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what does the Q2 do in this CCD preamplifier?

Discussion in 'Electronic Design' started by [email protected], Oct 11, 2012.

  1. Guest

  2. speff

    speff Guest

    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
     
  3. 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.
     
  4. Tauno Voipio

    Tauno Voipio Guest


    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.
     
  5. Jamie

    Jamie Guest

    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
     
  6. Guest

    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.
     
  7. speff

    speff Guest

    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".

    http://instrumentation.obs.carnegiescience.edu/ccd/gcam/preamp.html

    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
     
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