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Registered: ‎09-19-2011

MDS with DDC?

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.


Many thanks for any help!


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