Wireless sensor nodes are emerging in a wide range of critical applications such as environmental monitoring, health applications, home automation and military surveillance and reconnaissance. The addition of low power wireless capability to such sensor nodes allows communication between a node and a base station or between nodes, resulting in the formation of wireless sensor networks. Sensor networks can use the information available from the distributed sensor nodes to determine the location and nature of a stimulus or environmental condition. The information collected by the base station can be used to determine the appropriate course of action for dealing with the stimulus. In chemical/biological defense or safety monitoring scenarios, wireless sensor networks can be used to identify and track harmful chemical or biological agents which might be present in a particular area. Due to the potentially remote areas that wireless sensor networks aim to cover, it is essential to minimize the power consumption of a sensor node so that it can operate over a long period of time without a connection to the power grid. Sensor nodes can contain multiple blocks, such as the readout circuit which interfaces with the sensor, an embedded processor, and the wireless transceiver circuits, all of which need to operate on a low power budget.
This thesis specifically focuses on design of low power mixed signal readout circuits which interface with chemoresistive chemical sensors, i.e. sensors that demonstrate a variation of resistance (or impedance) in the presence of chemical agents. For this thesis, the sensor can be either a chemoresistive bead or a nanowire. By integrating multiple non-specific chemoresistive sensors together in arrays, a cross-reactive array can be realized, where the combined response of the arrayed sensors can be used to determine analytes present in a mixture even if their concentrations are low.
In this thesis, a CMOS resistive readout circuit based on a sigma-delta ADC is presented. The design is used to measure the resistance of chemoresistive beads and nanowires with respect to time. The frequency of the ADC output varies as the resistance of a sensor changes and, based on the magnitude and duration of the variation, the type of chemical agent and its concentration can potentially be estimated. For future cross-reactive sensor applications, an array of 16x16 sites is also included in the readout circuit design. Individual sites in the sensor array can be accessed using addressing blocks which designed to select a particular row and column using an 8-bit addressing system. This thesis also covers the techniques used for integration of chemoresistive beads and nanowires into the array locations provided on the prefabricated CMOS IC. Measurement results that demonstrate the operation of the resistive readout circuitry are presented.
Finally, a second readout circuit is proposed to measure complex impedance variations of a sensor device. Measurement of magnitude and phase changes of a sensor device can provide another degree of freedom in the analysis of chemical mixture. Simulation results demonstrating the functionality of the proposed impedance measurement system are also presented. / Master of Science
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/36247 |
| Date | 20 January 2011 |
| Creators | Kakkar, Nikhil |
| Contributors | Electrical and Computer Engineering, Raman, Sanjay, da Silva, Claudio R. C. M., Ha, Dong Sam |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | Kakkar_Nikhil_T_2010.pdf, Kakkar_Nikhil_T_2010_Copyright.pdf |
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