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ADC-Will this setup work ?

Discussion in 'Electronic Basics' started by brazingo, Nov 15, 2005.

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  1. brazingo

    brazingo Guest

    I have been building a data acquisition system to be connected to a
    pentium 4 system. But the problem is scanning rate of ADC (Analog to
    Digital) is about 3MHz. But I need a higher rate. Will connecting 2
    such ADCs in parallel and scanning them alternatively using my pc give
    me a better rate like 6MHz (3+3). Also can this be extended ?
    All opinions are welcome. And, thanks in advance.
    BZ
     
  2. Bob Masta

    Bob Masta Guest

    Ihave heard of this trick being used on high-speed digital scopes.
    Note that each ADC needs a sample/hold that is at least as fast
    as the overall desired sample rate would require. I don't think
    you should try using the PC to provide the interleaved scans at
    these rates; you'll need a dedicated clock with accurate phases.

    Just out of curiosity, how are you going to move the data to the
    PC at these rates? Or is this for some intermittent application
    like a scope that can ignore data between trace updates?

    Best regards,





    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  3. brazingo

    brazingo Guest

    Hi Bob,
    I was going to use a USB 2.0 interface to connect this ADC system
    to my computer - P4 2Ghz. I thought this should work fine given that
    6MHz is just a fraction of 2000MHz. I even planned to use assembly
    programing to get the clock cycles right with an optimum code.
    I'm not too intent on 6GHz though. Anything around 600Khz to 1MHz
    scan rate would do fine for me so that I can capture a sine wave of
    atleast 100Khz with 6 to 10 samples per cycle respectively. I just
    wanted to push this system to the limit and get 6GHz.
    I was really surprised by your question. Am I missing something here
    ?
    -BZ
     
  4. Bob Masta

    Bob Masta Guest

    I am not sure about your expertise in programming, so please
    forgive me if this seems condescending. However, in a modern
    protected mode OS (Windows XP, for example) you have essentially
    no control over timing issues. The OS can jump in at any time and
    run some other thread. When you start some operation, Windows
    may wait several msecs before actually doing anything about it.
    Some of these issues can be dealt with by Ring 0 drivers, but it
    would be a big can of worms unless you are already a pro at it.

    The normal way A/D systems work under protected mode is to
    buffer everything. You would collect a block of data to your own
    memory buffer, then signal Windows that it is full (while you
    switch to filling another buffer), and Windows will transfer the
    data whenever it feels like it. You have to insure that the rate
    of buffer filling doesn't exceed the average transfer rate.

    But it sounds like in your case you are making a scope, so
    you could get by with a single buffer. When the buffer is
    full (one scope-face sweep), you let Windows have it.
    The few msec of lag would be no problem, since your
    eyes don't need display updates all that fast anyway.
    Note, however, that this supposes you do all the triggering
    in hardware. If you want to do it in software, you have to
    pass Windows the entire data stream, which means you
    need to support sustained throughput. Or at least a big
    enough block of data that you can be sure it included at
    least one trigger event plus enough data for display.

    Best regards,



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  5. Jasen Betts

    Jasen Betts Guest

    it'll give a higher rate (there shouldn't be bus speed problems at that
    rate), but not neccessarily higher resolution.

    maybe have a look into ADCs designed for video capture
     
  6. Jasen Betts

    Jasen Betts Guest

    usb 2.0 is 480M bits per second, so it should work.


    Bye.
    Jasen
     
  7. brazingo

    brazingo Guest

    Hi Bob,
    I confess I'm not a pro at system programming. Just been
    learning pentium programming limited to only real mode and also
    restricted to DOS. I thought I could make myself a nice little scope.
    Guess I overestimated certain things. Thanks for the inputs though.
    I'll try working with the buffering concept.
     
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