An ADC's resolution is set by the maximum input voltage divided by 2b where b is the number of bits. In that way, a 16-bit ADC with 20V input can measure a voltage of 20/65536 = 0.3mV (corresponding to the ADC level 0000000000000001). However, I know an ADC can be used to measure voltages down to the ADC's own quantisation noise, which is typically a number more like 5uV.
I guess you could measure a very noisy signal with mean value lower than 0.3mV, as long as you averaged many measurements. Is this true? Does the noise have to be large enough to tip the signal above 0.3mV (in this case) so that the ADC can register a non-zero signal? Does this relate to dithering?