Fully Digital Transmit Equalizer With Dynamic Impedance Modulation

This paper analyzes the energy efficiency of different transmit equalizer driver topologies. Dynamic impedance modulation is found to be the most energy-efficient mechanism for transmit pre-emphasis, when compared with impedance-maintaining current and voltage-mode drivers. The equalizing transmitter is implemented as a digital push-pull impedance-modulating (RM) driver with fully digital RAM-DAC back-end for pattern lookup. This back-end compensates for both duty-cycle distortion and driver nonlinearity, while providing a programmable pre-emphasis. A testchip fabricated in 90-nm CMOS process shows relatively small signal degradation from dynamic modulation of driver output impedance over a variety of 20" backplanes at 4 Gb/s, with energy efficiency of 2 pJ/bit at 100 mV of receiver eye. Despite this signal degradation, at the same performance point, the impedance modulating driver shows better energy efficiency than impedance-maintaining current and voltage-mode drivers.