Circuit Approaches to Nonlinear-ISI Mitigation in Noise-Shaped Bandpass D/A Conversion

This paper focuses on the analysis of nonlinear inter-symbol interference (ISI) in RF bandpass delta-sigma power digital-to-analog converters (DACs). Digital RF transmitters based on direct delta-sigma modulation have been proposed for efficient linear multi-functional radios. We show that a realistic one-bit DAC limits the linearity of such a transmitter. The linearity is discussed in terms of ISI, for both single-ended and differential DAC circuits. Theoretical and experimental comparisons are given with a three-tone delta-sigma test signal at a 1.5 GHz clock frequency with 60 MHz possible signal bandwidth centered at 375 MHz. The signal-to-noise-and-distortion (SINAD) is shown to be improved for the differential case. It is also shown that circuit design and bias voltages have a dramatic effect on signal linearity.