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Multilayer PCB - Shielding - Which supply on each layer?

Discussion in 'Electronic Design' started by Bill, Jul 29, 2009.

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  1. Bill

    Bill Guest

    Hi,

    I need to design a 4-layer PCB for an analog circuitry that measures
    EEG (10 uV to 1000 uV peak to peak, 0.5 Hz to 50 Hz).

    My supplies are:
    +5 V (for analog)
    +3.3 V (for digital)
    0 V (for analog and digital)
    -5 V (for analog)

    I know about (and have experience with) star topologies, to minimize
    contamination from one section to others. That doesn't worry me.

    I'm concerned about SHIELDING. I don't have much experience working
    with uV signals, so I need to pay attention to what supplies/signals I
    place on each layer.

    I'm also not concerned about the +3.3 V supply, since it doesn't need
    to power any IC close to the analog input.

    I'm concerned mainly about +5 V, 0 V and -5 V. Where would you place
    them, in the 4-layer PCB?

    I was thinking of something like this:
    Layer1 : 0 V plane + soldering pads for SMD parts.
    Layer2 : +5 V and -5 V planes, sharing the total surface.
    Layer3 : Signal tracks.
    Layer4 : 0 V plane.

    What do you think?

    Thank you very much.
    Bill
     

  2. Make a bluetooth device for each sensor that sends the digitized value
    to a receiver. Already having been quantified at the transducers removes
    all error. Bonus: No need for shields, and small circuit size,at least
    for the sensor senders. Also, you would incorporate a handshake in each
    that would keep another nearby unit from passing false data to your "data
    logger".

    Have an inductive charging circuit for the battery device that powers
    each "sensor xmitter". They would all charge overnight and after each
    use while in the storage drawer of the receiver cart.

    Sell tens of thousands of units.

    Have a nice day...
     
  3. Bill

    Bill Guest

    Hi again,

    Please read the last paragraph of the first article ("Input filter
    prevents...") in this pdf:
    http://www.bmed.mcgill.ca/reklab/ma...r_prevents_in-amp_RF-rectification_errors.pdf

    It says: "You should build the RFI filter using a pc board with a
    ground plane on both sides."

    As I said, I don't have much experience with shielding uV, but that
    made some sense to me. Intuitively, I think I would prefer having more
    capacitive pickup, but it happenning to ground planes, where it
    doesn't do any harm, than having less pickup, but it happenning to
    signal tracks. Doesn't it make sense?

    I now debugging will be more difficult (because the signal tracks will
    be buried), but reducing interference is a higher priority for me.

    I also understand Bill's comment (about a track right next to a
    conductive plane radiating less at far distances --by image theory),
    but I'm not sure that the benefit from that[*] will be higher than the
    benefit from two ground planes at layers 1 and 4.

    [*] Actually, from the reciprocal of that, since I'm not concerned
    about radiating, but about picking up noise. By reciprocity, an
    antenna that radiates less at infinity also picks up less from
    infinity.

    So... I don't know what to do. Are you guys convinced of what you say?
    Can you convince me, too? :-(


    Thank you,
    Bill
     

  4. Sorry, but I'll bet that examination rooms are already RF rich, and I'll
    also bet that there is no influence on low impedance transducers at all,
    particularly when the value gets converted right at the source. From
    then on, the data that gets sent is digital, and 100% error free. We can
    sense picoamp fluctuations in a PMT to see a missile launch 80 miles
    behind an aircraft without error introduction from local RF, I think we
    can handle this.

    So, the transducer and ADC would be right there local to the sensor (ie
    no interference whatsoever), and the signal sent over the air is the
    quantisized values from sampling to sampling and are digital, so they
    cannot carry error either. Absoluetely a better solution. No more need
    for wired patch sensors. Far better, and it IS or WILL BE the future.
    Why not have it be him that goes wireless with body sensors?

    Transparent Aluminum, anyone?
     
  5. Why would one ever want sensitive signals taking long paths? Nowadays
    converting a signal to digital is cheap, even when 8 or 16 channels are
    involved.

    Hell, a simple (or complex) data logger would be an excellent PCB layout
    to examine.
     
  6. Bill

    Bill Guest

    No one wants sensitive signals taking long paths, but sometimes you
    need to amplify before A-to-D conversion is possible. And sometimes,
    you need to amplify so much, that you need to break that gain into
    several stages, not to have output-to-input feedback. That's how you
    end up needing shielding.
    It depends on the voltage levels and frequency bands it deals with. My
    application is not RF, but it deals with uV, so I'd rather learn from
    RF PCBs than from PCBs of data loggers.
     
  7. Bill

    Bill Guest

    I also think that is the future, and there are several companies that
    start offering wireless electrodes. I don't know how they do the
    strong filtering that is needed at the input to avoid saturation of
    the first stage, due to (self) HF interference.
     
  8. Bill

    Bill Guest

    Hey, thanks for bringing this up. I never thought of it.
    I'll hope thermal changes won't be as fast as 0.5 Hz.
     
  9. When testing your circuit and closely inspecting something on the PCB,
    don't be surprised, if the voltages fluctuate with your breathing
    every few seconds :). Any transparent semiconductor package can cause
    problems due to ambient light variations.

    Paul
     
  10. Artemus

    Artemus Guest

    Any transparent semiconductor package can cause
    Even hermetically sealed packages can be a problem. Way back
    when I was a test engr for a semi manufacturer I discovered that the
    TO-99 packages we were using leaked light up the frit seal around
    the legs. This wreaked havoc with Vos and Ib on OA's and IA's.
    Art
     
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