Sensors and Actuators B 95 (2003) 232–243 Design and simulation of a smart ratiometric ASIC chip for VOC monitoring Jesús Garc´ ıa-Guzmán a , Nicola Ulivieri b , Marina Cole a,∗ , Julian W. Gardner a a School of Engineering, University of Warwick, Coventry CV4 7AL, UK b Department of Information Engineering, Universita’ di Siena, 53100 Siena, Italy Abstract This paper reports on the design and simulation of a novel ratiometric application specific integrated circuit (ASIC) chip for the monitoring of volatile organic compounds (VOCs) or gases. The design integrates two polymeric chemoresistors in a ratiometric configuration, together with smart circuitry, into a single chip fabricated through a standard silicon CMOS process. The circuit provides automatic compensation of signal from variations in both supply voltage and ambient temperature. On-chip control of the operating temperature of the sensors is also an option. The response of the ratiometric set of polymeric chemoresistors to different concentrations of gases at different temperatures and humidities was simulated with the aid of a novel parametric Cadence model. Simulations confirm that the ratiometric configuration is less sensitive to temperature variations and that it also has a better performance in terms of humidity dependence when compared to an individual chemoresistor. These features, together with its ability to compensate for a large range in values of polymer resistance, make us believe that the circuit offers relevant smart capabilities at a very low-cost and so it can be used as the main component for the mass production of a self-calibrating, programmable, palm-top instrument. © 2003 Elsevier B.V. All rights reserved. Keywords: Smart sensor; ASIC; Resistive gas sensor; Ratiometric sensor array; Behavioural models 1. Introduction Further advances, in the field of polymer resistors for the sensing of vapours, require the development of low-cost smart devices capable of addressing the problems of both process variation and changes in environmental conditions. The use of integrated circuits capable of self-calibration and compensation within a single chip unit can provide a good solution. The adoption of a standard fabrication process al- lows the integration of smart interface circuitry and arrays of polymer-based sensors, leading to the development of novel intelligent sensor systems for gas or vapour monitoring. Some of the problems associated with conducting poly- mers, such as temperature dependence, have already been addressed through the design of four-element and five- element microbridge devices [1–3] and a significant reduc- tion in the values of temperature and humidity coefficients was achieved. In this paper we describe an application specific integrated circuit (ASIC) chip for gas monitoring that provides several smart features and incorporates a pair of polymer resistors ∗ Corresponding author. Fax: +44-1203-418922. E-mail address: mvc@eng.warwick.ac.uk (M. Cole). URL: http://www.eng.warwick.ac.uk/srl. in a ratiometric configuration [4]. In order to perform de- tailed analyses of the behaviour of this ratiometric chip, a parametric model was developed to simulate the polymer re- sistance on exposure to a given gas at different temperatures and humidities [5]. This model is particularly useful because it permits the response of polymeric chemoresistors to be predicted before CMOS processing. Cadence software was used for the design and simulation of the ASIC chip as well as for the implementation of the model. An Alcatel 0.7 m CMOS process was chosen for the design and fabrication of the chip through the low-cost Europractice scheme. 2. Description of the design Fig. 1 shows the overall structure of the new smart gas sensor system. The main component is the ASIC chip, which performs two basic functions: (a) sensing the gas presence and concentration and (b) controlling the operating tempera- ture of the gas sensor. Within the ASIC chip, the top section corresponds to the gas sensor circuit, whereas the bottom sections correspond to temperature control and monitoring. A microcontroller unit is used for processing the inputs and outputs of the ASIC chip. Additionally, three digitally controlled potentiometers (available in the Xicor’s X9241A 0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00432-5