MSc Z. Cai

PhD student
Electronic Instrumentation (EI), Department of Microelectronics

Expertise: CMOS-compatible Carbon Dioxide-sensor interfaces

Biography

Zeyu Cai received his BEng degree in Communication Engineering from Nankai University, China, in 2004, and a MSc Degree With Distinction in Electronics: Analogue System Design from the University of Edinburgh in November 2011. From 2005 to 2010, he was a product engineer at RF Micro Devices (Beijing), working on the test, characterisation and qualification of RF power amplifiers and front-end modules. From July 2012, he was with Tsinghua Tongfang Microelectronics Company in Beijing as an analogue design engineer. He joined the Electronic Instrumentation Laboratory at TU Delft in October 2012 to continue his research interests in CMOS integrated circuit design and smart sensors.

Projects history

CMOS-Compatible Hot-Wire CO2 Sensors

In the project, we have developed CMOS-compatible CO2 sensors that detect the CO2-dependent heat loss of a suspended hot-wire transducer using dedicated precision readout electronics.

  1. A Phase-Domain Readout Circuit for a CMOS-Compatible Hot-Wire CO$_2$ Sensor
    Z. Cai; R. van Veldhoven; H. Suy; G. de Graaf; K. Makinwa; M. Pertijs;
    IEEE Journal of Solid-State Circuits,
    Volume 53, Issue 11, pp. 3303--3313, November 2018. DOI: 10.1109/JSSC.2018.2866374
    Abstract: ...
    This paper presents a readout circuit for a carbon dioxide (CO2) sensor that measures the CO2-dependent thermal time constant of a hot-wire transducer. The readout circuit periodically heats up the transducer and uses a phase-domain modulator to digitize the phase shift of the resulting temperature transients. A single resistive transducer is used both as a heater and as a temperature sensor, thus greatly simplifying its fabrication. To extract the transducer’s resistance, and hence its temperature, in the presence of large heating currents, a pair of transducers is configured as a differentially driven bridge. The transducers and the readout circuit have been implemented in a standard 0.16-μm CMOS technology, with an active area of 0.3 and 3.14 mm2, respectively. The sensor consumes 6.8 mW from a 1.8-V supply, of which 6.3 mW is dissipated in the transducers. A resolution of 94-ppm CO2 is achieved in a 1.8-s measurement time, which corresponds to an energy consumption of 12 mJ per measurement, >10× less than prior CO2 sensors in CMOS technology.

  2. CMOS-Compatible Carbon Dioxide Sensors
    Z. Cai; R. van Veldhoven; H. Suy; G. de Graaf; K. A. A. Makinwa; M. Pertijs;
    In Low-Power Analog Techniques, Sensors for Mobile Devices, and Energy Efficient Amplifiers,
    Springer Science \& Business Media, November 2018. DOI: 10.1007/978-3-319-97870-3
    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.

    document

  3. A Phase-Domain Readout Circuit for a CMOS Compatible Thermal-Conductivity-Based Carbon Dioxide Sensor
    Z. Cai; R. van Veldhoven; H. Suy; G. de Graaf; K. A. A. Makinwa; M. A. P. Pertijs;
    In Dig. Techn. Papers IEEE International Solid-State Circuits Conference (ISSCC),
    pp. 332-333, February 2018. DOI: 10.1109/ISSCC.2018.8310319

  4. CMOS-Compatible Carbon Dioxide Sensors
    Z. Cai; R. van Veldhoven; H. Suy; G. de Graaf; K. A. A. Makinwa; M. Pertijs;
    In Proc. Workshop on Advances in Analog Circuit Design (AACD),
    pp. 68-91, April 2018. (invited paper). DOI: 10.1007/978-3-319-97870-3
    document

  5. A tunable THz wave modulator based on graphene
    Feng Zhang; Xian-Ping Chen; Qi-qin Wei; Li-Bo Yuan; Miao Cai; Huai-Yu Ye; Kouchi Zhang Jing Xiao;
    In 19th International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2018,
    2018.

  6. Color shift modeling of light-emitting diode lamps in step-loaded stress testing
    M Cai; D Yang; J Huang; M Zhang; X Chen; C Liang; S Koh; Kouchi Zhang;
    IEEE Photonics Journal,
    Volume 9, Issue 1, pp. 1-14, 2017.

  7. A CMOS Readout Circuit for Resistive Transducers Based on Algorithmic Resistance and Power Measurement
    Z. Cai; L. Rueda Guerrero; A. Louwerse; H. Suy; R. van Veldhoven; K. Makinwa; M. Pertijs;
    IEEE Sensors Journal,
    Volume 17, Issue 23, pp. 7917-7927, December 2017. DOI: 10.1109/JSEN.2017.2764161
    Abstract: ...
    This paper reports a readout circuit capable of accurately measuring not only the resistance of a resistive transducer, but also the power dissipated in it, which is a critical parameter in thermal flow sensors or thermal-conductivity sensors. A front-end circuit, integrated in a standard CMOS technology, sets the voltage drop across the transducer, and senses the resulting current via an on-chip reference resistor. The voltages across the transducer and the reference resistor are digitized by a time-multiplexed high-resolution analog-todigital converter (ADC) and post-processed to calculate resistance and power dissipation. To obtain accurate resistance and power readings, a voltage reference and a temperature-compensated reference resistor are required. An accurate voltage reference is constructed algorithmically, without relying on precision analog signal processing, by using the ADC to successively digitize the base–emitter voltages of an on-chip bipolar transistor biased at several different current levels, and then combining the results to obtain the equivalent of a precision curvature-corrected bandgap reference with a temperature coefficient of 18 ppm/°C, which is close to the state-of-the-art. We show that the same ADC readings can be used to determine die temperature, with an absolute inaccuracy of ±0.25 °C (5 samples, min–max) after a 1-point trim. This information is used to compensate for the temperature dependence of the on-chip polysilicon reference resistor, effectively providing a temperature-compensated resistance reference. With this approach, the resistance and power dissipation of a 100 transducer have been measured with an inaccuracy of less than ±0.55 and ±0.8\%, respectively, from −40 °C to 125 °C.

  8. Ratiometric device
    Z. Cai; M. A. P. Pertijs; R. H. M. van Veldhoven; K. A. A. Makinwa;
    Patent, United States 9,835,575B2, December 2017.

  9. A Ratiometric Readout Circuit for Thermal-Conductivity-Based Resistive CO$_2$ Sensors
    Z. Cai; R. H. M. van Veldhoven; A. Falepin; H. Suy; E. Sterckx; C. Bitterlich; K. A. A. Makinwa; M. A. P. Pertijs;
    IEEE Journal of Solid-State Circuits,
    Volume 51, Issue 10, pp. 2453‒2474, October 2016. DOI: 10.1109/jssc.2016.2587861
    Abstract: ...
    This paper reports a readout circuit for a resistive CO2 sensor, which operates by measuring the CO2-dependent thermal conductivity of air. A suspended hot-wire transducer, which acts both as a resistive heater and temperature sensor, exhibits a CO2-dependent heat loss to the surrounding air, allowing CO2 concentration to be derived from its temperature rise and power dissipation. The circuit employs a dual-mode incremental delta-sigma ADC to digitize these parameters relative to those of an identical, but isolated, reference transducer. This ratiometric approach results in a measurement that does not require precision voltage or power references. The readout circuit uses dynamically-swapped transducer pairs to cancel their baseline-resistance, so as to relax the required dynamic range of the ADC. In addition, dynamic element matching (DEM) is used to bias the transducer pairs at an accurate current ratio, making the measurement insensitive to the precise value of the bias current. The readout circuit has been implemented in a standard 0.16 μm CMOS technology. With commercial resistive micro-heaters, a CO2 sensing resolution of about 200 ppm (1σ) was achieved in a measurement time of 30 s. Similar results were obtained with CMOS-compatible tungsten-wire transducers, paving the way for fully-integrated CO2 sensors for air-quality monitoring.

  10. An Integrated Carbon Dioxide Sensor for Air-Quality Monitoring
    Z. Cai; R.H.M. van Veldhoven; A. Falepin; H. Suy; E. Sterckx; C. Bitterlich; K.A.A. Makinwa; M.A.P. Pertijs;
    In Proc. Conference for ICT-Research in the Netherlands (ICT.OPEN),
    The Netherlands, March 2016.

  11. First-principles study of the effect of functional groups on polyaniline backbone
    X P Chen; J K Jiang; Q H Liang; N Yang; Huaiyu Ye; M Cai; L Shen; D G Yang; T L Ren;
    Scientific Reports,
    Volume 5, pp. 16907, 2015.

  12. A ratiometric readout circuit for thermal-conductivity-based resistive gas sensors
    Z. Cai; R. H. M. van Veldhoven; A. Falepin; H. Suy; E. Sterckx; K. A. A. Makinwa; M. A. P. Pertijs;
    In Proc. European Solid-State Circuits Conference (ESSCIRC),
    IEEE, pp. 275‒278, September 2015. DOI: 10.1109/esscirc.2015.7313880

  13. An integrated carbon dioxide sensor based on ratiometric thermal-conductivity measurement
    Z. Cai; van R. H. M. Veldhoven; A. Falepin; H. Suy; E. Sterckx; K. A. A. Makinwa; M. A. P. Pertijs;
    In Proc. International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS),
    IEEE, pp. 622‒625, June 2015. DOI: 10.1109/transducers.2015.7181000

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Last updated: 27 Dec 2018