Why is a bias level applied to CCD?

OK, I know why the bias level is applied, which is so that the
analogue-to-digital convertor doesn't get any random negative input,
which would happen if the baseline signal was zero. Some would be
negative on average, but if you apply a constant positive level, then
all the ADC input would be positive and can be just subtracted later.

But why is the concept of negative input bad? Can anyone help me?

Thanks,
Steve

On 5 Mar 2004 13:22:07 -0800, (Steve Shorlin)
wrote:

OK, I know why the bias level is applied, which is so that the
analogue-to-digital convertor doesn't get any random negative input,
which would happen if the baseline signal was zero. Some would be
negative on average, but if you apply a constant positive level, then
all the ADC input would be positive and can be just subtracted later.

But why is the concept of negative input bad? Can anyone help me?

Most cameras produce unsigned 16-bit data. That means there is no way to
distinguish between a small negative value and a value near saturation. Other
than that, there would be no reason you would have to provide a positive bias.
It makes working with the data simpler if you stay positive; naturally you could
force that once the data is retrieved from the camera if a bias hasn't already
been applied (assuming you have some mechanism to figure out if the data is
negative).

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Chris L Peterson" wrote in message
...
On 5 Mar 2004 13:22:07 -0800, (Steve Shorlin)
wrote:

OK, I know why the bias level is applied, which is so that the
analogue-to-digital convertor doesn't get any random negative input,
which would happen if the baseline signal was zero. Some would be
negative on average, but if you apply a constant positive level, then
all the ADC input would be positive and can be just subtracted later.

But why is the concept of negative input bad? Can anyone help me?

Most cameras produce unsigned 16-bit data. That means there is no way to
distinguish between a small negative value and a value near saturation.
Other
than that, there would be no reason you would have to provide a positive
bias.
It makes working with the data simpler if you stay positive; naturally you
could
force that once the data is retrieved from the camera if a bias hasn't
already
been applied (assuming you have some mechanism to figure out if the data
is
negative).


I'd like to elaborate on this. My two cents.

Since you're not expecting negative signals fromt he CCD (less photons than
zero?! What a concept! ^_^ ) it's desirable to use the full scale
available on the AtoD converter, so if it's 16-bits, you want as much of
that 16-bits as possible. Unfortunately, due to noise in the analog stages,
you might get negative going signals, and this produces what appears to be
large positive pixel values after you convert, hence the bias.

So one idea is to reduce the noise. This leads to low noise amplifiers and
careful construction to shield all analog lines from noise. By the way,
this is why Meade took two years to get their 216XT out the door once they
announced it. I talked to a contractor they hired to solve their problems,
and he finally was able to convince them that placing the power supply so
close to the analog circuits was the main reason they saw so much noise from
the camera. But anyways, you have to do it before it gets converted,
otherwise it's impossible to process the
noise-looking-like-large-pixels-values out later. You might reject
legitimate signals out of the image.

--
Sincerely,
--- Dave
----------------------------------------------------------------------
It don't mean a thing
unless it has that certain "je ne sais quoi"
Duke Ellington
----------------------------------------------------------------------

Thanks Dave and Chris. Big help there guys to a PhD candidate who has
used many CCDs, but has had the fiddly finer points pass him by.