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Thread: DAC Voltage Suddenly Unstable

  1. #1
    Junior Member
    Join Date
    Oct 2016
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    5

    DAC Voltage Suddenly Unstable

    I am sending voltage directly from the DAC on the Teensy to the Control Voltage Input on a analog voltage control oscillator.

    The voltage was stable for months yet now the output voltage seems to be unstable.

    Hardware: Teensy LC and Teensy 3.6

    Circuit:

    Teensy Synth

    Ground ---> Ground
    DAC Pin ----> CV Input

    Issue occurs on both

    Issue occurs with my original code along with the following simplified code.

    I guess this instability could have always been there and I simply did not notice til now, but it seems to be new.

    I will edit the post and post a picture of the signal on my oscilloscope as soon as I have it.

    Is it expected that I send the DAC output through an opamp?

    Or would using an opamp as a buffer simply output a buffered voltage with the same issue?

    Code:
    void setup() {
    analogWriteResolution(12);
    }
    
    void loop() {
      for (int i =0; i < 4; i++){
    
       float dacU = i*1279.6875;
       dacU = round(dacU);
    analogWrite(A21, dacU);
    delay(2000);
    }}
    The voltage fluctuates noticeably at all times.

    Thanks in advance

  2. #2
    Junior Member
    Join Date
    Oct 2016
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    5
    While taking a picture of the signal on an oscilloscope I realized that reading the signal without the ground connected to the scope looked the same as my issue....

    The problem was fixed by connecting ground.

    I was using my first ever self designed and printed PCB, looks like I have some grounding issues to fix.
    I would delete this if I could, but I guess is good for others to see that people make stupid mistakes....

    Use a multimeter FIRST before spending hours checking other things....

    I am shocked that the voltage was able to effect the synthesizer and oscilloscope so much without a ground connected??!!

  3. #3
    Senior Member fdaniels's Avatar
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    Oct 2020
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    Ostwestfalen, Germany
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    While you are fixing the pcb its a good idea to at least buffer the DACs output with a 740x adding a 68R at the output of the OpAmp. Its standard for Eurorack that every device protects itself as you cant be sure the other side is nice to your module....

  4. #4
    Senior Member fdaniels's Avatar
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    Oct 2020
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    Ostwestfalen, Germany
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    While you are fixing the pcb its a good idea to at least buffer the DACs output with a 740x adding a 68R at the output of the OpAmp. Its standard for Eurorack that every device protects itself as you cant be sure the other side is nice to your module....

    I am shocked that the voltage was able to effect the synthesizer and oscilloscope so much without a ground connected??!!

    Its "grounded" via your Teensys USB connection

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