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LMH0387
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SNLS315G –APRIL 2010–REVISED APRIL 2013
The SMPTE 424M, 292M, and 259M standards have stringent requirements for the input and output return loss
of receivers and transmitters, which essentially specify how closely they must resemble a 75Ω network. Any non-
idealities in the network between the BNC and the LMH0387 will degrade the return loss. Care must be taken to
minimize impedance discontinuities both for the BNC footprint and for the trace between the BNC and the
LMH0387 to ensure that the characteristic impedance is 75Ω. Best return loss performance is achieved with the
LMH0387 placed closely to the BNC to minimize the trace length between the BNC and the LMH0387's BNC_IO
pin. Please consider the following PCB recommendations:
• Place the LMH0387 in close proximity to the BNC.
• 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 LMH0387.
• Select a board stack up that supports both 75Ω single-ended traces and 100Ω loosely-coupled differential
traces.
• Maintain symmetry on the complimentary signals.
• Route 100Ω 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 device power and
ground pins to the respective power or ground planes.
• Remove ground plane under input/output components to minimize parasitic capacitance.
Typical Performance Characteristics
Figure 6. Equalizer Output at 2.97 Gbps with 120m Belden Figure 7. Cable Driver Output at 2.97 Gbps
1694A, H: 100 ps / div, V: 50 mV / div
H: 62.5 ps / div, V: 100 mV / div
(SDO output shown)
(BNC_IO output shown)
Figure 8. Output Return Loss
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