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Questions about 2nd Order Sigma Delta Modulator

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xujing2003

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Hi guys:
I am designing a 2nd order sigma delta modulator in cadence.
The problem is that I found the low frequency noise components within signal bandwidth are much higher than they should be according to the noise shaping function as shown the figure below.

My input bandwidth is 24KHz, and input signal is 24KHz, sampling rate is 6.144MHz, and the oversampling ratio is 128.

The bins on the left of the signal bins should be lower than the bins on the right of the signal as they are shaped by noise shaping function.

Could anyone propose what could cause the abnormally low frequency high noise components?

I really appreciate.
SDM2ndPSD4096Best.jpg
 

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