Yawei,
The problem is that you are using the wrong tool to evaluate the
frequency response. If you are simulating a DAC and you want to
evaluate the effect of glitch on the response then you should use
the Fourier integral, .four, not the FFT to evaluate the frequency
response. The Fourier integral evalutes the entire waveform while
the FFT only samples the waveform. So the FFT is great for ADCs,
continuous analog --> discrete analog --> digital since the output
waveforms are sampled. However for DACs, the output waveforms
are not suited for the FFT, digital -->continuous analog. The bottom
line is that FFT is inefficient if you are sampling in small enough time
intervals to capture the glitch. The downside of the Fourier integral
is that it does not support window functions so if you want to use
for Sigma-Delta DACs, you need to write your own window function
Verilog-A module.
BTW, there was alittle bit if discussion of this in a previous thread,
http://www.designers-guide.org/Forum/YaBB.pl?num=1113231964 Best Regards,
Sheldon