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PCM1864: Why does the obtained data have a 4dB attenuation and clipping?

Part Number: PCM1864

I input 0V for the IN+ terminal of this ADC and 3.3V for the IN - terminal .

I set channel gain As 0dB, No high pass filter.

The data getting from ADC is: 0xD25080。

As the input range of ADC is 4.2Vrms, the data must be 0xB8DFE1. Why is there approximately 4dB attenuation?

I change channel gain to 11.5dB.

The data getting form ADC is: 0xAF3C80.

Obviously, the data is clipped 4dB from the full range. Why?

I had set the register page0, reister 5 As 0x06, which disabled the automatic clipping suppression.