An Energy-Efficient High-Swing PAM-4 Voltage-Mode Transmitter

Abstract

As the data rate of high-speed I/Os continues to increase, four-level pulse amplitude modulation (PAM-4) is adopted to improve the bandwidth density and link margin at 50 Gb/s and beyond. Compared to non-return-to-zero (NRZ) signaling, however, the PAM-4 eye height is reduced, which calls for larger transmitter swing to maintain signal-to-noise-ratio. A new energy-efficient transmitter is proposed to generate large swing PAM-4 signals with a cascode voltage-mode driver and supporting pre-drivers and logic circuits. By reconfiguring the pull-up and pull-down branches based on the transmit data and steering the bypass currents, the proposed voltage-mode driver significantly reduces power consumption compared to conventional implementation while maintaining impedance matching. Voltage stacking technique is adopted for pre-drivers to further improve energy efficiency. To demonstrate the new transmitter design, a prototype 56 Gb/s PAM-4 transmitter is designed using a generic 28-nm CMOS technology with a 2-V power supply voltage. It achieves a overall output swing of 2 V and a minimum eye height of 490 mV with good linearity (98.7% level separation mismatch ratio). Compared to a conventional voltage-mode transmitter design with the same swing, the static power consumption of the new transmitter is reduced almost by half (from 30 mW to 16 mW), and its overall energy efficiency improves from 0.7 pJ/b to 0.5 pJ/b.