Hi Tony, Austin.
Tony Sleep wrote:
> Austin Franklin ([EMAIL PROTECTED]) wrote:
>
> > Devices are not really linear. There are a number of 'distortions'. One is
> > offset, the second is linearity, and the third is gain.
>
> CCD's are AIUI inherently very linear. 'Offset' = CCD noise in this context, gain
> = 1.0
I think Austin was refering to the analogue pre-amplifiers built into a lot of A/D
converters.
These allow control of gain, and can also insert or remove a deliberate analogue
offset voltage.
An offset voltage is the only way to alter the 'slope' of the A/D conversion, by
changing the analogue value that corresponds to the LSB. Altering the gain simply sets
the maximum input voltage that the A/D can handle.
If you think of the A/D conversion as an anti-logarithmic (to the base 2) process,
then it's easy to see that an addition or subtraction, at the input, is converted to a
multiplcation or division at the output.
(apologies if this isn't what you meant, Austin)
> as fancy analogue amplification seems to be viewed as creating more problems
> than it solves, and such things are better (and cheaper) done to the digitised
> signal.
I think the difficulty is in predicting or calculating the exact value of any offset.
Since it'll vary with individual CCD, and with temperature.
Regards, Pete.
