Can someone tell me what the mininum detectable signal (MDS) should be using a digital down converter (DDC)? Assume I have a 14-bit A/D with 74 dB SNR which feeds the DDC. Say the A/D operates at 65 MHz (fs) and then the DDC down-converts a 10 kHz BW (bw) anywhere in the first Nyquist zone to baseband. I think this means a processing gain of 10 log (fs / bw), for 10 log (65 MHz / 10 kHz) = 38 dB extra reduction in the A/D noise floor, but I'm not quite sure what this means. Does it mean I would be able to resolve a signal just above 74 + 38 = 112 dB below the full-scale of my A/D (well below the LSB)? Would my MDS be limited by the A/D noise floor, or the A/D floor lowered by the DDC processing gain?
There is dither too. I know that the spurious free dynamic range (SFDR) of the A/D can be increased by adding dither. Is this necessary to take advantage of the 38 dB of processing gain? Do the spurs of a NON-dithered A/D remain after the DDC processing? I think they would.