Low power biosensor and decimator design
Date
2013
Authors
Scholfield, Kristin, author
Chen, Tom, advisor
Collins, George, committee member
Tobet, Stuart, committee member
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Abstract
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.
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Subject
analog-to-digital converter
low power circuits
electrode
decimator
biosensor