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The general definition of each are the following:

\$ENOB=\frac{SNDR_{max}-1.76}{6.02}\$

Where

\$SNDR_{max}=10 \log{\frac{S_{s,max}}{S_n}}\$

And

\$S_{s,max}\$ : maximum signal power

\$S_n\$ : noise and harmonics power

In the context of a sigma-delta ADC, \$S_n\$ is easily found by integrating the PSD from 0 to half of the sampling frequency ( \$f_s/2\$ ). What I want to know is if \$S_{s,max}\$ is the power of the sinus with maximum amplitude that can be processed by the sigma-delta ADC or the power of a hypothetical sine with an amplitude equating the rail voltage. There is a big difference because in these ADCs, for stability reasons a margin should be left. For instance in a second order sigma-delta ADC, the maximum signal amplitude should not exceed 80-90% of the full scale.

The general meaning would be for a hypothetical signal with an amplitude equating the rails voltage. This works for any Nyquist rate ADC but for a sigma-delta ADC that would mean that SNDR and ENOB are actually not measurable since the circuit would become unstable (or marginally stable).

Anyone can give me his opinion? Maybe with a reference. Thanks!

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The reference level for an ADC is the maximum signal that meets some quality criterion, for instance distortion.

In an ordinary ADC, exceeding the rails results in a distorted signal. In a sigma delta ADC, exceeding the stability threshold results in a distorted signal.

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