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This paper presents a new highly sensitive (10 μV/ppm) bidirectional current excitation method for piezo-resistive sensor measurements, through excitation of two half bridges for ΔR measurement. The proposed circuit is insensitive to thermoelectric and stray noise effects since it measures the peak-to-peak value of the generated voltage. Measurement results using resistors show that variations as low as 0.3 ppm are measurable from 15°C to 80°C with resistors. As an experimental application, 40 parts per billion variation in gas concentration using piezo-resistive SU-8 microcantilevers is measured by the proposed circuit at room temperature.