But 9Vpp (provided that is the full scale range of the ADC) is only 3.182VRMS, for a full scale sine wave. Then the SNR is 20*log10(3.182/0.005) = 56.075 dBFS, or equivalently (56.075-1.76)/6.02 = 9.0224 ENOB.
On the 1V range: 20*log10(3.535/0.005) = 56.998dB = 9.17 ENOB
Hmm -- pity that you deleted the screenshots taken at 50 mV/div. Those actually had a higher ENOB
On the 50mV range I get ... 64.95dB = 10.5 ENOB
Martin posted a link to article about ENOB measurements. It seems not all read it, but here we are, magic numbers are coming out of the hat.
Measuring ENOB in scopes is made in accordance to IEEE Std 1057â„¢ standard "IEEE Standard for Digitizing Waveform Recorders".
There are many nice articles by various scope manufacturers on the topic.
ENOB is calculated from SINAD (Signal to noise and distortion) ratio. It is measured by inputting low distortion sinewave (better than scope measured) and sweeping across a frequency BW of scope. ENOB is measured as a curve, a graph, not a single number.
It will vary with with frequency.
On scopes it is measured with signal that is 90% of full screen.In this case it would be 7.2V P-P for DHO800 1V/div.
That curve than gets statistics treatment and some kind of average or worst case scenario number is picked by marketing. Siglent publishes worst case scenario ENOB number with some reserve included, for instance, making it very conservative number. They guarantee it will be better than number published. It is always better than that. Most manufacturers are the same in that regard. Rarely manufacturers (any of them) publish full characterisation of ENOB curves directly in datasheet.
It is definitely
not a ratio of scope AC RMS measurement of noise floor and theoretical RANGE of ADC.
Since really measuring ENOB is out of reach of many (no wide BW signal source of sufficient quality) we can compare and talk about just noise floor levels VS full screen scale. Which is definitely not the same as ENOB.In which case it is simply SNR (signal to noise ratio). Expressed in dB.
We could talk about bits equivalent of SNR but that is not usually made because it has no practical meaning.
But this is not ENOB. ENOB needs signal applied and includes distortions (nonlinearities, sampling jitter...) . It is already a simplified metrics....
One another note: DHO800 has 8v P-P range on screen. That equates to 2,8369 V RMS sinewave.
That is your usable full dynamic range. The rest is reserve to prevent clipping.