Energy-Efficient Capacitive Sensor Interfaces
Capacitive sensor systems are potentially highly energy efficient. In practice, however, their energy consumption is typically dominated by that of the interface circuit that digitizes the sensor capacitance. Energy-efficient capacitive sensor interfaces
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Energy-Efficient Capacitive Sensor Interfaces Michiel A.P. Pertijs and Zhichao Tan
Abstract Capacitive sensor systems are potentially highly energy efficient. In practice, however, their energy consumption is typically dominated by that of the interface circuit that digitizes the sensor capacitance. Energy-efficient capacitive sensor interfaces are therefore a prerequisite for the successful application of capacitive sensors in energy-constrained applications, such as battery-powered devices and wireless sensor nodes. This chapter derives lower bounds on the energy consumption of capacitive sensor interfaces. A comparison of these bounds with the state-of-the-art suggests that there is significant room for improvement. Several approaches to improving energy efficiency are discussed and illustrated by two design examples.
8.1
Introduction
In low-power and energy-constrained applications, such as battery-powered systems and wireless sensor networks, capacitive sensors are an attractive choice since they do not consume static power, and the energy required to read them out can be very low [1, 2]. Energy-efficient capacitive sensor systems for the measurement of e.g. pressure [3], acceleration [4] and humidity [5] have been reported. In most cases, however, the energy consumption of such systems will be dominated by that of the interface circuit that digitizes the sensor capacitance. Hence, energyefficient capacitance-to-digital converters (CDCs) are essential to make the most of the low-energy potential of capacitive sensors. This chapter explores the limits on the energy consumption of CDCs. It will be shown in Sect. 8.2 that there is a significant gap between these limits and the energy
M.A.P. Pertijs (*) • Z. Tan Electronic Instrumentation Laboratory, DIMES, Delft University of Technology, Mekelweg 4, 2628 CD Delft, The Netherlands e-mail: [email protected] A.H.M. van Roermund et al. (eds.), Nyquist AD Converters, Sensor Interfaces, 129 and Robustness: Advances in Analog Circuit Design, 2012, DOI 10.1007/978-1-4614-4587-6_8, # Springer Science+Business Media New York 2013
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consumption of most designs reported in literature. Various reasons for this gap will be explored, some of which are linked to the fundamental limitations of practical capacitive sensors, others to the limitations of practical interface circuits. In Sect. 8.3, various ways to realize more energy-efficient interface circuits will be discussed. In Sect. 8.4, these approaches will be illustrated using two design examples, one based on period modulation, the other based on delta-sigma modulation. Finally, conclusions are presented in Sect. 8.5.
8.2 8.2.1
Limits on the Energy Consumption of CDCs The Energy Efficiency Potential of Capacitive Sensors
To better appreciate the energy efficiency of practical CDCs, it is helpful to first establish a fundamental lower bound on their energy consumption. For this, we will consider the class of CDCs based on switched-capacitor techniques, a simplified representation of wh
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