Interface Circuit Design And Test For MEMS Resonant Microcantilever Biological Sensors

Biological sensors based on resonant microcantilever possess ultrahigh sensitivity andsuperb mass-detecting resolution, thus empowering trace detection on a single-cell scale.Additionally, such sensors can help to realize portable on-site detection, hence avoiding theutilization of bulky equipment widely employed in conventional biological detection.Therefore, microcantilever-based biosensors are increasingly attended to and related studiesare intensified. Aiming at the specific requirement of biological detection and theenhancement of mass-detecting resolution, this work focuses on the design of closed-loopinterface circuit urgently needed by the aforementioned sensors.Our investigation indicates that the resolution of as-prepared sensors closely relates to threecritical elements, i.e., the effective mass, resonant frequency and phase noise of themicrocantilever: either a smaller effective mass, or a higher resonant frequency or a lesserphase noise materializes a higher resolution. The effective mass and resonant frequency canuniquely be optimized via changing the structures, sizes and manufacturing processes,whereas the phase noise can be controlled via enhancing the performance of interface circuit.The main features of this paper is that improve the resolution of the resonant microcantileverby optimum design of the interface circuit. The closed-loop interface circuit ofmicrocantilever-based biosensors features Phase-Locked Loop (PLL), meeting therequirement of closed-loop self-excitation via tuning the modules of operation-amplifier, filter,phase-shifter, etc. Additionally, in this paper, the operational characteristics of as-preparedmicrocantilever and circuit were analyzed in detail on a systematic scale. Also, in order toeliminating the poor stability and strong noise-perturbation, related methods were explored.Not least, portable on-site application necessitates the design of specialized frequency counter for the abovementioned biosensors. To meet the stern demand of high precision offrequencies, a frequency measurement featuring an equal precision was employed. Thefrequency counter utilizes a 89C51 singlechip as a master chip, and 8-bit LCD to display thefrequencies, and assembler language to design the programme. The singlechip may constructa function-expanding platform if incorporating some circuits (e.g., a data-processing one) viathe well-known interface technology.Finally, for characterizing the frequency stability more scientifically and minimizing thenegative influence induced by frequency excursion, the measured data were processed viaHadamard Variance, thus satisfying the mandatory requirements of trace detection throughas-produced biosensors, and the resolution of the sensor can reach 69fg.