Sensors and Actuators B 179 (2013) 313–318 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb A CMOS integrated low-voltage low-power time-controlled interface for chemical resistive sensors A. De Marcellis a , A. Depari b , G. Ferri a , A. Flammini b,∗ , E. Sisinni b a Department of Industrial and Information Engineering and Economics, University of L’Aquila, Via G. Gronchi 18, 67100 L’Aquila, Italy b Department of Information Engineering, University of Brescia, Via Branze 38, 25123 Brescia, Italy a r t i c l e i n f o Article history: Available online 2 November 2012 This paper is dedicated to the brilliant scientific carrier of Prof. Arnaldo D’Amico which, since 1986, has been for the authors a precious guide in the development of smart sensor systems. Keywords: Wide range resistive sensors Fast sensor readout Parasitic capacitance estimation Single-chip front-end a b s t r a c t In this paper, an innovative integrated interface circuit, suitable for wide range resistive sensors, such as chemical devices for gas sensing, is presented. The key characteristic is the ability to overcome the main limit of the circuits based on the resistance-to-time (R–T) conversion, that is the long measuring time occurring in the evaluation of high-value resistances. The proposed solution is based on a tricky oscillating circuit architecture performing a sort of “compression” of the higher part of the resistive range, thus limiting the measuring time. The interface is oriented to the development of a single chip for resistive chemical sensor applications, thus it is designed to be as simple as possible, utilizing only operational amplifiers (OAs) and passive components. The proposed front-end is capable to estimate both the sensor resistance over a wide range (five decades) and, through the AC excitation voltage of the sensor, the in-parallel parasitic capacitance, for diagnosis purposes or to provide a more complete characterization of the sensor. Preliminary experimental measurements, conducted through a fabricated discrete component prototype PCB and utilizing commercial sample resistors and capacitors to emulate sensor behaviour, have shown the feasibility of the proposed approach in a wide resistive range. Further experimental results, achieved through the fabricated integrated circuit, developed in 0.35 m standard CMOS technology, have shown good performance both for resistance and capacitance estimations. The on-chip integrated solution, requiring a silicon area of about 0.9 mm 2 , supplied at 1.8 V and showing a low power consumption (lower than 600 W), results to be suitable for resistive sensor array configurations and portable applications, as also witnessed by an experimental test with CO gas and a sample commercial sensor. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Resistive chemical sensors for gas sensing usually show a baseline value which can fall in a range of several decades (from kilohms to gigohms), depending on different physical and chemical parameters, sensor operating conditions and fabrication processes/materials. The high sensitivity shown towards some ana- lytes determines a resistive sensor behaviour which can also span in a wide range starting from the sensor baseline value. In the optic of low-cost artificial olfactory systems and, more in general, for the realization of low-power portable and smart sensory systems [1–3], the sensor electronic interface should be designed to be as univer- sal as possible, without the need of calibration or tuning operations for a specific sensor. In this way, the same front-end can be repli- cated for the use with multiple sensors (for example, for electronic nose applications [4–6]) and thus the implementation in integrated ∗ Corresponding author. E-mail address: alessandra.flammini@ing.unibs.it (A. Flammini). circuits for the realization of single-chip solutions is furthermore simplified. Together with the measurement of wide range of resistance variation, the electronic interface should also quantify the sensor parasitic effects. Typically, these sensors are modelled by a parallel of a resistor and a parasitic capacitor (on the order of tens of pico- farads), accounting for behaviour at the surface, bulk, contact, etc. [7–12]. The front-end should be able to estimate the sensor resis- tance without being affected by the parasitic effects. However, the estimation of the parasitic capacitance value can be useful for diag- nostic purposes (e.g., sensor connection failure detection) as well as to provide more features which can be used for the sensor response analysis. Concerning resistive sensor front-ends, different electronic read-out circuits have been proposed in the literature to properly interface this kind of sensors [13–31]. Generally, the voltamper- ometric approaches, such as the simple voltage divider or the resistive DC-excited Wheatstone bridge, perform a resistance-to- voltage (R–V) or to-current (R–I) conversion, but can be adopted only for reduced resistive variations, otherwise scaling factors, 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.09.104