A new 0.35 μm CMOS electronic interface for wide range floating capacitive and grounded/floating resistive sensor applications

In this paper we propose a novel interface circuit suitable for the read-out of both wide range floating capacitive and grounded/floating resistive sensors. This solution, employing only two Operational Amplifiers (OAs) as active blocks and some passive components, is based on a square-wave oscillating circuit topology which, instead of a voltage integration typically performed by other solutions in the literature, operates a voltage differentiation. Therefore, the proposed circuit, performing an impedance-to-period (Z–T) conversion, results to be suitable as first analog front-end for both wide variation capacitive (e.g., relative humidity) and resistive (e.g., gas) sensors. Its sensitivity and dynamic range can be easily set through external passive components. Preliminary experimental measurements, which have characterized and validated this solution, have been conducted through a suitable prototype PCB fabricated with discrete commercial components. Then, the proposed interface has been also designed at transistor level, in a standard CMOS technology (AMS 0.35 um), developing a single-chip integrated circuit with low-voltage (1.8 V, single supply) low-power (about 350 μW) characteristics in a very small silicon area (lower than 0.6 mm2) which results to be suitable for sensor array configurations and portable applications. Further experimental results, achieved utilizing commercial sample resistors and capacitors to emulate sensor behavior, have shown a linear trend and a satisfactory accuracy in the evaluation of floating capacitive (in the range 10 pF–1 μF), grounded resistive (in the range 150 kΩ–1.5 MΩ) and floating resistive (in the range 10 MΩ–1 GΩ) variations, also when compared to other solutions presented in the literature. The satisfactory interface behavior has been also confirmed by the measurement of both relative humidity through the commercial sensor Honeywell HCH-1000 (capacitive) and carbon monoxide CO through the commercial air quality sensor FIGARO TGS-2600 (resistive).