A Precision Capacitive Rotary Encoder With Multistage Offset Calibration

This paper presents a precision capacitive rotary encoder that integrates a shared charge amplifier, and a multi-stage offset calibration scheme. Recently, capacitive encoders have gained significant attention due to their intrinsic advantages over optical and magnetic counterparts, including simple structure, low production cost, potential for miniaturization, and robustness in harsh environ- ments. However, improving their resolution, precision, and linearity to minimize signal distortion caused by inter-channel interference or mismatch remains a highly challenging issue. To address these limitations, a 6-bit programmable capacitor array and a 12-bit R-2R digital-to-analog converter (DAC) were implemented at the analog front-end, achieving effective DC offset cancellation. Additionally, a quadrature modulation/demodulation scheme was adopted to enhance channel isolation. The entire encoder IC was fabricated using a standard 180-nm CMOS process. Measurement results demonstrate a low angular nonlinearity of 0.189%, an SNR of up to 74.1dB, and stable operation across a wide speed range up to 4000 RPM. The system consumes 20 mA in total, of which 7.12 mA is drawn by the chip, offering a favorable balance between performance and power efficiency. The fabricated encoder prototype achieves a highly compact form factor with an outer diameter of just 21.6mm, making it one of the most miniaturized implementations among comparable designs.