10.1 Layout Guidelines

When dealing with an RF amplifier with relatively high gain and a center frequency of 2.6 GHz, certain board layout precautions must be taken to ensure stability and optimum performance. TI recommends that the LMH9226 board be multi-layered to improve thermal performance, grounding, and power-supply decoupling. Figure 23 shows a good layout example. In Figure 23, only the top signal layer and its adjacent ground reference plane are shown.

• Excellent electrical connection from the thermal pad to the board ground is essential. Use the recommended footprint, solder the pad to the board, and do not include a solder mask under the pad.
• Connect the pad ground to the device terminal ground on the top board layer.
• Verify that the return DC and RF current path have a low impedance ground plane directly under the package and that the RF signal traces into and out of the amplifier.
• Ensure that ground planes on the top and any internal layers are well stitched with vias.
• Do not route RF signal lines over breaks in the reference ground plane.
• Avoid routing clocks and digital control lines near RF signal lines.
• Do not route RF or DC signal lines over noisy power planes. Ground is the best reference, although clean power planes can serve where necessary.
• Place supply decoupling close to the device.
• The differential output traces must be symmetrical in order to achieve the best linearity performance.

A board layout software package can simplify the trace thickness design to maintain impedances for controlled impedance signals. To isolate the affect of board parasitic on frequency response, TI recommends placing the external output matching resistors close to the amplifier output pins. See the LMH9226 Evaluation Module user guide for more details on board layout and design.