Development of a low-cost piezoresistive compass on CMOS

This paper presents the design and the prototyping of a low-cost monolithic CMOS compass. The system is based on a pair of piezoresistive cantilevers actuated by the Lorenz force. A special focus is given to the conditioning circuitry, which has been designed to improve sensor performances with respect to various parasitic phenomena such as noise, thermal effects and process uncertainties. It includes a tunable digital frequency synthesizer in the driving block, an original auto-centered filter in the readout circuit and an A/D converter. An analytical model of the sensing parts is proposed and described in a behavioral Analog-HDL module to support mixed-mode system level simulations. Validations of the complete system have been performed this way. The prototyping on CMOS is achieved using a low-cost (auto-aligned) bulk etching process. First experimental results are in perfect accordance with initial specifications. The obtained performances are a resolution inferior to 4° for a total surface of 10.6 mm2, a time response of 20 ms and a power consumption of 56 mW.