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Thread: what makes AGND "analog" ?

  1. #1

    what makes AGND "analog" ?

    Hello,

    I am looking at voltage regulators for a different project and I was trying to understand what exactly was AGND on my Teensy. Looking at the schematics it seems that there is only one power line and AGND is only separated from GND by a ferrite. Is that what makes it better for the ADC ?

    I know there are regulators with several LDO, can that be a solution to keep the analog and digital power rails separated ?

    Thanks !

  2. #2
    Senior Member+ Theremingenieur's Avatar
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    The ferrite has two functions. First, it shall remove digital noise from GND. Second, it is intended to be the one and single (to prevent ground loops) required DC reference connection point between GND and AGND.

    Power rails (as opposed to GND rails) and voltage regulation are a completely different topic and depend strongly from the project specifications and the planned peripheral circuits. There is no general rule.

  3. #3
    Crystal clear, thanks !

    As for a separate power rail for digital and analog, do you think it is worth it if I am using force sensitive resistors (hence low frequency) for my analog application ? I am not sure how I can calculate/anticipate the noise. Everything works fine on my Teensy which has only one power rail so I would be tempted to keep it simple but I have been told that having 2 power rails was good practice... As you said, no general rule.

    BTW what are the ferrite characteristics on the Teensy board ?

    Cheers

  4. #4
    Senior Member+ Theremingenieur's Avatar
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    If you "read" these FSRs at low rate, you might, if needed, still apply some low pass filtering in your code. I wouldn't overthink that for the moment and keep it simple.

    The ferrite is (IIRC) small, around 0.6uH and 0.08R. Just enough to reduce that ugly digital noise >= 1MHz. If you need more precise info than from my old brain, check with a LCR meter between GND and AGND on an unpowered Teensy with nothing connected...

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