2500

1500

2000

1000

500

-5

-10

-15

-20

-25

-30

-35

0

-40

FREQUENCY (MHz)

5

3000

SMPTE Limit

LMH0387

www.ti.com

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 75

Ω 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 75

Ω. 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 75

Ω single-ended traces and 100Ω loosely-coupled differential

traces.

•

Maintain symmetry on the complimentary 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 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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Product Folder Links: LMH0387