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Frequency modulation technique can be applied to microelectromechanical systems (MEMS) transducers that require some form of resistive sensing. For example, electrothermal sensing is being investigated as a viable means of measuring displacement in micromachined transducers. This paper proposes a highly sensitive readout circuit, which can convert 10 Ω change of resistance in a 400 Ω electrothermal sensor to more than 200 kHz frequency variation (350-550 KHz). The frequency variations are then converted to voltage values by means of a frequency demodulation. In addition, the proposed technique achieves high linearity from the voltage applied to the actuator to the voltage measured at the sensor's output, which can potentially eliminate the need for an additional linearization if the sensor is used in a feedback loop. The proposed approach leads to high sensitivity in the MEMS electrothermal sensing since the method is not affected by amplitude variations that could arise from the readout circuit.
Date of Publication: Aug. 2012