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

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,