An adaptive feedback circuit for MEMS resonators

The first adaptive feedback circuit capable of detecting resonant frequencies for a wide range of MEMS resonators is presented. The feedback system presented implements a hill-climbing algorithm that sweeps actuation frequencies, locking onto the resonance condition at maximum cantilever amplitude response without limitations on the frequency range. To demonstrate its adaptability, a circuit implementation of this feedback algorithm was used to detect the resonant frequency of eight different cantilever-based sensors (width (W) = 1.4 mu m, length (L) = 40-75 mu m, and thickness (T) = 1.8 mu m), resonating at 201.0 to 592.1 kHz. Additionally, the same circuit was used to track resonant frequency shifts due to isopropanol adsorption on three different chemical sensors with no modifications. The feedback electronics integrated with these resonator sensors provide a mass resolution limit of 123 femptograms. The realization of this system will enable real-time chip-scale sensor systems, providing an alternative to external instrumentation modules that perform sensor control and monitoring.