ADAU1761 ADC noise floor

hi all,

I am working with an ADAU1761.

In the datasheet of the ADAU1761 it is mentioned that the ADC is 24-bit and that the noise level of the ADC is at least >98 dB.

My question are:

1) are those 98 dB referring to the accumulated noise on all frequencies (overall power of the noise) or to the independant average level of each independant frequency?

2) why is the noise floor of the ADC that high. For a 24-bit ADC I would expect a quantisation error of around 144 dB.

What else is contributing to the increase of the noise floor when using the ADC?

thank you for your time

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