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Magnetic encoders are widely used for speed or position measurement. This research presents a suitable method to correct the quadratic signal from a magnetic sensor. A new quadrature all digital phased-locked loop (QADPLL) method is presented. The method minimizes the effect of amplitude imbalance, noise, phase shift, and signal offsets. It also can solve waveform distortion and time-lag problems. Moreover, this paper proposes an interpolation technique to improve the accuracy of position information. By deriving the high-order signal from a sinusoidal signal, a high-resolution position can be obtained from a low-resolution encoder. Simulation and experiment on a linear motor were conducted. The results verify the performance of the proposed methods.