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Removing ground planes under small capacitors

Discussion in 'Electronic Design' started by Joel Kolstad, Jul 18, 2006.

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  1. Joel Kolstad

    Joel Kolstad Guest

    Does anyone around here bother to have the ground plane underneath, say, an
    0402 capacitor removed? I ran through some numbers, and it looks like you can
    easily end up with something in the ballpark of many tens to the low hundreds
    of femtofarads by not doing so -- if your "circuit" capacitors are only in the
    couple pF range anyway, this seems like it could be significant. I'm
    operating the hundreds of MHz ballpark, so I'm not too worried about
    "non-lumpedness" yet by doing this.

    I've seen data sheets such as that of the Analog Devices AD8045 1GHz GBW
    op-amp suggest removing the ground planes from underneath the input and output
    pins to avoid creating a pole that might noticeably reduce the device's GBW...

    ---Joel Kolstad
  2. Chris Carlen

    Chris Carlen Guest

    I just did something like that with an OPA355 photodiode amp. I put
    cutouts under the RC feedback path, and the photodiode and it's pin
    which connects to the opamp.

    Good day!

    Christopher R. Carlen
    Principal Laser&Electronics Technologist
    Sandia National Laboratories CA USA

    NOTE, delete texts: "RemoveThis" and
    "BOGUS" from email address to reply.
  3. Ken Smith

    Ken Smith Guest

    I've done it on larger circuits but not 0402s.

    When making a crystal oscillator, it often makes sense to move the ground
    to the bottom of the PCB. PCB material doesn't make very good capacitors.
    They drift more than NPO/COG.

    When making very high impedance circuits, the capacitance to the ground
    plane can be too low of an impedance to allow in the circuit.
    You really want to reduce the capacitance at the input pins. The feedback
    resistor and this capacitance create a pole that can cause peeking or
    oscillation. To prevent this oscillation, the op-amp's compensation must
    reduce the bandwidth.

    The output pin of an op-amp has a lowish open loop impedance so the
    capacitances here have less effect.
  4. Joerg

    Joerg Guest

    Hello Joel,

    For high-Q resonant circuits and for high impedance nodes such as the
    collector or drain of an RF transistor I do. For decoupling or RC
    filters I don't. Also not on controlled impedance traces where the cap
    serves as a DC block (such as in T/R switches). However, the actual size
    of the cap doesn't really matter in those decisions.

    Same if you didn't void the plane under the parts in the collector/drain
    path of an RF stage. On stuff in your frequency range you can see a
    noticeable drop in bandwidth if you don't.
  5. John Larkin

    John Larkin Guest

    I measured some FR4 at +950 ppm/K. I wonder what the prop delay tc is
    like... I guess I should measure that some day, too.

  6. Joel Kolstad

    Joel Kolstad Guest

    Hi Joerg,

    Thanks for your feedback...

    This is a standard LC filter (50 ohms) with various LC series and parallel
    resonators; I'm tempted to try a layout both with and without the ground
    plane judiciously removed and see what happens.
    Agreed -- I've noticed that if all you're after is a DC block, you can
    typically get away with some awful 100nF cermaic cap that costs under a
    penny and the various parasitics will often do nothing but help make the
    path look like more of an RF short. (Seeing the effect of SRF on an S21
    plot usually takes some effort, and it helps a lot if you've cheated and
    measured the SRF previously so that you know where to look. :) )

  7. Joerg

    Joerg Guest

    Hello Joel,

    You could keep the area underneath the ground plane void of traces and
    use one layout. Then take one board and mill away the plane. Or, if you
    are really daring, chip it away with a Rotozip or Dremel grinder (but be
    careful...). Another option is to use thermal relief type connections
    at, say, two sides where there is nothing on the other side you could
    hit. Then grind these away with a little diamond disc.
    Normally yes but that often doesn't work in T/R switches. There you have
    to trade off between switch transients (often called ringdown) and
    mismatch at the low end of the RF band.
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