A 6-bit segmented RZ DAC architecture with up to 50-GHz sampling clock and 4 Vpp differential swing

A novel, distributed, return-to-zero (RZ) power digital-to-analog converter (DAC) architecture is presented. The circuit features 14 independent data bits - 7 for the 3 most significant bits (MSB) and 7 for the 3 least significant bits (LSB) - each running at up to 50 Gb/s. It is fabricated in a production 130-nm SiGe BiCMOS technology and operates as a large swing arbitrary waveform generator suitable for a variety of wireline, fiber optic, and instrumentation applications. The measured small signal differential gain and bandwidth of the clock path are 19 dB and 43 GHz, respectively. An output swing of 2 Vpp per side is observed. Spectral measurements demonstrate multi-bit modulation at carrier frequencies as high as 50 GHz. To the best of our knowledge, this marks the highest clock frequency, highest voltage swing 6-bit DAC.

I. Introduction

Modern 40–110 Gb/s optical transmitters increasingly rely on Digital Signal Processing (DSP) and a high speed (56–60 GS/s) non-return-to-zero (NRZ) DAC [1]–[3] to provide higher order amplitude modulation of the optical carrier. The DAC output signal is first amplified by a large swing linear driver before it is applied to the optical modulator. To avoid gain compression which distorts the DAC signal, the driver is operated with several dB of back-off from its I-dB compression point. In turn, this further increases power dissipation and limits the bandwidth achievable even with the most advanced III-V large power transistor technologies. In principle, two DAC's, each with 2 effective bits of resolution at 28 GS/s and driving an I/Q optical modulator can produce a 16-QAM optical carrier with an aggregate throughput of 112 Gb/s. In practice, each DAC should have at least 2 extra effective bits of resolution to allow for pre-distortion and pre-emphasis at the nominal sampling rate. As a result, the DAC is designed with at least 6-bit resolution to allow for performance degradation with increasing sampling rate.