A common-mode suppression structure for absolute optical encoders

Abstract

In traditional absolute optical encoders, the DC levels of the absolute code output signals are easily influenced by factors such as light intensity and temperature, leading to fluctuations in the duty cycles and errors in reading absolute code information. This paper presents a novel common-mode suppression structure for the readout of absolute code signals. By eliminating the common-mode component of the signals, the structure effectively suppresses common-mode interference and ensures that the duty cycles of the output signals remain close to the ideal value of 50%. To verify the effectiveness of the proposed common-mode suppression structure for absolute optical encoders, a photonic readout chip based on a 0.35μm CMOS process was designed, and good common-mode suppression effects can be achieved by combining the differential-distributed code disk. Test results show that when the LED’s normal operating current is 3.5 mA, the output duty cycle error is less than ±4%, which reduces 50% compared to the traditional structure. Under a variation of ±1 mA in LED operating current, the corresponding changes in output duty cycles remain within 5%. Similarly, when the temperature shifts from 25 °C to 125 °C, the changes in output duty cycles are contained within 3% It can be seen that this structure can suppress most of the common-mode interference caused by external factors, ensuring the stability of the output signals and reducing the bit error rate of the absolute code signals.