Dac verilog ams output FFT plot issue

Hi All,

I have a DAC implemented in  verilog ams.

I am giving a 405Mhz digital hex code based digital input.

But when taking the spectrum of that it doesn't look like a sinewave means I don't see a single tone in the output. I am using Spectre as the simulator with AMS option selected and ++APS turned on in conservative mode.

The code looks to be fine as it is giving expected single tone output when using other utilities.

can anyone please suggest where I might be screwing up ? Should I be using XPS instead of APS or how can I debug the issue.

thanks