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AD9970 and CCD in "waiting mode" - problem

Question asked by Al_Jumper on Nov 2, 2014
Latest reply on Nov 14, 2014 by Al_Jumper

Hello!

 

I use AD9970 in scientific CCD camera design. It works fine in "continuous" mode, when the camera captures images continuously frame by frame. But we need the camera to work also in "waiting mode". It means, that the CCD waits for the external trigger pulse for indefinite time, clocking the vertical registers without starting the exposition. After the trigger comes, exposition is started and the frame is read out and digitized in ordinary way. In this mode I have a problem with AD9970. Several (2-5) frames after the pause are darker than normal frames, captured in continuous mode. I suppose the reason is that CCD in "waiting mode" doesn’t output valid optical black (OB) pixels, because the charge is not transferred from photodiodes to V-registers. OB clamping loop inside AD9970 continues to operate, but it no longer matches  the real OB level. I tried to stop clocking HD, VD and both during "waiting mode". Also tried to use CLPOB masking (no idea how it can help, because length of “pause” is undefined). All that gave no result. Is there any way to “freeze” OB clamping loop in AD9970 to prevent it from clamping invalid OB pixels during pause?

 

Thanks in advance

Alex.

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