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LM9822 won't play

Discussion in 'Electronic Components' started by Lars, Sep 23, 2008.

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  1. Lars

    Lars Guest

    Hi all!

    We are trying to use the LM9822CCWM Analog Front End to digitize the
    output from a linear colour CCD but with poor result. Initially the
    device did not output any digital data att all (all zeros). After
    limiting the analog input voltage, (the reset puses from the CCD
    exceeded the 5.3 V max input voltage), things looked a bit better, but
    still all data bits after some time begin to exhibit transitions on
    both edges of the clock and none resemble the expected data, going
    gradually after several minutes running to no data output at all (all
    zeros). It is not a thermal phenomena since a very breif power-off/
    power-on restarts the behaviour from scratch.

    My best clue after we got the analog voltage in hand was the we had a
    clock reflection on MCLK causing false clocking on the wrong edge, but
    that was not the case. Careful termination of MCLK had no effect.

    The next clue is that the device is extreamly sensitive to over- and
    undershots (deduced from the fact that behaviour changed when analog
    input was limited). I do not have any simple way to verify this, short
    of adding a buffer stage close to the device. We drive the clock
    signals from a small FPGA (XC3S50AN) and the traces to the device are
    quite long (approx 10 inches) due to mechanical limitations in the
    application. I have limited the drive to minimum (2mA slow LVTTL
    drivers, have also tested CMOS) and we are running at a fairly low
    clock speed of 4MHz MCLK in 6 bytes per pixel colour mode, which is
    the default setting, but there is moderate over- and undershoot on all
    signals.

    Va is 5V and Vd is 3.3V. Both are regulated by LDO's in series from
    the same 12V source so they ought to ramp up in sync (verified on
    scope).

    Anyone out there with a clue as to what is going on? Did we break the
    device when we had too large analog input or are there any quirks with
    this device that we are missing in the data sheet? We have exactly
    the same behaviour on the two different individualls we have tested so
    far.

    Regards, /Lars

    P.S. Remove the obvious from my email address in case you want to
    email me directly. D.S.
     
  2. neon

    neon

    1,325
    0
    Oct 21, 2006
    it is a temperature problem. turn it on wait for the data to colapse then repeat with cooling see if it exentds the time.
     
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