The TMDS inputs to the TFP401/401A receiver have a fixed single-ended termination to AV_DD. The TFP401/401A is internally optimized using a laser trim process to precisely fix the impedance at 50 Ω. The device functions normally with or without a resistor on the EXT_RES pin, so it remains drop-in compatible with current sockets. The fixed impedance eliminates the need for an external resistor while providing optimum impedance matching to standard 50-Ω DVI cables.

Figure 9-1 shows a conceptual schematic of a DVI transmitter and TFP401/401A receiver connection. A transmitter drives the twisted-pair cable via a current source, usually achieved with an open-drain type output driver. The internal resistor, which is matched to the cable impedance at the TFP401/401A input, provides a pullup to AV_DD. Naturally, when the transmitter is disconnected and the TFP401/401A DVI inputs are left unconnected, the TFP401/401A receiver inputs pull up to AV_DD. The single-ended differential signal and full-differential signal is shown in Figure 9-2. The TFP401/401A is designed to respond to differential signal swings ranging from 150 mV to 1.56 V with common-mode voltages ranging from (AV_DD – 300 mV) to (AV_DD – 37 mV).

Figure 9-1 TMDS Differential Input and Transmitter Connection

Figure 9-2 TMDS Inputs