SNLS233O April 2007 – July 2015 LMH0344
PRODUCTION DATA.
10 Layout
10.1 Layout Guidelines
For information on layout and soldering of the WQFN package, please refer to the following application note: AN-1187 Leadless Leadframe Package (LLP) (SNOA401).
The ST 424, 292, and 259 standards have stringent requirements for the input return loss of receivers, which essentially specify how closely the input must resemble a 75-Ω network. Any non-idealities in the network between the BNC and the equalizer will degrade the input return loss. Take care to minimize impedance discontinuities between the BNC and the equalizer to ensure that the characteristic impedance of this trace is 75 Ω.
Please consider the following PCB recommendations:
- Use surface-mount components, and use the smallest components available. In addition, use the smallest size component pads.
- Select trace widths that minimize the impedance mismatch between the BNC and the equalizer.
- Select a board stack up that supports both 75-Ω single-ended traces and 100-Ω loosely-coupled differential traces.
- Place return loss components closest to the equalizer input pins.
- Maintain symmetry on the complementary signals.
- Route 100-Ω traces uniformly (keep trace widths and trace spacing uniform along the trace).
- Avoid sharp bends in the signal path; use 45° or radial bends.
- Place bypass capacitors close to each power pin, and use the shortest path to connect equalizer power and ground pins to the respective power or ground planes.
10.2 Layout Example
Figure 6 and Figure 7 demonstrates the LMH0344EVM PCB layout. Ground and supply relief under the return loss passive components and pads reduces parasitic - improving return loss performance. Note in Figure 7 that the five vias between the four solder paste squares do not have solder paste. This practice improves both thermal performance and soldering during board assembly.
Figure 6. LMH0344EVM Top Etch Layout Example
Figure 7. LMH0344EVM Top Solder Paste Mask