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AD9850 performance

Discussion in 'Electronic Design' started by Garrett Mace, Feb 13, 2004.

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  1. Garrett Mace

    Garrett Mace Guest

    I can find lots of projects designed around the AD9850, but haven't
    seen one that actually compared the output with that of a commercial
    function generator. Anybody know how well it stacks up against, say, a
    decent Agilent model? Only on the basis of signal quality...not the
    usability or special modes available on a "real" function generator.
    It would be nice to see pairs of sine, triangle, and square waves on
    the same scope, at several frequencies from about 50Hz to 50MHz. If
    you want to get really fancy, perhaps some spectrum analysis at the
    same frequencies.
     
  2. John Larkin

    John Larkin Guest

    But the AD9850 only makes sine waves.

    John
     
  3. What is visible on a scope may not be the obvious stuff.
    As already mentioned, it only does sinewaves.
    The frequency accuracy is dependent on your oscillator.
    The amplitude is not really adjustable.
    Well with one resistor ... meaning to get a decent generator
    you'll need a digital attenuator.
    The phase noise is not that bad, see page 6.


    Rene
     
  4. I never used the AD9850, but I do have experienced the AD9852. Basically the
    answer is : "The signal quality can be very good as long as your design is
    very good !". More specificaly the first thing is to design a very good
    output low pass filter. I mean fully shielded, filtered power supply, etc.
    On my first design I wanted to use both the sinus output and a square output
    (using an on-chip comparator that is in the 9852), and the result was more
    than bad : heavy high frequency noises on the sinus output, just because I
    was routing back my nicely filtered sinus output to a noisy digital
    environment. Then I suppresed this feature, and installed a clean low pass
    filter in a two-cans shielded box with proper power supply filtering, with
    the output amplifier in the second can of the same shield, and then I got a
    signal as clean as on the documentation provided by AD...

    Hope's that will be helpful,

    Friendy yours,
     
  5. Garrett Mace

    Garrett Mace Guest


    Not strictly true, it can also produce square waves. I got carried away and
    wasn't really looking for triangle waves in the product spec.

    And sine and square waves are 99% of most people need anyway. The use here
    is for a fast-ish function generator, 50 or 60 megahertz; sine and square
    are pretty much what hobbyists use most anyway.

    Anyway, if I ever need triangle waves I know how to make a halfway decent
    auto-centering square-to-triangle integrator with gain control, as long as I
    don't need more than 200khz or so.
     
  6. Garrett Mace

    Garrett Mace Guest


    Can do square waves too...

    For amplitude, I was going to use a variable gain amp anyway. Also want to
    add in some offset control.
     
  7. FRom memory the 9850 has a 10 bit DAC, while the 9852 is a 12 bit
    device, with corresponding improvement in spurs. Phase noise is nearly
    as good as the clock you use, with a penalty for using the PLL clock
    multiplier in the 9852/4
    I found the 9852/4 gave very good performance on a spectrum analyser for
    most frequencies. However spur amplitude can vary dramatically with even
    a small change in output frequency. This can be predicted though it is
    complex and I havent tried to do it. I doubt you would get useful
    information from a CRO unless there were serious problems - the output
    is a very clean looking sine wave. I was able to get 1 Hz to 70 MHz with
    the board at http://members.iinet.net.au/~richardh/VK6BRO.htm
    and the Atmel driver board.

    I am looking to do a board using the new 9951/9954 series - has anyone
    done this yet? I would be keen to get a colaborative group going as I
    dont have the time for these things nowadays! It should be possible to
    get 160 MHz out directly, with much lower current drain than the 9854,
    though the supply is 1.8V so I guess you would have to use a SMPS to get
    the power savings from a 12V supply. It is a 14 bit DAC, so spurs are
    better still. Anyone interested?

    Richard
     
  8. John Larkin

    John Larkin Guest

    OK, but you have to properly lowpass filter the DDS output and then
    square it up with a comparator... just like any other sinewave source.
    DDS chips don't make usable square waves all by themselves. The
    filter/comparator thing can be a little tricky over a wide frequency
    range, too.

    John
     
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