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Thread: Noise on lower end of DAC trace - can I fix this?

  1. #1
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    Default Noise on lower end of DAC trace - can I fix this?

    Hi,

    I'm building my own DAC controller - I put together a simple R-2R DAC driven by a 595 shift register.

    The output stage is through the same op-amp config used in norm's laser show DAC. (Parallel port DAC)

    I noticed that I was getting noise at the lower end of the cycle - this is +5 -> -5v.

    Does anyone know if R-2R DACs are prone to this, or is there something wrong? Either way, what would you recommend to try to fix it? It looks way too noisy to send to a galvo.

    Steve
    Attached Thumbnails Attached Thumbnails scope1.gif  


  2. #2
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    OK I'll reply to myself...


    I think I'm entering the "noise zone" so I'll apply a liberal sprinkling of caps to the power supply and see what happens...

  3. #3
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    the output amplifier have enought power supply to output +/-5v (supply rails @ 8v minimum for 5v output)? or there is an output load? what is the op amp reference?

  4. #4
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    I'm using +/-15v to power the op-amp stage - these are the JFET TL082 opamps as used in norm's laser dac - I don't have the diagram to hand, but it's been posted here before.

    I don't have a load on the output, running straight into my scope inputs (differential inputs, as the GND (center) is actually at +10v to my PC GND running the scope software)

  5. #5
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    ok this is perhaps before the op amp at the DAC output when the output fall near the ground but this is strange.
    do you have the wiring diagram to help?

  6. #6
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    Wow! - someone trying to do the same thing as me...

    I've been trying to put together a working laser harp for about 25 years

    I built a really simple one a couple of years with a frame around it, but now I'm trying to build a scanner-based frameless harp.

    The trace you see above is after the op-amp output stage which is what is shown in the attached diagram -

    I tried to use the same DAC chip but I could not get the Arduino to talk SPI properly - so I built an R-2R ladder and controlled it from a 595 shift reg.

    The output of the r-2r is a fairly clean 0-5v signal, with no distortion around 0v. So the noise you see on the scope is being added by the TL084 stage.

    I cleaned up the r-2r output with a bypass cap (0.01uF) but this lead to spikes in the final AC output. It did not help with the noise so I removed it.

    I reprogrammed the Arduino to output values in the 25-225 range and this avoided the noise area completely. I've lost resolution, but I'm going to open and close the harp with a digital potentiometer, so I don't think it matters. I might try scaling the final output back to -5 <-> +5v and see if the noise reappears.

    regarding your laser harp project - did you get a working reflection trigger? Mine are all very difficult to calibrate.

    Steve
    Attached Thumbnails Attached Thumbnails DAC0832.gif  


  7. #7
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    this is strange because tl084 is good op amplifier and in this wiring (invert add) there's theorically no noise


    about my laser harp yas i have a good sensor system and it's an open frame
    here is a video take last week end:
    http://www.dailymotion.com/relevance...rpe-laser-test

    and free laser show
    http://www.dailymotion.com/relevance...w-laser_events
    Last edited by Genesis; 11-15-2007 at 07:30.

  8. #8
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    Can you explain the sensor system? I have problems with the light reflecting from the ceiling of the room I am in. This interfers with the signal.

    What is the power of your laser?

    Steve

  9. #9
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    the laser is a 150mw dpss and the schematic need to be made under software to post it on the web but on paper you just need to follow my link "laser harp" at the bottom of this post.
    you have the schematic in picture on the first page

    about your problem when you have -5v at the output you need to have 0 volt at the output of the current to voltage converter. there is noise at this point?

  10. #10
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    If I understand you correctly then yes the DAC output is 0v and the TL082 output is -5v (approx).

    The DAC output is not very noisy, and seems to be OK over the whole range. So the noise is being introduced in the '82 section.

    Tonight I will plot the output of the first phase of the '82 and see if there is noise there.

    Laser Harp: I've been looking at the schematic for your sensor - can you explain what the last stage is for? It has unmarked components / output.

    Would an LM324 op-amp chip be OK for this circuit?

    (What kind of photodiode did you use?
    One sensitive to the laser frequency?)

    (Also what is the component '103' - a 10K resistor?)

    Steve

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