A DAC With an Impedance Attenuator and Distortion Analysis Using Volterra Series

An impedance attenuator is implemented to reduce digital-to-analog converter (DAC) distortion, and two techniques to further reduce the distortion are discussed: the first is to use cross-coupled capacitors to cancel nonlinear currents; the second is to optimize the common-mode bandwidth of the differential amplifier for cancellation of distortion. To support these techniques, DAC distortion has been analyzed with the Volterra series, and the analysis is compared with simulation results and measurement data. The analysis uncovers a previously unknown nonlinear cancellation mechanism for the DAC and confirms the proposed techniques. Each technique can contribute 3-6 dB of improvement, and about a 6-dB improvement when combined. The 0.708-mm
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DAC is fabricated in a 28-nm metal-gate digital CMOS process with the proposed impedance attenuator, and it achieves -89 dBc of average HD3 at a 5-MHz input frequency and sampling rate of 230.4 MHz.