How many 10-bit measurements are you feeding into your FFT? Typically it takes more than one reading in the time domain to attain enough of a waveform to transform into a meaningful result into the frequency domain (as an example - I don't exactly know what you are trying to do). So those many measurements that you take at 10-bits get processed to achieve a higher resolution transformer value. Think about it, if you have 10-bit data across many (hundreds) of waveform cycles, you can very accurately determine the frequency of the waveform once it is transformed (via FFT).

How many 10-bit measurements are you feeding into your FFT? Typically it takes more than one reading in the time domain to attain enough of a waveform to transform into a meaningful result into the frequency domain (as an example of a FFTransform). So those many measurements that you take at 10-bits get processed to achieve a higher resolution FFTransformed value. Think about it, if you have 10-bit data across many (thousands) of waveform cycles, you can very accurately determine the frequency of the waveform once it is transformed (via FFT).