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Thread: Filter for on-board Teensy 3.6 DACs

  1. #1
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    Filter for on-board Teensy 3.6 DACs

    Hi all,


    Does anyone have any advice/suggestions on suitable filters for the on-board DAC?

    Butterworth/bessel/sallen-key etc

    I'm drawing up a board to help me play with the Teensy and a filter on the audio sounds like a good idea.


    James

  2. #2
    Senior Member MickMad's Avatar
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    I suppose you are talking about hardware filters.

    Anyway, it depends on the kind of filter you want to implement. Is it a low pass only? And I would suggest using a bandpass filter on the outputs, to cut away spurious low-frequency 1/f noise and every sampling frequency harmonics. In an ideal world you would cut above 22.05 KHz but in reality you might want to start cutting around 15-18 KHz.

    I've been looking into filters like these in the last weeks and in my opinion a good topology for an output filter would be a 2nd order high pass to remove the low freq noise, and a 6th order lowpass (either sallen key or multiple feedback topology).

    This could be done with a quad opamp IC "easily".

  3. #3
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    Thanks @MickMad

    That all sounds about right.

    Simplicity is reasonably important to me — so being able to do it with a quad opamp would be good.

  4. #4
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    A few thoughts and suggestions.

    You can get off-the-shelf switched capacitor based filters that are more or less a single chip solution with a few setup pins. I wonder if that might work for your application.

    You mention audio- What is the source signal's bandwidth? A synth? I wouldn't rely on the internal DAC for that unless you purposely want 8-bit or old school ensoniq sampler type sounds.

    If it's wav or similar audio you just need some low pass. I doubt it would have to be anything exotic or expensive. 12dB/oct from one op amp should do the trick.

    If you ARE building a synth, why not incorporate a proper synth diode ladder filter or whatnot and use PWM from the teensy to create the control voltages?

    What is your application?

    Jamie

  5. #5
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    Quote Originally Posted by imaradiostar View Post
    A few thoughts and suggestions.

    You can get off-the-shelf switched capacitor based filters that are more or less a single chip solution with a few setup pins. I wonder if that might work for your application.

    You mention audio- What is the source signal's bandwidth? A synth? I wouldn't rely on the internal DAC for that unless you purposely want 8-bit or old school ensoniq sampler type sounds.

    If it's wav or similar audio you just need some low pass. I doubt it would have to be anything exotic or expensive. 12dB/oct from one op amp should do the trick.

    If you ARE building a synth, why not incorporate a proper synth diode ladder filter or whatnot and use PWM from the teensy to create the control voltages?

    What is your application?

    Jamie
    WAV playback but synth stuff would be useful too.

    I don't mind slightly lo-fi.


    James

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