Absolute Optical Encoder Chip With Self-Reference Compensation

Abstract

The characteristics of the analog waveform generated by the absolute photodiodes in traditional absolute optical encoders are highly affected by changes in environmental conditions, primarily fluctuations in light intensity and temperature. While a constant direct current (dc) voltage is set as the reference, maintaining stable duty cycles of the digital square waves converted from the generated analog waveforms at the ideal 50% can be challenging, potentially resulting in incorrect absolute code information. This article introduces an innovative approach for generating a reference level based on the additional current produced by the incremental photodiode array (INCP). By aligning consistently with the offset variations of absolute signals, the proposed method effectively stabilizes the duty cycles of the digital square waves. To validate the approach, a photonic readout chip utilizing a $0.35~\mu $ m CMOS process was developed, and the proposed method was successfully assessed. Test results show that the internal reference voltage remains nearly constant. And the method of generating the internal reference level can effectively maintain the stability of the duty cycle and the accuracy of absolute code information when the environment changes.