A Ratiometric Readout Circuit for Thermal-Conductivity-Based Resistive Gas Sensors

This paper presents a readout circuit for thermal-conductivity-based resistive gas sensors. It digitizes the sensor's heat loss to its environment, which is a function of gas concentration, relative to that of a reference transducer, which is made of the same material and acts as a thermal-conductivity reference. Thus, dedicated voltage, power or temperature references are not needed. The ratiometric interface is based on a reconfigurable delta-sigma modulator that digitizes both the temperature and power ratio of the sensor and reference transducers, from which their thermal-conductivity ratio is calculated. It uses a dynamic baseline-resistance cancellation technique to relax the required dynamic range. In addition, dynamic element matching and 6-bit bias-current trimming are used to suppress errors due to transducer mismatch. The interface has been implemented in a standard 0.16 mu m CMOS technology. Experimental results obtained in combination with CMOS-compatible tungsten-wire transducers show a CO2 resolution of 228 ppm (1 sigma), which is the highest resolution reported for thermal-conductivity-based CO2 sensors.