Paper MN-TuM13
An Adaptive Feedback Control Circuit for Resonator Sensors

Tuesday, October 16, 2007, 12:00 pm, Room 615

Integration of smart electronics with MEMS sensors will enable systems to be versatile, compact, and portable. MEMS resonator sensors are powerful tools for the detection of target analytes due to the high sensitivity of resonant frequency to absorbed mass. We present an adaptive feedback control circuit to detect and trace the resonant frequency of MEMS resonator sensors. The purpose of our feedback circuit is its integration with a fully developed III-V optical resonator system for chemical and biological sensors, facilitating testing and data acquisition. Feedback circuits with similar functions, such as self-excitation systems, have been reported before in literature. These systems, however, require phase and amplitude compensation stages that require separate designs for each resonator measured. Our feedback circuit utilizes a hill climbing algorithm which is valid for any resonator sensor that exhibits any range of resonant frequency, thus broadening the applicability of the circuit. The hill climbing algorithm sweeps the driving frequency searching for maximum cantilever response. The algorithm is implemented using a four-stage CMOS circuit consisting of an amplitude detector, a differentiator, a digital logic circuit, and a voltage controlled oscillator (VCO). The feedback circuit receives the displacement output of the resonator and supplies the actuation signal to the resonator from the VCO output. Utilizing the hill climbing algorithm, the resonator is driven at its resonant frequency. By monitoring the VCO input voltage, the resonant frequency with respect to time can be measured. We have confirmed the adaptability of the design of the circuit with initial testing results. The results have demonstrated that the maximum amplitude of an input signal can be detected with input frequencies ranging from 100 KHz to 500 KHz. This range is only limited by the frequency response of the CMOS components. A delay of 3 ms was observed between the input and output signal of the circuit, which is acceptable due to a significantly larger sensor time constant. We will present the test results of the combined circuit with indium phosphide MEMS cantilever sensors. The flexibility of the circuit and its improved capabilities over conventional measurement circuits will be demonstrated.