AGND and DGND seperation using star grounding connection???

Discussion in 'The Projects Forum' started by vivek20055, Feb 12, 2013.

  1. vivek20055

    Thread Starter Member

    Nov 4, 2012
    88
    0
    Hi,

    I need to use star grounding connection for seperation of DGND and AGND???

    Can anyone suggest me how to implement this using PADS software????





    Regards
    Vivek Alaparthi
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Very carefully... and manually. Don't expect an autorouter to get this correct.

    Netlist driven design rules tend not to work for star grounds, as any and all ground points are considered the same and thus interconnectable.

    One trick you can use is (that may even work with an autorouter) create some phantom component with two tiny SMD pads (mimic a zero ohm resistor) in series with each ground domain's connection to power ground. That makes each domain on a different net and allows you to control the connection point, and as the very last final board layout step delete the phantom component in both he schematic and the board.

    The deletion step may be skipped if PADS allows you to short the pads together in the phantom component.
     
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  3. vivek20055

    Thread Starter Member

    Nov 4, 2012
    88
    0
    Hi,

    I am designing a 4 layer pcb board.

    I am using analog ground(AGND) and digital gground(DGDN) seperately.

    I need to implement a star connection to the common GND separation???

    Can anyone suggest anything about this??



    Regards,
    Vivek
     
  4. vivek20055

    Thread Starter Member

    Nov 4, 2012
    88
    0
    Hi,


    Thanks for your reply.


    I am using seperate copper areas for both AGND and DGND. But I have analog components so spread in my design. They are mixed with digital components.

    what is the need of using star grounding(need of connecting analog and digital components)?

    If I use star grounding technique. Then can I GND the analog components in the copper plane itself.

    Regards
    vivek
     
  5. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    All digital switching is fast, and that produces current pulses in the ground trace. The ground always has some tiny bit of resistance and that causes a voltage when the current passes through. These little bumps in voltage get into the analog signal and make it noisy.
     
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  6. vivek20055

    Thread Starter Member

    Nov 4, 2012
    88
    0
    Hi,

    Thanks for your reply

    Then seperate AGND and DGND copper planes are enough. Right?(voltage fluctuations will not enter into AGND from DGND)

    what is the need of star connection????(connecting AGND and DGND)??


    regards
    Vivek
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Well, you've designed yourself into a corner by mixing A and D circuitry. If they were separated you'd only need one ground plane.

    The term "star" comes from the topology of the connections: there is one single point where all grounds join such that current from one domain returns directly to the power ground and doesn't transit thru another domain, ie, the digital currents do not flow under the analog parts but just back to power ground.
     
  8. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    And the parasitic inductance is even worse than the resistance, since the amplitude of the induced voltage depends on how fast the current is changing, not its amplitude.
     
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