Code-independent output impedance: A new approach to increasing the linearity of current-steering DACs

This paper proposes a novel complementary current approach to eliminating the code-dependence in the output impedance of current-steering digital-to-analog converters (DACs), and increasing the spurious-free dynamic range (SFDR) significantly. A 14bit 1.0GS/s current-steering DAC design example shows an SFDR increase of 10∼15dB. In traditional designs, one major effect that degrades the linearity is the code-dependence of the DAC's output impedance, which becomes a bottleneck at high frequencies because of the parasitic capacitance in the current branches. By adding additional current sources and switches to keep the DAC's output impedance nearly constant at different digital input codes, the proposed approach increases the DAC's SFDR by tens of dBs.