Digital Time-Domain Predistortion of Linear Periodically Time-Varying Effects and Its Application to a 100-GS/s Time-Interleaved CMOS DAC

In high-speed data transmission systems, CMOS digital-to-analog converters (DACs) at very high sampling rates are essential components allowing predistortion of the transmitted signal. Next to compensation of bandwidth limitations of the DAC and assembly, predistortion is also able to compensate other effects originating from time interleaving or from artifacts of the DAC circuit concept. Especially, linear, periodically time-varying (LPTV) effects are one source of impairments in the output signal causing spurious components and their compensation is of particular importance for time-interleaved systems. In this work, a universal predistortion concept including system identification based on the system’s reactions to unit impulses at all corresponding positions in a period is presented and applied to a 28-nm CMOS DAC with time interleaving by an analog multiplexer at sampling rates up to 100 GS/s. Measurements of this DAC reveal LPTV effects with a period of 32 affecting the analog output signal that may be attributed to time interleaving as well as architecture. A theoretical problem description is given and sources of LPTV distortions are identified. Starting from the theoretical description, a universal N:1 predistortion method in time domain is deduced based on a simple system identification method. Measurements of single-tone signals reveal a signal-to-noise and distortion ratio improvement up to 13.5 dB and around 7 dB for a broadband, two-level pulse-amplitude modulated signal at 100 GS/s compared to a linear, time-invariant filter. The proposed predistortion concept is a universal method to compensate for any N:1 LPTV artifacts with significant reduction of LPTV distortions and can be translated to a common transversal filter structure.