A Variation-Tolerant Voltage-Mode Transmitter With 3+1 Tap FFE in 28-nm CMOS

In this study, we introduce a voltage-mode transmitter (TX) equipped with a 3+1 tap feed-forward equalizer (FFE). The proposed FFE merges a 3-tap coefficient-error robust FFE (B-FFE) 1 with a 1-tap sliding-tap FFE, enabling both variation-tolerant performance for significant taps and flexible configuration for the auxiliary tap. The original current-mode B-FFE structure and its transition filtering scheme are modified to accommodate energy-efficient voltage-mode operation and avoid a reduction in output swing for multi-tap configurations. An auxiliary sliding tap is implemented to remove ISI with versatile coverage. The proposed 3+1 tap FFE TX is designed and fabricated in a 28-nm CMOS process, achieving 18-Gb/s data rate, 6.6-pJ/bit energy efficiency, and 43% lower sensitivity with respect to mismatches in tap coefficients.