Performance Tradeoffs of Three Novel GMR Contactless Angle Detectors

Three innovative solutions for contactless angle detection are described and compared. They use a giant magnetoresistive (GMR) sensor bridge which detects the direction of the magnetic field of a rotatable magnet placed near the sensor. The three solutions differ in the CMOS Application-Specific Integrated Circuit (ASIC) employed as sensor signal conditioning interface, leading to different cost-performance tradeoffs. The first ASIC is all-analog, being the simplest and cheapest one. The second ASIC is a mixed-mode solution providing a pulse width modulated (PWM) output, and its main innovation is the use of a compact piecewise-linear ADC to compensate for the sensor nonlinearity. This way, linearization and A/D conversion are performed simultaneously, saving power and silicon area. The third ASIC is mostly digital and fully programmable, being the most flexible and accurate (albeit costly) solution. It incorporates as main novelties a new offset compensation technique that shares most of required circuits with the successive-approximation ADC, and a new combinational circuit for linearization. Measurement results for the three implementations are provided, and their performance tradeoffs discussed.