What I do is for a Sigma - delta modulator design a digital filter, the digital filter by the CIC filter, filter and HB filter compensation of cascade,

I designed the three filter respectively in the FDA and generate module in simulink, to amplitude frequency response of the observation after they cascade (mainly consider the compensation effect of observation compensation filter), but using linear analysis tool mapped bode diagram is a perpendicular to the horizontal axis line (pictured),



Then I observed amplitude frequency response of the single CIC filter found mapped is a straight line perpendicular to the horizontal axis, but in the FDA observation waveform is normal, can you tell me how to solve this problem? If I wrong in the process of design and simulation operation?

Here to help you, thank you,

System module chart below



Specific operation process:

Analysis - & gt; The control design - & gt; Linear analysis... -> Analysis of I/Os to choose root level I/Os - & gt; bode