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MDS with DDC?

Question asked by Reson8r on Sep 19, 2011
Latest reply on Oct 5, 2011 by Reson8r

I'm not sure if I'm in the right section here -- please move this post if not.

 

Can someone tell me what the minimum 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, but maybe that means that I can't resolve anything lower than the A/D noise floor.

 

Many thanks for any help!

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