What sets the minimum detectable signal level for a direct-sampling software defined receiver in the absence of atmospheric or thermal noise?

In a direct-sampling software defined receiver (SDR), incoming analog signals are digitized by an Analog-to-Digital Converter (ADC). When there is no external or thermal noise to consider, the absolute minimum detectable signal is determined by the ADC's smallest quantization step. This minimum step size is a function of the ADC's reference voltage and its sample width in bits (resolution). The reference voltage sets the maximum measurable signal, while the number of bits divides that voltage range into discrete steps. A larger number of bits creates finer steps, allowing the receiver to detect much weaker signals. Distractors like sample clock phase noise or data transfer rates affect overall receiver performance or bandwidth, but they do not dictate the absolute lowest voltage threshold a direct-sampling receiver can register.