter for Biomedical Instrumentation

There are an estimated 15 million babies born preterm every year and it is on the rise. The complications that arise from this can be quite severe and are the leading causes of death among children under 5 years of age. Among these complications is a condition known as apnea. This disorder is defined as the suspension of breathing during sleep for usually 10 to 30 seconds and can occur up to 20-30 times per hour for preterm infants. This lack of oxygen in the bloodstream can have troubling effects, such as brain damage and death if the apnea period is longer than expected. This creates a dire need to continuously monitor the respiration state of babies born prematurely. Given that the breathing signal is in analog form, a conversion to its digital counterpart is necessary.In this thesis, a novel low power analog-to-digital converter (ADC) for the digitization and analyzation of the respiration signal is presented. The design of the ADC demonstrates an innovative approach on how to operate on a single polarity supply system, which effectively doubles the sampling speed. The ADC has been realized in a standard 130 nm CMOS process

Design of a 16-bit 50-kHz low-power SC delta-sigma modulator for ADC in 0.18um CMOS technology

This Master Thesis work aims to design a low power high-resolution Delta-Sigma modulator for ADC in a low-cost standard mixed-mode CMOS technology. For this purpose, a single-bit single loop Delta-Sigma architecture will be selected in order to mitigate distortion issues caused by technology mismatching. Also, the switched capacitor (SC) circuit implementation of the Delta-Sigma modulator will avoid the use of any internal voltage supply bootstrapping for biasing critical switches in favor of extending IC lifetime. The designer will take benefit of the low-power Class-AB OpA general purpose 16 Bits Sigma-Delta modulator ADC for double precision audio 50 kHz bandwidth, targeted for Low-power operation, involving no additional digital circuit compensation, no bootstrapping techniques and resistor-less topologies, and relaying on Switched Capacitor Sigma-Delta modulator topologies for robust operation and insensitivity to process and temperature variations, is presented in this work. Designed in a commercial 180 nm technology, the whole circuit static current is calculated in 620 uA with a nominal voltage supply of 1.8 V, performing a Schreier FOM of 174.16 dB. This outstanding state-of-the-art forseen FOM is achieved by the use of architectural and circuital Low-power techniques. At the architectural level a single loop Low-distortion topology with the optimum order and coefficients have been chosen, while at circuit level very novel OTA based on Variable Mirror Amplifiers allows an efficient Class-AB operation. Specially optimized switched variable mirror amplifiers with a novel design methodology based on Bottom-up approach, allows faster design stages ensuring feasable circuit performance at architectural level without the need of large iterative simulations of the complete SC Sigma-Delta modulator. Simulation results confirms the complete optimization process and the metioned advantages with respect to the tradicional approach

Robust low power CMOS methodologies for ISFETs instrumentation

I have developed a robust design methodology in a 0.18 [Mu]m commercial CMOS process to circumvent the performance issues of the integrated Ions Sensitive Field Effect Transistor (ISFET) for pH detection. In circuit design, I have developed frequency domain signal processing, which transforms pH information into a frequency modulated signal. The frequency modulated signal is subsequently digitized and encoded into a bit-stream of data. The architecture of the instrumentation system consists of a) A novel front-end averaging amplifier to interface an array of ISFETs for converting pH into a voltage signal, b) A high linear voltage controlled oscillator for converting the voltage signal into a frequency modulated signal, and c) Digital gates for digitizing and differentiating the frequency modulated signal into an output bit-stream. The output bit stream is indistinguishable to a 1st order sigma delta modulation, whose noise floor is shaped by +20dB/decade. The fabricated instrumentation system has a dimension of 1565 [Mu] m 1565 [Mu] m. The chip responds linearly to the pH in a chemical solution and produces a digital output, with up to an 8-bit accuracy. Most importantly, the fabricated chips do not need any post-CMOS processing for neutralizing any trapped-charged effect, which can modulate on-chip ISFETs’ threshold voltages into atypical values. As compared to other ISFET-related works in the literature, the instrumentation system proposed in this thesis can cope with the mismatched ISFETs on chip for analogue-to-digital conversions. The design methodology is thus very accurate and robust for chemical sensing