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Thread: Analog Ground and Multiplexers / separating ground planes on PCB

  1. #1

    Analog Ground and Multiplexers / separating ground planes on PCB

    Hi there!

    I'm designing a PCB with 39 digital and 46 analog inputs, 11 multiplexers (M74HC4851) a few more inputs and mosfet-switches and connectors for other devices. The board has a socket for a teensy 3.6 which is used as Midi USB Controller and controls the mosfets. The multiplexers are decoupled by one 0.1uF capacitor per multiplexer close to the VCC pin.

    1. As far as I understand it is advisable to connect the analog inputs to analog GND on the teensy and the rest of the board to GND to minimize noise. What about the multiplexers? I could connect their GND pin to Teensy GND instead of AGND but this would lead to longer signal traces.

    2. Since AGND and GND are connected within the teensy board it is better to not connect them externally to avoid ground loops. True?

    3. I'm designing a 4 layer board where the first innner layer is reserved for ground planes. Since the layout of the board is mainly determined by the layout of the user interface it's kind of hard to properly divide separate the AGND plane from the DGND plane. This is what in my view would be possible:

    Click image for larger version. 

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    Is it ok like this or would it be better to go for a single ground plane for all inputs?

    Thank you in advance!

  2. #2
    Senior Member+ Theremingenieur's Avatar
    Join Date
    Feb 2014
    Colmar, France
    1. The multiplexer's GND is the reference potential for the digital control. Thus, it has to go to the digital ground.
    2. Yes, you are most probably right.
    3. The central ground plane of your layout should be DGND only. You are not forced to use the Teensy's AGND pin signal, though. You might create as many local AGND planes/pours/islands on other layers as you wish, connecting them to DGND through a similar ferrite as on the Teensy 3.6 original board.

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