Temperature control and readout circuit interface for Mox based NH₃ gas sensor

This work demonstrates a handheld, micro-heater integrated metal oxide (MOX) based gas sensor interface system that includes digital set point, a precise closed loop temperature control and a wide range resistance readout circuit. The implementation of proposed system, constitutes two circuits. First, the temperature control loop circuit, able to set the platinum micro heater temperature over a range from ambient temperature to 300 °C, with a maximum peak to peak deviation less than 2.0 °C. An ON–OFF control technique is used to control the temperature of microheater since the heat transfer characteristics of micro heater are difficult to model. Second, a resistance to time conversion based resistance readout circuit is designed that measures the resistance over a range of 1 KΩ to 100 MΩ. The overall system is flexible and can be interfaced with integrated sensors having different power requirements and fabrication parameters. Gas sensing film used for gas detection is made of Tin oxide (SnO2). Since, the metal oxide sensor works at elevated temperatures, it can detect different gases like, ammonia (NH3) at 230 °C, nitrogen dioxide (NO2) at 80 °C and carbon mono-oxide (CO) at 270 °C. Various experiments were conducted to test the above mentioned circuit by first connecting discrete known resistive components and also, its response with NH3 is recorded with various concentrations. The resistance readout circuit output has an excellent accuracy with an error of less than 2 % in sensor resistance measurement and hence in measurement of gas concentration.