CCD-world: cross-talk in multi-channel CCDs
Tom Droege
droege at wwa.com
Thu Mar 23 08:01:39 CLT 2000
The following was posted to CCD-world:
Roy and all,
First I will second Barry's recommendation. The Analog Devices technical
notes on grounds are just great!
Here is a suggestion on how to test. Remove the CCD. Apply a pulse
generator to one of the outputs and run the system. You will probably have
to kludge up something to generate the signal you need. But this is a
tough problem and will require some preperattion. Now get a fast
differential scope probe. Tektronix makes some nice ones. Now you can go
around taking differences between grounds. With say a 1 volt pulse (or
whatever the front end amplifiers will stand withoug saturation) and 100 uv
or so sensitivity on the differential probe, you should be able to see a
lot of stuff on the grounds. This just may lead you to a solution.
Cross talk is a tough business at this level. Don't expect an easy
solution. Don't hesitate to build up something to test properly. It is
easy to spend a lot of time on little quick fixes that do not work when a
more elaborate carefully executed plan will solve the problem.
Errrr! Possibly you are using a multi-layer board with a common ground
plane under the four channels. I would not do this. Four separate ground
planes, connected at one point is the way I would go. Of course separate
analog and digital ground planes with the digital plane separate from the
analog plane. You can use one layer for both but separate them between
analog and digital.
I repeat my suggestion of making a good diagram. Every place there is a
common tie between two grounds draw in an inductor. Put capacitors between
everything on the diagram. Remember that the ground plane (if you have
one, I would not) couples capacitively between everything.
Sigh! A tough problem. Good luck with it and please keep us informed. It
is often more important to learn how not to do things than how to do things.
Tom Droege
At 11:27 AM 3/22/00 -1000, you wrote:
>The following was posted to CCD-world:
>
>Hi Roy,
>
>Photon-forager, I like that, maybe Photon-phorager or Foton-forager...
>
>Anyway back to the matter at hand...
>
>The first thing I'd ask is have you tested your acquisition system
>performance separately from the CCD. If you still have the problem it's
>not the device (or only the device at least at the level your currently
>experiencing) if it goes away then either it's the CCD or your interface
>to the CCD. Split the chain if you can to see what's your offending
>party....
>
>Regarding decoupling of video amplifiers, often the addition of more
>capacitance does not improve your decoupling due to the resonant
>frequency issues of non-ideal capacitors. Often you need to parallel
>0.01uf ceramics with 1uF tantalums (surface mount best, extremely short
>leads mandatory) to get solid wideband decoupling. There is a wonderful
>application note by Analog Devices AN-202 "An IC Amplifier User's Guide
>to Decoupling, Grounding and Making Things Go Right for a Change" by
>Brokaw that's a classic (great title as well). But there's a load of
>good info for Analog Devices, Burr-Brown, and Linear Technology
>application notes in this area.
>
>
>Good Luck,
>
>Barry
>
>Roy Tucker wrote:
>>
>> The following was posted to CCD-world:
>>
>> Dear Fellow Photon-Foragers,
>>
>> I am participating in the up-grading of the GONG
>> (http://www.gong.noao.edu) video data system which includes a new camera
>> (Silicon Mountain Designs 1M60-20) with a Thomson THX7887A imager
>> (frontside 1024 square array, four channels of 1024 x 256, 14 um pixels, 60
>> frames per second, 12 bits per pixel). Our data acquisition algorithm
>> consists generally of co-adding images for 60 seconds into three or six
>> accumulation buffers in synchrony with the angular orientation of rotating
>> optical elements in the system. Co-adding 600 or 1200 12-bit numbers
>> produces approximately 23-bit results.
>>
>> We are noticing a low-level (far below the 12-bit resolution of an
>> individual image) cross-talk between the four output channels of the
>> camera. The phenomenon is non-linear in that it does not appear until a
>> threshold luminance level is reached. Our initial hypothesis that there
>> were insufficient bypass capacitors associated with the video amp power
>> supply rails and CCD bias voltages appears to have been disproven by an
>> experiment involving the installation of larger caps. There appeared to be
>> no beneficial effect.
>>
>> Although not a serious problem, we would like to reduce or
eliminate the
>> effect if possible. Before proceeding further with our testing, we thought
>> it might be wise to consult the CCD engineering community to see if anyone
>> else has seen such an effect and if they could indicate to us if it is due
>> to the CCD support electronics or an inherent property of this particular
>> CCD imager or multi-channel CCDs in general.
>>
>> Thank you very much for your attention.
>>
>> Best regards,
>> Roy Tucker
>>
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>
>--
>_____________________________________________________________________________
>
>Barry Michael Starr
>starr at cfht.hawaii.edu
>Canada France Hawaii Telescope Ph: 808-885-3139
>PO BOX 1793 Kamuela, HI 96743 Fax:
>808-885-7288
>_____________________________________________________________________________
>- -- CCD-world -- --
>CCD-world is fully moderated. Send posts to CCD-world at astro.ku.dk
>Standard replies will go to the list; address personal replies manually.
>For more information, please go to: http://www.not.iac.es/CCD-world/
>
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