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Thread: ADC questions

  1. #1

    ADC questions

    Hello,

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    - does the AGND also need to be connected to the output of my audio applicance when using one of the DAC of the teensy

    ( or is it only a AGND for ADC input ports ? so my applicange output connector should be connected to GND instead? )

    - can that AGND be connected to the general GND of my applicance or better not ?

    I guess that the top left 47k cap is used to maintain a default input to 0V and the 10F at the top is to decouple DC (not sure)

    - it seems like 10k+2.2k create 0.595V as a midpoint ? so I have to set the VREF to internal 1.1V ?

    - so then what does 10F+47k (at the right) do ? it looks like some sort of filter

    thanks

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Usually you can use either GND or AGND. Sometimes AGND will be better, so best to use AGND unless you have some reason to use GND.

    Often there's no difference, so if you have some need for GND, using it is fine.

    But if you're using motors or PWM driving powerful LEDs or other high-power stuff, their power consumption can tend to interfere with your audio signal. AGND usually helps in those situations.

    - it seems like 10k+2.2k create 0.595V as a midpoint ? so I have to set the VREF to internal 1.1V ?
    Yes, that's correct, except the Teensy 3.x internal ref is 1.2V, not 1.1V. (some Arduino boards have 1.1V internal ref)

    If using the audio library, the internal 1.2V ref is used by default, which is why this circuit is recommended by the audio lib documentation.

    - so then what does 10F+47k (at the right) do ? it looks like some sort of filter
    The 10uF capacitor forms a low-pass filter with the 10K+2.2K resistors (effectively ~1.8K), so the 0.6V will have a more stable voltage. The corner frequency is ~8.8 Hz. If there's noise present on the 3.3V power line, it gets filtered away so the 0.6V is (hopefully) a low-noise DC voltage.

    Both 47K resistors act as load resistors for the signal. So whatever equipment you connect will see an AC input impedance of ~23.5K. Without a load resistor, the "floating" voltage would be undefined and the circuit could be very susceptible to pick up hum or other interference.

  3. #3
    awesome explanation :-)

    I was intriguied that the output (DAC) counterpart had no particular recommendation

    so if i add to it a decoupling 10F and use the AGND, is it enough ?

  4. #4
    no need to add a 47k for the output ?

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