Mohamed,

Sorry if we have hi-jacked your question to discuss weighing semantics! In answer to one of your questions, if you use a 24-bit ADC, you will barely see better than, say, 19-bits, even if implemented perfectly, and with a perfect input signal. In reality, it is often much worse than this. I'm not really surprised at your 13-bit variation from the ADC output. You will see from our earlier (supposedly off-topic) discussion, that further severe digital filtering is essential before the output becomes practically useful.

Dave