This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMH6505 - Cut away the ground plane, if any, under Rg.

Other Parts Discussed in Thread: LMH6505, LMH730066

Hi all,

Would you mind if we ask LMH6505?

The datasheet P18 indicates "Cut away the ground plane, if any, under RG."

<Q1>
Why is cut away the ground plane required?

<Q2>
About, our customer board, LMH6505 is the forward side(component side) and ground layer is under the forward side.
20150526_LMK6505_Layer.pdf

Our customer board is cut away the ground plane like as the file "LMH730066^NOPB.jpg"



Is it required to cut away the ground plane under this ground layer?
Should the ground of the B-side(back side) be cut away the ground plane?
Should all of the ground layer be cut away the ground plane under Rg?

We need your help.

Kind regards,

Hirotaka Matsumoto

  • Hi team,

    Could you give us reply?
    We have to hurry it.

    Kind regards,

    Hirotaka Matsumoto

  • Hello Hirotaka,

    Cutting the ground plane under the resistor reduces the stray capacitance from the resistor body, pads and signal trace to ground. Ideally, the entire sensitive node (resistor, input pin and any other devices) have the ground plane removed from around and below them.

    It is possible to knock-out 1pF or more of stray capacitance doing this - and at the speeds of the LMH's, that can have a significant effect on the overall performance.

    The ground plane should be removed from the layer directly under the resistor. It is also good to remove copper away from the resistor pads (and use the smallest area resistor pads).

    Ideally, you would also remove the copper on lower layers on a multi-layer board, but doing this could carve up ground/power planes. A judgement needs to be made if the reduction of capacitance is worth the possible carving/cutting-up of underlying ground planes.

    The '66 eval board you reference does not have a ground plane under the entire device and feedback resistors (on either side) to reduce the strays of the entire circuit.

    This needs to be done with caution, as removing the ground plane can also remove the "stripline" effect of matched impedance traces.

    It is all part of the "black magic" and compromises of high-speed layout.

    Regards,
  • Paul san,
    Thank you for your excellent reply!

    Kind regards,
    Hirotaka Matsumoto