An Ultrahigh-Speed Low-Power DAC Using InP HBTs for Multi-Level Optical Transmission Systems

This paper presents an ultrahigh-speed low-power digital-to-analog converter (DAC) for multi-level optical transmission systems. To achieve both high-speed and low-power operation, we used a simple R-2R ladder-based current-steering architecture and devised a timing alignment technique. The 6-bit DAC test chip was fabricated with our InP HBT technology, which yields a peak f_t of 175 GHz and a peak f_max of 260 GHz. The measured differential and integral non-linearity (DNL and INL) are within +0.68/-0.16 LSB and +0.17/-0.68 LSB, respectively. The measured spurious-free dynamic range (SFDR) remains above 36 dB up to the Nyquist frequency at a sampling rate of 13.5 GS/s, which was the limit of our measurement setup. The expected linear ramp-wave outputs at a sampling rate of 27 GS/s are also obtained. The DAC chip was assembled into a metal package for use in an optical transmission experiment, where it provides a clear 4-level signal for 16-QAM optical transmission at a sampling rate of up to 28 GS/s. The DAC consumes only 0.95 W and has a better figure of merit (FOM) (0.53 pJ) than any other previously reported DAC with a sampling rate above 20 GS/s. Our DAC module can also be applied to beyond-100-Gb/s/ch multi-level optical transmission systems.