16.3 A 330mW 14b 6.8GS/s dual-mode RF DAC in 16nm FinFET achieving −70.8dBc ACPR in a 20MHz channel at 5.2GHz

Abstract

Direct-RF synthesis has gained increasing attention in recent years [1] [2] as it simplifies the transmitter system by eliminating the intermediate frequency stage. It also offers the opportunity to address the extensive range of cellular bands with the same architecture and building blocks. Direct synthesis of carriers in the 5 to 6GHz unlicenced bands remains a challenge for RF-DACs operating in the 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> Nyquist band, as sampling rates in excess of 12GS/s are required. A more power efficient way to synthesize directly these frequencies is to use wideband mixing-DACs, which increase the output power in the 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> and 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> Nyquist bands [3]. In [3] the mixing is done using the quad-switch configuration, which doubles the number of switches and drivers, directly impacting the overall DAC width. In [4] the mixer is inserted in-line between the current cell switch and the output cascode, which requires additional headroom in the output stage. Both implementations impact the overall performance and power of the DAC even when the mixing operation is not used. This work presents an alternative implementation of the mixing-DAC using the traditional current switching cell. The mixing is realized in the data path, enabling full utilization of the DAC analog bandwidth across 1st, 2nd and 3rd Nyquist zones without compromising performance. This dual-mode RF-DAC is manufactured in a 16nm FinFET process and demonstrates ACPR better than -70dBc in a 20MHz channel centered at 5.2GHz while dissipating a total power of 330mW including shared biasing, clock receiver and clock distribution.