Why do I see mismatches in HDL simulation with the System Generator generated testbench when my design contains the DDS Compiler block using either the frequency synthesis or SIN/COS Lookup Table?
This is because of a known issue with differences of how floating point results are handled between Xilinx System Generator for DSP and ModelSim. There should only be subtle differences in the lowest 1 or 2 LSBs of the DDS Compiler output and can be safely ignored.
This issue is resolved in 11.3 with the latest version of the DDS Compiler v4.0.