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Help with basic design

Discussion in 'Electronic Basics' started by Robin Cull, Feb 3, 2004.

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  1. Robin Cull

    Robin Cull Guest

    Hi all,

    I'm pretty new to this electronics game having come from a software
    background. I am trying to design a basic interface board between the
    Analog Devices ADXL202E accelerometer and the LabJack U12 USB DAQ.

    I have a basic design sketched out on paper but I would like some help
    with some of the details.

    Firstly, the ADXL202E is an 8-pin LCC surface mount component
    (5mm*5mm). I have soldered surface mount before but it's a pain and
    as I only have two parts (and they're expensive to replace) I don't
    want to solder them directly unless I have to. Also using SMD makes
    the use of other through-hole components more difficult (two-layer
    board). I have been searching, so far in vain, for a socket type
    adaptor that will clamp onto the LCC package and have through-hole
    pins. I've seen these things for larger (more pins) parts but not one
    this small. Any tips?

    Second problem is capacitor choice. The ADXL202E analogue output is
    filtered by a capacitor. I've figured out that I need to use 0.47uF
    filtering capacitors (this will yield a bandwidth of 12.5Hz so I can
    sample on the DAQ at 25Hz). I am guessing that as these are being
    used for timing purposes they need to be fairly precise so normal
    electrolytics of +/-20% won't do. What kind of capacitors should I be
    using for this purpose?

    Thirdly, working on the assumption that I may not be able to get an
    adaptor for the LCC package I've created a part in Eagle so I can make
    a schematic. I'm having a little trouble with the layout. Initially
    I created a symbol as a rectangle with 8 pin-outs on the right hand
    side but doing it this way makes it really hard to layout the
    schematic without making a real birds nest out of it. Any
    recommendations on how I should tackle this?

    Finally, and this question is more specific to the ADXL202E and the
    Labjack, I plan to use the +5V and common 0V lines from the Labjack to
    power the board, the part maxes out at 5.25V and I've placed a 100Ohm
    resistor in series with Vdd to smooth the power. I assume this is ok,
    looks fine from the data sheet. The signals for the analogue sampling
    are coming in series after the filter capacitors. I think I've got to
    go to ground after this but I am not sure how. Also the datasheet for
    the ADXL202E mentions that it maybe necessary to op-amp buffer these
    signals to get sufficient power out of them to read, I am not sure
    whether I need to do this or not. Could anybody clarify these points
    for me?

    Thanks a bunch for putting up with these questions, I know they are a
    little varied.

    Cheers,

    Robin
     
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