1.6 Gb/s/pin 4-PAM signaling and circuits for a multidrop bus

A 1.6 Gb/s/pin 4-pulse-amplitude-modulated (PAM) multidrop signaling system has been designed. The motivation for multi-PAM signaling is discussed. The system uses single-ended+reference current-mode signaling with three dc references for maximum bandwidth per pin. A test chip with six I/O pins was fabricated in 0.35-μm CMOS and tested in a 28-Ω evaluation system using on-chip 2¹⁰ pseudorandom bit sequence (PRBS) generator/checkers. Two different 4-PAM transmitter structures were designed and measured. A high-gain windowed integrating input receiver with wide common-mode range was designed in order to improve signal-to-noise ratio when operating with smaller 4-PAM input levels. Gray coding allowed a folded preamplifier architecture to be used in the LSB input receiver to minimize area and power. In-system margins are measured via system voltage and timing shmoos with a master communicating with two slave devices