looks like Q2 is a clamp reset to zero voltage reference ,i.e. the ground. However, the ccd signal is changing rapidly , for example, within 1us the ccd has a signal like ILX511B
so this clamp reset doesn't work for 1us ccd?
Looks to me like a "black level clamp" to set the DC level prior to each scan line. That make sense? The CCD data sheets may offer more clues.
--sp
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ccd has a signal like ILX511B
I don't know much about CCd's. Do you need to reset the cap before reading each pixel, or just before a whole scan? (I think that's what Speff said.)
George H.
has a signal like ILX511B
Once for the scan, if the time constant is long enough, or, for a multiline scan, once for each line.
The point is that a CCD has a pretty large and drifting DC baseline, compared to the video signal. The purpose is to charge a reference DC level into the cap during the interval between the scans and hope that it stays well enough to give good video out.
-- Tauno Voipio
has a signal like ILX511B
That looks like the makings of a chopper amplifier, used in cases where supper sensitivity is required. The CCD reference most likely will include the pulses, but at a rate faster than expect and thus can be filtered later down the line. Looking at C2, it looks like the integrator needed to remove this or reduced BW.
That's just a guess...
Jamie
has a signal like ILX511B
If the circuit is driven by an unbuffered CCD shift register then because the output impedance is very large, Q2 does effectively GND the amplifier input. It is not necessary to charge C1.
The time constant with the switch 'off' is 1 second (so it won't change much in 100 or 200msec). With the switch 'on' it is about 40usec (so it will reset pretty well in a few hundred usec).
Here's the original article, which mentions the part under discussion in passing as a "DC restore".
There are actually two "reset" switches-- one is Q2 and the other is inside the CCD chip. The latter switches during each pixel time between a reference voltage (perhaps several volts from ground) and the video voltage (offset from the reference voltage). Q2, in conjunction with the 100n capacitor, gets rid of the several volts offset mentioned above (with its attendant drift and effect on signal to noise in subsequent processing).
--sp
On a sunny day (Wed, 10 Oct 2012 20:41:09 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :
has a signal like ILX511B
constant the
Normally the horizontal clamps work for a few us at the start of each line. There is no need to completely discharge a full capacitor in one clamp action, it just slowly changes the charge so black is at the right level. You are just removing bits of charge every time the clamp is active. The whole reason for the clamp is LF restoration. After equilibrium your clamp only corrects the little charge imbalance (from DC) added by each scan line, and that is sufficient.
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