SMPTE specifies several key requirements for the
Serial Digital Interface to transport digital
video over coaxial cables. Such requirements
include return loss, AC coupling, and data rate
dependency with rise and fall times.
- Return Loss: This specification details how closely the port resembles 75Ω
impedance across a specified frequency band. The LMH12x9 features a
built-in 75Ω return-loss network on SDI_IN, SDI_IN1, and SDI_OUT to minimize parasitics
and improve overall signal integrity.
- AC Coupling: AC-coupling capacitors are required for transporting uncompressed
serial data streams with heavy low-frequency content. The use of 4.7μF, AC-coupling
capacitors is recommended to avoid low-frequency DC wander.
- Rise/Fall Time: Output 75Ω signals are required to meet data dependent rise
and fall timing. This improves the eye opening observed for the receiving device. The
LMH12x9 SDI_OUT cable driver features automatic edge rate adjustment
to meet SMPTE rise and fall time requirements.
TI
recommends placing the LMH12x9 as close as possible (approximately 1 inch)
to the 75Ω BNC ports to meet SMPTE specifications.