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Thread: Hello everyone.

  1. #11
    Join Date
    Aug 2005
    Location
    Shiga, Japan / California, USA
    Posts
    50

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    hi Groover - I was just checking out your homepage, looks like you're doing some cool stuff over there!

    Welcome aboard and happy new year-
    -Josh

    p.s. Thanks for the link (photondream)!

  2. #12
    Join Date
    Nov 2005
    Location
    Sweden
    Posts
    133

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    Thanks josh.

    yaddatrance:

    Maybe you can help me with a problem. I get a terrible interference so I traced it back with the scope and finally found the source. It's coming from the DAC itself. Most irritating and I can’t figure out why I get it at the DAC output. I first thought it might be dataerror but it's off sync with the data transfer.

    I'm all out of ideas here.

    For clarification, this is the schedule:
    http://www.slackersdelight.com/laser/images/dac2.jpg
    You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads!

  3. #13
    Join Date
    Jun 2005
    Location
    SoCal
    Posts
    508

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    Hey Groover, great to hear from you...

    Assuming you are testing your TI chip after disconnecting the
    opamps completely...

    First thing I would do is buy 2 - 10uF tantalum capacistors
    and attach them as close to your DAC as possible (one between
    Vss and ground and the second between Vdd and ground)...
    Your schematic only shows the caps on the power supply itself...
    the "decoupling" caps on a DAC, especially a high-power DAC
    like yours isn't optional... This may in fact fix your problem
    completely... The "tantalum" part is important due to the ESR
    and low leakage. If you don't have one handy, throw an electrolytic
    one in, to see if it helps a bit...

    Another problem I see is that you have tied your analog ground to
    your digtal ground... Pins 5 & 6 should not be directly tied together,
    put a resistor between it... The digital ground should run to the parallel
    port, the analog ground should be run to the opamps...

    The second problem you may run into is possibly "railing" your voltages
    on the DAC itself... Your Vss is max -7V, but if you feed it from a 7905
    with -5V, then you'll see performance degradation or undefined behavior
    near the rails...


    You should follow the data sheet as shown above when running it bipolar
    as you seem to be doing...

    The way I personally like to do it is make the DAC run in unipolar
    configuration and then use opamps to correct the p-p voltage and offset
    before feeding to the inverting and unity gain opamps for the differential
    output.

    Hope some of this helps... I'm running out to Vegas to do a show, but
    I'll be back on the 2nd or so to see any replies/questions/etc...

    P.S. scope the ground lines on your PC parallel port itself... You may
    want an RC circuit between the two to filter noise... the crap noise may
    be coming from your PC and exhibiting as a ground loop. If you feel
    like doing it the correct way, use optoisolators on the inputs from
    the PC.

  4. #14
    Join Date
    Nov 2005
    Location
    Sweden
    Posts
    133

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    Your work is just not of this world. I'm very impressed. After I had a look at your 8 bit dacs I thought I might do a quick n dirty dac but still a little better than the first one.

    Just remember I am a amateur

    http://www.slackersdelight.com/laser/images/dac3.jpg

    I know the values for the amplifying op are a bit low but I plan to use what I can get a hold of on a short notice.
    You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads!

  5. #15
    Join Date
    Jun 2005
    Location
    SoCal
    Posts
    508

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    Thanks! At this analog stuff I'm quite new... I'm still learning as I go,
    and I tend to find things out the hard way Your first circuit is far
    more impressive than my first shot out the door, which used DAC0800s.

    Heh... here's a trick on the resistors... I just bought a metric assload of
    milspec precision 10K resistors, and when I need 2*R, I just use two
    of them in series

    That way I don't have to measure and do lots of work...

    Oh and it looks like you're doing an adjust to bump it to +/-10V (20Vp-p),
    which is not the differential spec. Differential is just 10Vp-p...
    A long time ago, we laserists ran differential at 20Vp-p, but we ran
    into big problems with the opamps railing, so we cut it down to
    a reasonable value... A side effect of the new standard was that it also
    kept the multiplying resistors very sane.

    Another tip (since 8-bit dacs aren't accurate) is to put in a voltage bridge
    onto the reference line between +5V and ground with a few caps on
    there to smooth it out. That way you can tune the projected images down
    without losing resolution. Keeping any pots and noisy passive components
    out of the analog AC side makes a huge difference in clarity.

    Oh yeah, you can use the ILE to turn off the inputs on the DACs when you
    want to start adding color and other channels too btw.

  6. #16
    Join Date
    Nov 2005
    Location
    Sweden
    Posts
    133

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    Thanks for the tip. I have a bad habit of leaving out things on my schematics. I've picked up some tantalum capacistors that are going in there too as decoupling.
    You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads!

  7. #17
    Join Date
    Nov 2005
    Location
    Sweden
    Posts
    133

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    Made another schematic and thisone is the actual schematic I'm using right now. Just got it working.

    http://www.slackersdelight.com/laser/images/dac4.jpg

    Well, it's time for bed. 7:30 AM here.
    You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads!

  8. #18
    Join Date
    Jun 2005
    Location
    SoCal
    Posts
    508

    Default

    Beautiful!

    Optoisolation is definitely my more favorite fixes to problems
    It always works!

    You're missing the current limiting resistors off the top 3 optos, but seeing
    as you used in on the rest, I'm sure you already know...

    But great design!

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