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Common mode chokes and grounding

Discussion in 'General Electronics' started by AJ, Aug 18, 2005.

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  1. AJ

    AJ Guest

    Hi,

    I am looking for opinions on grounding techniques, particularly in
    automotive circuits. I have designed a circuit that uses both digital and
    analogue circuits and I was planning to connect these grounds at a single
    point to attempt to keep digital noise away form the analogue section. I am
    also using a switch mode power supply operating at 300KHz so I was thinking
    of using a common mode choke on the input to help EMC. I was wondering the
    best way to connect this, I am a bit confused......

    I connect positive and ground to one side of the choke so on the other side
    I will have a filtered positive and a filtered ground. Does this then give
    me a 3rd ground? I was thinking about connecting the unfiltered ground to
    the Chassis and running a ring around the outside of my PCB on both layers.
    I was then going to connect the digital ground the filtered output after the
    choke and connect the analogue ground like described above Some circuits I
    have seen appear to have the ground connected together before and after the
    choke so I cant see what the pint is of filtering the ground in the first
    place in that example..

    I hope I have been clear on what it is that I am trying to do and really
    appreciate any feedback

    Best Regards

    AJ
     
  2. Harold Ryan

    Harold Ryan Guest

    Hi AJ:

    Try to use star grounding where all the grounds connect together at one
    point. Usually the AD converter is the grounding point because they are not
    isolated like in a PC. Make sure only one ground and power trace connect to
    this point. This is also where the power supply connects. Also connect the
    filter caps like 0.01uf to each of the electronic sections. The trick is to
    separate the currents from one section from another. As for the switcher,
    its impossible to remove the ringing from the analog circuits due to RF from
    the wires near by. Always use a liner for analog if possible. The choke
    should be used to separate the digital and switcher from the analog
    circuits. The switcher and the digital circuitry are both noisy.

    The ring around the board only causes trouble like an AM antenna. The
    protection you want is to protect the board from RF as it passes thru the
    board, not from the edges. In other words, add a ground plane and a Vcc
    power plane with traces on the other 2 layers. If you are making a 2 layer
    board, create a ground plane with signal traces on the other side. Its ok
    to run a few traces on the ground side but you need a ground plane.

    Harold
     
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