This article presents an energy-efficient resistor-to-digital converter (RDC) intended for use in a gas sensor system to detect harmful benzene, toluene, ethyl-benzene, and xylene (BTEX) compounds. The sensor is based on a resistive micro-electromechanical systems (MEMS) sensor, whose selectivity is tuned by different heating levels of micro-heater, allowing a single sensor to detect multiple gases. To achieve both high resolution and energy efficiency, the sensor’s output current is digitized by a continuous-time (CT) delta–sigma ( $\Delta \Sigma$ ) RDC. It combines the bias circuit for the sensor and the RDC’s first integrator, and then, its charge balancing between the sensor resistor and the 16-level feedback resistive digital-to-analog converter (RDAC) allows the use of a low bias current. Fabricated in the 110-nm CMOS process, the prototype RDC achieves an 18.8-bit resolution in a resistance range from 20 $\text{k}\Omega$ to 500 $\text{k}\Omega$ . The sensor system is validated through a transient response of BTEX and a principal component analysis (PCA). It achieves 0.1-to-5-ppb gas resolution in the BTEX concentration range of 9 ppm with a measurement time of 0.5 ms, while consuming 95 $\mu \text{W}$ from a 1.5-V supply. This corresponds to an energy consumption of 47.5 nJ.