ENOB from SINAD on dBc

Hello,

I'm trying to calculate the ENOB of a 16-bits 100kSPS digitizer card. I applied a sine signal and use Matlab to get the SINAD value (using the sinad function).

I know the equation

ENOB = (SINAD - 1.76) /6.02

However, I'm a little bit confuse with the SINAD value from Matlab being expressed on dBc. Should I use that value as SINAD on the previous equation? Or do I need to convert it to dB somehow?

I attached as an example the result that I got from Matlab from a 40kHz sine waveform (SINAD = 39.5 dBc). In this case, if I use directly the value on the equation I get ENOB = 6.27; which seems very low for a 16-bits digitazer card.

Thanks,

Jesus

[Robin Getz]

That is not actually the correct equation for ENOB with a lower than full scale signal (which is what the dBc is telling you).

Have a look at: http://www.analog.com/MT-003

equation 2, should help you out.

-Robin