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 75-Ω impedance across a specified frequency
band. The LMH1297 features a built-in 75-Ω return-loss network on SDI_IO 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 4.7-μF, AC-coupling capacitors is recommended to avoid low-frequency DC wander.
- Rise/Fall Time: output 75-Ω signals are
required to meet certain rise and fall timing depending on the data rate.
This improves the eye opening observed for the receiving device. The LMH1297
SDI_IO (in CD mode) and SDI_OUT cable drivers feature automatic edge rate
adjustment to meet SMPTE rise and fall time requirements.
TI recommends placing the LMH1297 as close as possible to the 75-Ω BNC ports to meet SMPTE specifications.