This thesis discusses the theory, architecture and circuit design and measurements of an ultra-low-energy prototype interface circuit for a resonant gas sensor in standard 0.35-?m CMOS technology. A transient measurement method is used here. The resonant sensor is driven at a frequency close to its resonance frequency by an excitation source that can be intermittently disconnected causing the sensor oscillation amplitude to decay exponentially. From the ring-down signal, the frequency of the freely oscillating sensor and the quality factor are obtained. Test chips are fabricated to measure the resonance frequency and quality factor of the resonant sensor using ring-down measurement approach. The obtained results(resonance frequency and quality factor) show good consistency compared with what we obtained using an alternative approach (impedance analysis). The circuit consumes an energy of 237nJ per measurement.