AD7171's ENOB and Noise Free bit is calculated by datasheet p.9 equations.
These results is over 16bits.
And ENOB is 16bits in specification.
Is there over bits sampling?
Yes, the result shows that you have no flicker code or noise in the LSB and you can maximize the entire 16 bit resolution assuming that you have the ADC input referred noise and Reference listed on the table. The noise for sigma delta ADC is very low due to oversampling technique, the signal is oversampled at a rate higher than the minimum required sampling rate (Nyquist frequency). Oversampling will spread the noise over a much wider bandwidth and with sigma delta modulator added feature of noise shaping, the quantization noise of the analog-to-digital conversion is shaped by the modulation scheme, shifting it (typically) from a low bandwidth up to a higher frequency, allowing a low-pass digital filter to eliminate it from the conversion result. Thus, key concepts of oversampling, noise shaping, averaging (filtering), and decimation, is used to increase the effective resolution of Sigma delta ADC.
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