SDI Electrical Interface - SDI Electrical Interface

External SDI cable equalizers and cable drivers are required to convert the serial signals into and out of the GTX transceivers to SDI electrical standards.

An external SDI cable equalizer must be used to convert the single-ended 75Ω SDI signal to a 50Ω differential signal compatible with the receiver input signal requirements of the GTX transceiver. Appropriate SDI cable equalizers are available from several manufacturers. The differential outputs of these cable equalizers usually must be AC-coupled to the GTX receiver input signals. An example of interfacing a typical SDI cable equalizer to a GTX receiver is shown in the following figure. 12G-SDI cable equalizers typically have built in reclockers, but that does not change any of the requirements for electrical interfacing to the GTX.

Important: The capacitance values of the AC coupling capacitors between the outputs of the external SDI cable equalizer and the serial inputs of the GTX RX must be large enough to pass the SDI pathological signals without significant signal droop. AC coupling capacitors with values of at least 1.0 μF are required and 4.7 μF capacitors are recommended. Some new generation SDI cable equalizers default to 600 mV differential swing levels on their outputs instead of the traditional 800 mV differential swing. When using an equalizer with 600 mV differential swing, 4.7 μF capacitors may not be sufficient to prevent excessive signal droop at SD-SDI rates. It is recommended that cable equalizers be configured with 800 mV differential swing.

The differential inputs of the GTX RX have built-in differential termination. As described in 7 Series FPGAs GTX/GTH Transceivers User Guide (UG476), RX Termination Use Mode 3 is the recommended termination mode for the GTX RX inputs in SDI applications. The GTX internal programmable termination voltage should be set to 800 mV for SDI applications.

Figure 1. Interfacing an SDI Cable Equalizer to the GTX Receiver Inputs

Notes relevant to the previous figure:

Consult the SDI cable EQ manufacturer's information for the network between the SDI cable EQ and the BNC connector.

Similarly, the differential serial outputs of the GTX transmitter are connected to the inputs of an SDI cable driver, usually with AC coupling as shown in the following figure. The cable driver converts the differential signal from the GTX transmitter into a single-ended signal with electrical characteristics meeting the SDI standards.

Important: The capacitance values of the AC coupling capacitors between the GTX TX serial outputs and the inputs of the SDI cable driver must be large enough to pass the SDI pathological signals without significant signal droop. AC coupling capacitors with values of at least 1.0 μF are required and 4.7 μF capacitors are recommended.

Figure 2. Interfacing an SDI Cable Driver to the GTX Transmitter Outputs

Notes relevant to the previous figure:

Consult the SDI cable driver manufacturer's information for the network between the SDI cable drive and the BNC connector.

SDI Electrical Interface - SDI Electrical Interface - XAPP1249

Implementing SMPTE SDI Interfaces with 7 Series GTX Transceivers (XAPP1249)