Resistive-type sensors such as gas sensors are widely used to monitor the environment. In most resistive-type sensor applications, none or very simple signal processing circuits are used. In this work, we apply modem signal processing circuitry to the resistive-type gas sensors. We tailored the signal processing circuitry to the special nature of these SnO₂, based resistive-type sensors, with the eventual goal of integrating the signal processing circuitry and the CMOS compatible gas sensor on a single silicon chip. _{2 2}...[ Read more ]

Resistive-type sensors such as gas sensors are widely used to monitor the environment. In most resistive-type sensor applications, none or very simple signal processing circuits are used. In this work, we apply modem signal processing circuitry to the resistive-type gas sensors. We tailored the signal processing circuitry to the special nature of these SnO₂, based resistive-type sensors, with the eventual goal of integrating the signal processing circuitry and the CMOS compatible gas sensor on a single silicon chip.

SnO₂, is a popular metal-oxide sensor used to detect combustible gases such as CO, H₂ and methane. The operation of the metal-oxide sensor is based on the decrease of resistance when these gases are introduced in the ambient atmosphere. The resistive sensor can be modelled as a gas sensitive variable resistor. Analysis shows that the resistance varies inversely with the square root of the partial pressure of the gas which gives

R_sensor[proportional to]1[division slash][square root[gas]]

A current-mode interface circuit for integrated resistive-type gas-sensor application, followed by signal processing circuits, is described. Analysis shows that current mode operation out performs other operation modes. The base-band resistive signal is first chopped to higher frequency to minimize the interference of flicker noise, converted to current and amplified. This configuration takes advantage of the property of Equation (l), to increase the dynamic range. After filtering, the chopped signal is demodulated back to base-band signal. Unlike conventional lockin amplifier technique that the AC signal is demodulated by only a single demodulator, two demodulators with 90[spacing ring above] phase difference are used in the circuit to (1) increase base-band magnitude, (2) reduce ac component, (3) double the frequency of the ac fundamental and harmonics and (4) reduce noise caused by difference in phase. Measurement shows that AC harmonics interference over base-band signal ratio at output is reduced by two thirds.

The circuit is realized with 1.2μm CMOS process. The use of switched-current technique in filter and phase shifter design requires only a single poly layer. Hence, it can be implemented on standard digital CMOS process, with other digital control logic, without extra cost of an analog process. Finally, employment of class-AB structure helps to achieve low power consumption of 5mW. The active circuit area measures 2mm x 2mm.