Digital-to-Analog Converter up to 118 GS/s in 28-nm CMOS

The on-going growth of global data traffic drives optical data transmission systems beyond 1 Tbit/s per wavelength. Digital-to-analog converters (DACs) in transmitter front-ends are critical parts for the realization of such systems requiring sampling rates in the range of 100 GS/s and beyond. For monolithic integration with digital signal processors, CMOS DACs are essential.

In this project, a full CMOS integration of two 8-bit sub-DACs and an active analog multiplexer (AMUX) on a single chip is realized in 28-nm FD-SOI CMOS for 100 GS/s and beyond for the first time. An additional on-chip 256-kB SRAM completes the system to a universal, single-chip arbitrary waveform generator (AWG). It comprises more than 14 million transistors. Generally, an AMUX is able to shift the limits of DACs in well-established CMOS technologies toward higher frequencies independent of technology advances and therefore, it opens a second, conceptual path for achieving higher sampling rates. The AWG is developed for optical communication experiments as well as for application in experimental setups requiring an ultra-fast AWG.

Broadband pulse-amplitude modulated (PAM) signals up to Nyquist frequency are demonstrated up to 108 GS/s (PAM-2) with data rates up to 240 Gbit/s (80 GS/s, PAM-8). At, 100 GS/s, PAM-4 signals are shown. Moreover, the DAC system can be used for oversampling applications and pulse shaping for sampling rates up to 118 GS/s at lower symbol rates.

Finally, a universal time-domain predistortion concept including system identification is developed for compensation of deterministic, linear, periodically time-varying (LPTV) effects.