A 3-Bit DAC With Gray Coding for 100-Gbit/s PAM Signal Generation

An integrated generator for 3-bit pulse amplitude modulation (PAM-8) is presented, supporting symbol rates above 30-GBaud. To optimize the generated pulses for a transmission, the digital input word is mapped to the binary-weighted segments of a digital-to-analog converter using the binary reflected Gray code, which minimizes the total bit error probability. The mapping is realized using full-rate digital logic based on exclusive-or gates. For validation, the circuit was fabricated in a 130-nm SiGe BiCMOS technology and measured on a PCB in combination with three pseudo-random bit sequence generators. At a symbol rate of 33.4-GBaud, the circuit can generate Gray-coded signals at 100.2-Gbit/s with PAM-8 and 66.8-Gbit/s with PAM-4.