The TMDS inputs to the TFP401A-Q1 receiver have a fixed single-ended termination to AV_DD. A laser trim process internally optimizes the TFP401A-Q1 device to fix the impedance precisely 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 7-1 shows a conceptual schematic of a DVI transmitter and TFP401A-Q1 receiver connection. A transmitter drives the twisted-pair cable through a current source, usually using an open-drain type of output driver. The internal resistor, matched to the cable impedance at the TFP401A-Q1 input, provides a pullup to AV_DD. Naturally, with the transmitter disconnected and the TFP401A-Q1 DVI inputs left unconnected, the TFP401A-Q1 receiver inputs pull up to AV_DD. Figure 7-2 shows the single-ended differential signal and full-differential signal. The design of the TFP401A-Q1 device is for response 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 7-1 TMDS Differential Input and Transmitter Connection

Figure 7-2 TMDS Inputs