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TLV320ADC6120: why does SDOUTADAC have output without a sound source?

Part Number: TLV320ADC6120

Dear Expert: 

We connected a simulated MIC at pins 1 and 2, as shown in the following figure,

In a stationary state, i.e. without external sound sources, the waveforms of BCLK-ADC, FSYNC-ADC, and SDOUT-ADC are measured as follows: the blue signal is BCLK-ADC, the yellow signal is FSYNC-ADC, and the green signal is SDOUT-ADC

After zooming in on the red box, as shown in the following figure:

My question is, why does SDOUT_ADC have output without a sound source?
We grounded the input terminals, MIC-ANA+and MIC-ANA -, and still saw data output in SDOUTADAC, but it is unclear where the problem lies.