A low power re-configurable ring oscillator (RO) based interface circuit for SnO₂ gas sensor is presented. The period of RO is dominated by the sampling voltage from gas sensor which is dependent on gas concentration. This proposed circuit improves the performance and power efficiency for resistive sensors. The interface circuit is implemented in 0.18 μm logic process with an area of 4000 μm²and 0.5 mW power consumption. The SnO₂ thin films deposited on a micro hotplate machined by a MEMS process and is finally integrated with the interface chip in the form of a multi-chip package. Detection of Sub-ppm-Level ethanol gas has been demonstrated, the total power consumption of the sensor is less than 25 mW, which is suitable for IoT gas detection applications.