DC Offset

Discussion in 'The Projects Forum' started by CathalGriffin, Jan 26, 2014.

  1. CathalGriffin

    Thread Starter New Member

    Jan 26, 2014
    2
    0
    Hi All,

    I'm looking for some help eliminating noise from a DAC (Digital to analog converter). The unit is being used in a cinema to decode audio for blind people. One channel of audio is coming from this device and the second channel is coming from our audio processor.

    When they are both connected, I am getting noise in one channel and attenuation in the next. I can stop the noise by powering the DAC by battery (5v) and grounding the DC- to the sound processor chassis. I think this means that I have a DC offset problem? Using batteries isn't a long term solution, just a means to determine what the problem is. The power supply that is supplied is an SMPS.

    Any suggestions?

    Thanks,
    Cathal
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    Try replacing the SMPS with a linear power supply.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    Did you try grounding the negative terminal of the SMPS -DC output to the sound processor chassis, the same as you did the battery?
     
  4. CathalGriffin

    Thread Starter New Member

    Jan 26, 2014
    2
    0
    Hey Folks,

    Thanks for the input so far.

    MrChips, I got myself a linear PS and am getting the same thing.

    crutschow, I have grounded the DC- of both my SMPS and Linear PS and this stops the DAC from operating.

    Any other suggestions?

    Thanks again!
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,786
    Words are insufficient to convey enough information to evaluate the situation. A schematic would be a big help. The more detailed the better.

    By using the battery you have identified path by which the "noise" is entering the system. It is not inherent to the power supply topology (linear vs. SMPS). My thought would be conducted noise from the AC mains which you can try to eliminate with a common mode choke or possibly a ground loop in the cabling that is acting as an antenna.

    Since you have a DAC I'm going to assume that you have both a Digital Ground and an Analog Ground. The proper way to do a layout is to connect the two grounds at a single point as close to the power supply and Earth ground as is physically possible. Please tell me you have a PCB and this is not a breadboard.
     
    Last edited: Feb 4, 2014
  6. TANDBERGEREN

    Member

    Jan 20, 2014
    74
    4
    It seems as if the noise is generated by the power for the DAC.
    As it is functioning without noise when using battery to power it, You tell us it works fine.
    Please supply us with the schematics for the dac and its powersupply.

    Also beware of signalground and powerground.
    Don't mix them, anywhere outside the DAC.
    Grounding is a serious buisniss, and there is always a better way.
     
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