A 60-GS/s 6-Bit DAC in 0.5-µm InP HBT Technology for Optical Communications Systems

This paper presents a 60-GS/s 6-bit digital-to-analog converter (DAC) for beyond-100-Gb/s/ch optical communications systems. The DAC was designed and fabricated using our in-house 0.5-im InP HBT technology, which yields a peak f_t of 290 GHz, a peak f_max of 320 GHz, and a BV_CEO of approximately 4 V. We used a simple R-2R ladder-based current-steering architecture to achieve both high-speed and low-power operation, and a timing alignment technique to suppress glitch noise and improve dynamic linearity. The DAC can provide clear multilevel signals for quadrature amplitude modulation (QAM) transmission at a sampling rate of up to 60 GS/s (60 Gbaud). To our knowledge, our DAC offers the highest sampling rate of any previously reported DAC, and it can be applied to 400-Gb/s/ch-class optical communications systems.