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To pad or not to pad (q. about using pad vs. straight surface)

Discussion in 'CAD' started by Vitaliy, Aug 11, 2006.

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  1. Vitaliy

    Vitaliy Guest

    Hello,
    Could someone please explain me the difference between making a pad or
    having a straight surface. A picture is worth a thousand words, so I am
    attaching a picture with the pad and without the pad.
    My application of the pad: ground of analog circuit. So, I am placing
    the
    capacitor on power rail (between power rail and gnd) to filter the
    noise
    from power supply and bond wires. My current to power up opamp and for
    diode biasing is in milliamps and my voltage is +-5V.

    Does it make a difference what I use (as in terms of noise, etc.), if I
    have a few of these in the circuit? Should I use instead of pads just
    the
    straight surface for the ground surface? I know that a pad has some
    parasitic capacitance, but then wouldn't the surface have some as well?
    I
    will be soldering on my own, so I won't be confused at hich components
    should go where.

    Currently have:
    --power---cap-----
    --line------pad-----
    ||||||
    ||||||
    cap
    ||||||
    ||||||
    cap
    |||---gnd---|||
    ||| pad |||
    ||| | |||
    |||||||||||||||||||||
    gnd
    surface

    Thinking about:
    --power---cap-----
    --line------pad-----
    ||||||
    ||||||
    cap
    ||||||
    ||||||
    |||||||gnd|||||||||
    ||||||surface|||
    |||||||||||||||||||||||


    I care because I need to convert very small current (in uA) to volts.
    Everything is working fine as it is, but I'm looking to reduce noise as
    much as possible to avoid oscillations for higher gain/bw.

    Thanks,
    Vitaliy
     
  2. Joerg

    Joerg Guest

    Hello Vitaliy,


    Sorry, but I do not find that picture below too clear. Actually rather
    confusing.


    The old rule is that there be the least amount of parasitic inductance
    from the ground plane to one side of a decoupling cap, and from the
    supply rail to the other side. If with "pad" you mean the SMT solder
    lands then the connections must be as short as possible while
    maintaining thermal relief rules.

    Well, we all do :)

    But if you already have it running you should be able to gain some extra
    margin by short bypasses, clever placement and shielding.
     
  3. Vitaliy,
    With what you have explained I would offer the following.

    Since you are putting a decoupling/filtering cap on the signal/power, of
    what worry is the parasitic capacitance? The parasitic capacitance is
    undoubtedly much smaller than the capacitor you are using, particularly on
    the power rails. More capacitance is better in that case, that is also why
    closely spaced gnd/vcc planes work very well for the higher frequency
    decoupling, not quite the same as your case but just presented the concept
    for your consideration.

    As for the assembly of such capacitor placement, as long as you can
    solder then anything is fair game. The only concern would come in in the
    form of reliability. Soldering the end cap of an SMT cap on top of a trace
    would not be as reliable as using a pad where the solder can form a full
    meniscus off the end of the cap and thus supply a highly reliable joint.
    This assumes that your trace is not as wide as the pad would have been
    length wise, otherwise there is no point in this whole discussion if the pad
    was smaller than the trace to begin with because adding the pad would have
    made absolutely no difference to the circuit whatsoever.

    Connections to large blocks of copper are generally always better for
    general circuit performance. Ever wonder why all of those sample/evaluation
    boards from chip manufacturers are typically done removing only isolation
    channels between non-connected components/signals? Some of those circuits
    are difficult to get operating to the full spec'd performance if you use
    more typical traces to connect the circuitry.

    The largest problem for oscillations in a circuit like you are
    describing is typically dealt with through good part placement and thus good
    routing. Keeping inputs away from outputs, keeping feedback loops very short
    and direct, etc.. Since your currents are so low, you might consider guard
    tracks between the input signal(s) and other nearby copper. After making you
    board you should make sure it is well dried (bake it in an oven for several
    hours at up to 100C), then conformal coat it so that contaminants can't
    interfere with your small currents. Although these days uAmps aren't that
    small, they could still give you problems doing this yourself using (I
    assume) hobbyist techniques/materials/processes.
     
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