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Thread: Trying to understand this...

  1. #1
    Join Date
    Aug 2007
    Location
    Indianapolis, IN
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    196

    Default Trying to understand this...

    Hi,

    I'm trying to understand the following glavo controller circuit:
    http://elm-chan.org/works/vlp/ldpctl2.png

    I see that a 12-bit DAC is being used to create the output voltages, but why are they then passed through an op-amp (TA75072) stage?

    Also is the following multiplexer (TC74HC4053) only to provide "emergency off"?

    What does the LM358 section do? It's tied to the inverting input of the TA75072, but I'm not sure why.

    Finally, why does the section with the Schmitt triggers below this not seem to be connected to anything?

    Any help would be greatly appreciated...

    Thanks,
    Steve

  2. #2

    Default

    Not a coding question. And that URL links through to an unrelated site if you're hiding the referrer header. (If unhiding that won't work, go to http://elm-chan.org/works/vlp/report_e.html and click 'Circuit diagram' for 'Controller Board 2' at page end)



    Ok, the op-amps are likely filters, to remove the shock of a point-by-point output's effects on the scanners. They appear to be unity gain so the important thing is what those capacitors are doing after the resistors on the + input.

    The LM358 is a unity gain buffer to stabilise a well-filtered reference voltage made by U1. This is to offset the 0 to 5V from the DAC to make it -2.5, to +2.5. The diagram shows output from -2 to +2 which bears that out. I'm not sure about ILDA but I thought that wants -5 to +5, and if so it would be worth changing the resistors on both channels of U3 so it has a gain of 2. If you did this, take care that you get rail-to-rail output voltage changes.

    The multiplexer seems to be as you say, probably chosen for speed. HC logic is much faster than most op-amps.

    The Schmitt trigger thing looks like an oscillator built on the digital ground to generate a negative supply for the analog stuff. Note that the supply for the galvo amps on the supply connector doesn't route either + or - to the board, so that has to get its - from somewhere else. The + for the input source to that oscillator comes from the inverter IC's 5V supply input.
    Last edited by The_Doctor; 10-16-2007 at 06:56.

  3. #3
    Join Date
    Aug 2007
    Location
    Indianapolis, IN
    Posts
    196

    Default

    OK - thanks for fixing my link, and 'duly noted' about the topic - I thought this was controllers in general, not just the programming side

    (programming I can deal with)

    Steve

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