HSPICED ! Don't you mean HSPICE ?
Can you attach the FFT result figure ?
Anyway, the only reason to have distortion is a design error in one of the blocks. For example:
- Do you use S/H ? If yes, have you simulated it standalone (with its load, of course) ? Do you see any relevant distortion there ?
- Are you sure that the folding stages are settling fast enough ?
- Are there any DC deviations in the zero crossings of the folding stages ?
- Are your comparators fast enough ?
In what concerns the high noise floor, how many points have you used in the FFT ? If the number of points is low, then this may explain the high noise floor.
Regards