See the calculation Andre77 showed in the Magnova thread. This one is correct. You need to go from the full dynamic range of the ADC which is 8Vpeak-peak in case of 1V/div. Your error is using the RMS value for full scale ADC range.
Andre's calculation is wrong.
SINAD (SNR if we have no distortion) is ratio of RMS full range and RMS noise level.
The IEEE has defined terminology and test methods for analog to digital converters in [1]. This includes the following definition of ENOB:
Equation 4
ENOB=0.5log
2(SINAD)-0.5log
2(1.5) - 0.5log
2(A/V)
The following definitions apply:
V : full-scale range of the device under test.
A : peak to peak amplitude of the sine wave fitted to the output.
SINAD : signal to noise and distortion ratio.
SINAD is defined as
Equation 5
SINAD = P
S/P
NADThe following definitions apply:
P
S : signal power; power in the FFT bin corresponding to the input frequency
P
NAD : noise and distortion power; sum of powers in all other frequency bins excluding the 0 frequency bin, up to and including the bin at Nyquist frequency
Here it is noted that SNR and SINAD, as defined in [1], are ratios of ‘
rms’ (root mean square) values and not a ratios of power values which is for example typical for communications engineering.
[1] IEEE Standard for Terminology and Test Methods for Analog-to-Digital
Converters, IEEE Standard 1241-2010
*** R&S
The Effective Number of Bits (ENOB) of my R&S Digital Oscilloscope
Technical Paper