Quick decision between split ground pane or a solid plane

Discussion in 'General Electronics Chat' started by chimera, Feb 23, 2012.

  1. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    Hello. I am designing a small pcb which contains a op-amp mosfet, a zener diode, micro-controller and a few passive components(no inductor).

    The micro-controller is being used for ADC and the pcb is through hole design

    My question is :

    Should i keep keep on ground plane for both digital and analog?

    OR

    Should i be using split grounds and some how make them meet at the power return terminal?

    1- I have never done a split ground plane--so might need guidance on that

    2- I am using internal RC oscillator on the micro, so no external crystal

    Thank you for the input!! :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    Arriving at the answer is not that simple. It depends on the frequency component of your signal and the resolution you are trying to achieve. In most cases, it is best to go with a split ground. But it also depends on the MCU. Does the MCU have separate AGND from DGND? If yes, go with a split GND. If no, go with a solid ground plane.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,977
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    Are there signals going between the analog and digital? If so you can use a split plane with a small connection between them somewhere on the board. I've know some designers who tie them together with a short ferrite bead connection to reduce high frequency noise transfer between the two grounds.
     
  4. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    Well when you 'depends on frequency', what frequency are you refering to.
    If it the ADC frequency, then its samplying at 16 Fosc, where Fosc is 8Mhz. The mcu does not have seperate grounds for analog and digital--it has one ground pin.


    I dont entirely follow what you mean when you say:

    Are there signals going between the analog and digital?

    Can you please explain a little bit.



    thanks
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
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    You do want the two grounds to be at the same voltage, but you don't want there to be any circulating currents in the analog side. So you divide the ground plane into two areas, joined by a narrow neck or even an added component, like the ferrite bead. Then you make sure that all the sensitive components are on the analog side, with any digital lines being kept as short as possible when they're on analog territory, and if you're really careful you try to avoid crossing any analog signals. You also make sure that you aren't creating islands in the ground plane. In general digital component are less fussy and more immune to noise, so you don't have to take great precautions there, though there are still design rules to follow.

    You might even want three ground planes, if there are high-current devices like motor controllers--so you'd have power, digital and analog.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    8 or 16 Mhz is far too slow for fancy grounding requirements, mainly keep high current tracks away from low current ones and do not combine such returns.
    Shunt each electrolytic by each chip (you do have them?) with a small ceramic as well.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    By that I mean are there any signal lines that go between the analog and digital sides of the board (?) or do the analog and digital side have completely separate inputs and outputs.
     
  8. chimera

    Thread Starter Member

    Oct 21, 2010
    122
    2
    okay..this might help explain what layout I am after. Please see attached .png

    You will see that digital is on the right and analog on the left. So bases on this layout, does everyone feel i should have two ground planes or just one.

    If its just one, i'll have it at the bottom of my pcb and connect that plane to the return of my power supply.

    If the consensus suggests that I have two ground (both on the bottom of pcb) then how much distance should i have betweeen them and in my mind I end up connecting both of them at the return on my power supply.


    :)
     
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