Design Of Integrated Converting Circuits For Resistive Gas Sensor Arrays

Resistive gas sensor is a kind of chemical components whose changes of the resistance or conductance of the sensitive thin film electrode are related to the concentration or composition of the ambient gas. This kind of sensor has the advantages of simple preparation process, convenience of measurement and being easy to implement the sensor arrays. It can also meet the requirements of preparation process of different materials, and be compatible with CMOS technology. Basing on CMOS technology, an integrated converting circuit with 3×2 electrode arrays of resistive gas sensor was designed. Through experiments, we realized the integration, miniaturization and low power consumption of the system, reduced the interference of external signal, and improved the sensitivity and stability of gas sensor.Firstly, the working mechanism and the noise of the resistive gas sensors were analyzed. We selected six kinds of sensitive materials and proposed three converting circuit modes combined with the work features of gas sensing films. Three modes are composed of integral amplifier mode, transimpedance amplifier mode and differential amplifier mode. These circuits were integrated onto a same chip in order to save costs, and we can select the required operating mode by closing the corresponding switch. Each converting circuit mode was designed, simulated, analyzed and optimized. The simulation results showed that three circuit modes can meet the design requirements. When the gas sensing resistance changes 20%, the linearities of integral amplifier mode, transimpedance amplification mode and differential amplifier mode are 1.12%, 0.88% and 1.04% repectively, the sensor resolutions of three conveting circuit modes are 0.02%, 0.02% and 0.024% repectively, and the single-channel powers of three modes are 11.5 mW, 10 mW and 7.5 mW repectively.Secondly, we designed interdigitated electrodes array based on the 0.5μm CSMC CMOS process rules. the fabrication of these interdigitated electrodes within a CMOS process can be achieved by exposing the top metal layer during the same process step used for bond pad opening. We connected together the electrodes array with its converting circuit. The final chip area of the sensor is 16.5cm×15.6cm.Finally, we designed a simple package about the chip after tapeout and tested the performance of the circuit. For the integrated circuit with three different converting modes, the sensing response to NH3 and the repeatability to formaldehyde were tested, respectively. Test results showed that the differential amplifier circuit has simple external interface and good linearity, but the preparation of its thin films is difficult; the transimpedance amplifier circuit can achieve real-time detection of gas sensing resistor, but the output has more obvious glitches; and the integral amplifier circuit output is more flat, but its input interface is more complex. Three kinds of circuits can achieve gas detection with good repeatability and restoration succeesfully.