The differences using FFT in MATLAB and SPICE

[j_c_lin]

I simulated my signal using FFT (Blackmanharris window) in MATLAB which gives me 16-bit reolution (96dB) but when i import the same signal to using sam FFT window in SPICE, the result only became 80dB left. Can any one answer why this happens and how should I avoid this problem. Thank you very much!