Analog Wheatstone bridge-based automatic interface for grounded and floating wide-range resistive sensors

Abstract

In this paper we present a suitable use of Wheatstone bridge-based circuits to employ a novel interface for both grounded and floating wide range resistive sensor estimation. The proposed topology is very simple and, through the use of only analog blocks in a feedback configuration, allows the automatic and continuous detection and quantification of the sensor resistance. Two circuit variations are proposed: both of them implement the commercial component analog multiplier AD633 as voltage controlled resistor placed in the feedback loop to compensate the sensor variations. Electrical measurement results, conducted on a prototype PCB, have shown that, even if the bridge autobalancing operation works in only 1.6 decades depending on the employment of the AD633, through a two-voltage-reading technique, it is possible to estimate up to 5-decades resistive variations, whose range is also settable, with a relative error within 1.5% and with a higher and supply-independent sensitivity, when compared to traditional bridge. This fact, together with its high resolution, has also allowed, in experimental tests performed by using an air quality resistive sensor, to reveal few ppm carbon monoxide particles.

Introduction

The demand to develop suitable low-cost systems (sensors and related electronic) for detecting air elements, for example gas concentrations, also in reduced quantities, in different environments as safety and security, is rapidly increasing. Recent studies have demonstrated that nowadays this issue is becoming important not only in special contexts (as industry and hospital), but also in everyday life (in houses, schools, offices, etc.). In the last years a new category of extremely sensitive chemoresistive gas sensors, called MOX (metal oxide semiconductors), have been developed [1], [2], [3], [4], [5], [6], [7]. They are able to detect a reduced particle gas quantity (ppm/ppb) but, as a drawback, their development process introduces lots of uncertainties about their baseline and sensitivity, making them difficult to their utilization also about a suitable choice (or design) for the first electronic interface; in fact, it must be reminded that chemical sensors reach the best sensitivity performances by opportunely setting their operative temperature through an electronic heater (external or integrated). The sensitivity value depends on the application, so different operative temperatures as well as different environmental conditions and also the measurand variation imply that the sensor baseline and/or its value can shift also of some decades, so classifying these sensors as wide-range devices. In these cases, an optimization of the readout circuit, related to the sensor operative point, is necessary and this initial setting is typically defined “calibration”.

In the literature, resistive sensor interfaces can be typically be grouped in two main categories according to the kind of the output signal: that performing the resistive-to-voltage (R–V) conversion (usually based on an open-loop bridge configuration) [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], utilized for small measurand variations and showing a high accuracy, and that operating a resistive-to-frequency (R–f) conversion (making use of oscillation circuits) [8], [19], [20], [21], [22], [23], [24], [25], [26], [27], employed for wide measurand variations (wide-range, more than 3 decades variations) and able to avoid non linearity errors and saturation conditions typical of the first solution, but suffering higher resistive estimation errors. The traditional Wheatstone bridge topology can be considered as the simplest R–V interface. It shows the advantage of being able to reveal only small measurand variations with a high accuracy and a linear trend but, as a drawback, requires the knowledge of the sensor baseline to employ the best resistive bridge components. Sometimes, to improve the sensitivity value, an operational amplifier (OA) is added at bridge output, but, due to its non ideal parameters (in particular: non exact gain value, noise and offset), its utilization worsen the circuit accuracy.

In order to improve the Wheatstone bridge features allowing also the determination of the sensor resistance, without knowing its base-line value, some solutions concerning autobalancing bridge-based topologies have been presented in the literature [28], [29], [30], [31], [32], [33], [34], [35], [36]. In these interfaces, one or more devices are driven by a control signal which depends on the system status. Usually these interfaces use topologies that make the interface very complex, as digital architectures, analog switches, and BJT, MOS and FET transistors operating as voltage controlled devices [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48].

For this aim, the authors have recently presented the basic ideas about a novel auto-balanced bridge-based circuit for wide-range resistive estimation [49], [50]. In the work here proposed, starting from the previous solution, suitable improved modifications as well as new analyses, experimental results and comparisons have been done, so to go toward the design of a fully analog high-dynamic resistive sensor interface. The design was aimed both to the possibility to apply a bridge topology also for wide-range resistive variations, which do not depend on the knowledge of sensor features in terms of baseline variations due to operative temperature and measurand changes (“uncalibrated” system), and to get higher accuracy, sensitivity and resolution [51], maintaining a simple circuit with a reading operation that does not need any scaling factor.

The dynamical tuning of one of the bridge elements has been performed by means of an unconventional use of the commercial device AD633 as voltage controlled resistor (VCR). This solution has been preferred to other ones based on transistor working in triode or saturation region, because in the latter it is not always possible to control and maintain accurate voltage references and often they cannot be characterized analytically; in fact, when a transistor changes its operating range, its constitutive relations also change. By means of the feedback loop, it is possible to avoid any calibration procedure at power-on and during circuit operation, so the bridge works at its equilibrium condition for very wide-range measurand variations, maintaining very good interface performances.

The paper is organized as follows: in Section 2 the VCR and the two proposed bridge configurations with relative theoretical calculations about interface sensitivity will be described; in Section 3 some experimental results (in particular, concerning accuracy and resolution) will be presented; in Section 4 measurement results using a commercial air quality sensor for carbon monoxide (CO) detection will be given and in Section 5 conclusions will be drawn.