> Of course it does, but the voltage to toggle the LSB of the A/D, *relative
to
> the maximum voltage from the CCD*
When talking about number of volts/bit (technically, volts/code) the
measurement is *USUALLY* done relative to the A/D input voltage range...but
if you want to reference to the CCD output voltage range, well, OK...as long
as that is stated.
> A/D converters vary more, of course, but I don't see what the input
voltage has
> to do with the usability of a 14 bit output.
Everything. If the input voltage to the converter isn't matched to the
converter, you will not get the full dynamic range of the converter.
> Anyway, adding a bit of gain is hardly the basis for an entire design
> philosophy.
> "Output of CCD = 2.8 volts, input of A/D converter = 4 volts. Ugh! Add
gain of
> 1.4."
> That's a no-brainer.
Well, it's possibly a little more than that. If the CCD output is 0-2.8V,
and the A/D input is -3V to +3V (typically, A/Ds take +- voltage swing), you
need to level shift it (negative offset of 1.4V), then apply gain...I don't
know the chips you are referring to, but perhaps they handle it
effortlessly... Obviously, you have to also consider how much distortion
these circuits introduce into the system, they may or may not be very
good... It all really depends on how good a system you want to design.
> And doesn't explain why all the scanner manufacturers are now moving to 14
bits.
Possibly because of cost. The lower bit converters aren't available any
more, and 14 bits are cheap... Sometimes this happens, where higher spec
parts are cheaper than older technology lower spec parts.
