A 10-Gb/sec unclocked current-mode logic (CML) analog decision-feedback equalizer (ADFE) in 0.18-mu m CMOS

Abstract

An unclocked analog decision-feedback equalizer (ADFE) is implemented in a 0.18-μm 40 GHz ft CMOS process to equalize legacy FR-4 backplane channels at 8-10-Gb/sec. The critical first feedback-loop latency requirement of the DFE is met by using a novel unclocked feedback topology and current-mode logic (CML) circuit building blocks. The circuit consists of a 4-tap linear analog feed-forward filter that cancels pre-cursor inter-symbol interference (ISI) to partially open the eye and a novel 1-tap analog tunable CML feedback filter that enables cancellation of the first post-cursor at 10-Gb/sec without the use of smaller process nodes or speculative techniques. The chip with pads occupies 1.04 mm2 and draws 240 mA DC current from a 1.8 V supply at a typical process corner. The ADFE is used to equalize 20 inches of FR-4 backplane traces at 8-Gb/sec and 10-Gb/sec.