A novel output transformer based highly linear RF-DAC architecture

Abstract

A major limitation of the linearity of Current Steering (CS) RF-DACs is the large output voltage swing (typically 1Vpp), which couples to sensitive internal nodes and thereby causes non-linear distortion. This paper proposes a novel approach for the linearization of the CS RF-DAC. An output transformer decouples the output from the circuit core and attenuates the voltage swing seen by the RF-DAC current cells. A lumped element model of a transformer is used in calculations and simulations to analyze the performance of the transformer in the Mixing-DAC application, and to select optimal design parameters for high linearity. Verification with a simulation model of an RF-DAC shows that the output related non-linearity (IMD3) of the CS RF-DAC improves with about 14dB when the proposed transformer parameters are used.