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.