[600MRG] Gain required before AD converter on 630M?

[lstoskopf at cox.net]

Tech question that from those with experience here.

I'm building a SoftRock for 630M and going a bit overboard.....  Using a Sound Blaster X-Fi HD  with a CS5361 with 114 dB DR.  Then I see a kit from Germany using the CS5381 with 120 dB native DR and a bit better pre-amp setup.  

http://beis.de/Elektronik/ADDA24QS/ADDA24QS.html

A bit pricey and it requires buying the (driverless) USB converter attachment, but as a kit with thru hole components easier to play with. Uwe has it set up with input to the LM4562 Op amp with a 10K resistor to ground to set the impedance.  The Tayloe mixer I'm using has a 200 ohm load requirement.  Seems to make more sense to set the 1st stage as an impedance converter with input to the negative Op Amp input.  Kit arrives early Feb at my request.

So the question to someone on the list that might know:

Given a 24 bit quality AD and the noise levels from a quiet location, how much gain do I need in front of the AD from antenna to AD?

This is a bit upside down from what we are used to with a total analog set.  We want to have enough signal at the AD to use the full 24 bit range.  Actually maybewe have only 20 useful bits (and maybe IF, say Simon with SDR-Console only uses 16 in his program) and the AD likes a maximum of 3 Volts.  You would think that the step size is below the noise?????

Also, from the data sheet for the LM4562 the best performance is with an output level of about 3 volts which matches the max input of the AD so we need to push to that level.

So I'm thinking that I need to go with ? gain in the two IC stage of the AD board and (heresy) put a bit of gain in front of the Tayloe if needed to bring the AD to peak voltage.

So back to the question.  What noise + signal levels are you guys with lots of listening experience seeing when the band is useful.  That tells me the gain needed.

Of course the answer to me is:  Get on and measure it yourself!  Its a cold afternoon.  Thanks for the time guys,

N0UU