Abstract
This chapter presents two cost-effective sensors that measure ambient carbon dioxide (CO2) concentration, intended for application in smart ventilation systems in buildings or in mobile devices. Both sensors employ a suspended hot-wire transducer to detect the CO2-dependent thermal conductivity (TC) of the ambient air. The resistive transducer is realized in the VIA layer of a standard CMOS process using a single etch step. The first sensor determines the transducer’s CO2-dependent thermal resistance to the surrounding air by measuring its steady-state temperature rise and power dissipation. A ratiometric measurement is realized by employing an identical but capped transducer as a reference. An incremental delta-sigma ADC digitizes the temperature and power ratios of the transducers, from which the ratio of the thermal resistances is calculated. The second sensor is based on a transient measurement of the CO2-dependent thermal time constant of the transducer. The readout circuit periodically heats up the transducer and uses a phase-domain delta-sigma modulator to digitize the CO2-dependent phase shift of the resulting temperature transients. Compared to the ratiometric steady-state measurement, this approach significantly reduces the measurement time and improves the energy efficiency, resulting in a state-of-the-art CO2 resolution of 94 ppm at an energy consumption of 12 mJ per measurement.
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Acknowledgments
This work was in part supported by NXP Semiconductors, The Netherlands, and in part by ams AG, The Netherlands. The authors want to thank Lukasz Pakula and Zu-yao Chang for their technical support.
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Cai, Z., van Veldhoven, R., Suy, H., de Graaf, G., Makinwa, K.A.A., Pertijs, M. (2019). CMOS-Compatible Carbon Dioxide Sensors. In: Makinwa, K., Baschirotto, A., Harpe, P. (eds) Low-Power Analog Techniques, Sensors for Mobile Devices, and Energy Efficient Amplifiers . Springer, Cham. https://doi.org/10.1007/978-3-319-97870-3_11
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DOI: https://doi.org/10.1007/978-3-319-97870-3_11
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