TMDS Voltage Swing in DVI Explained

Transition-Minimized Differential Signaling (TMDS) is the core physical-layer protocol used to transmit high-speed digital display data across Digital Visual Interface (DVI) connections. This article details the standard electrical characteristics and voltage swings defined by the DVI 1.0 specification, explaining both single-ended and differential peak-to-peak voltage amplitudes, Current Mode Logic (CML) architecture, and how these voltage levels ensure reliable video transmission.

Standard TMDS Voltage Levels

Under the DVI 1.0 specification, TMDS uses Current Mode Logic (CML) referencing a nominal supply voltage (\(AV_{cc}\)) of +3.3 V DC.

  • Single-Ended Voltage Swing: The nominal single-ended voltage swing is 500 mV (0.5 V), with an allowable operating range typically between 400 mV and 600 mV.
  • Differential Voltage Swing: Because TMDS relies on differential pairs (a true and an inverted signal), the nominal peak-to-peak differential voltage swing is 1000 mV (1.0 V), with an allowable range between 800 mV and 1200 mV (1.2 V).

Electrical Architecture and Signal Behavior

The TMDS transmitter consists of a switched current source, while the receiver incorporates 50-ohm termination pull-up resistors connected to the 3.3 V supply rail on each line of the differential pair.

When a TMDS line is in an inactive or high state, no current is drawn, pulling the line voltage to the 3.3 V termination level (\(V_{high} = 3.3\text{ V}\)). When the line switches to an active or low state, a current sink draws approximately 10 mA to 12 mA through the 50-ohm termination resistor. This current creates a 500 mV drop across the resistor, pulling the signal line down to approximately 2.8 V (\(V_{low} = 2.8\text{ V}\)).

Purpose of Low-Voltage Differential Signaling

The relatively small 500 mV single-ended swing provides two primary advantages:

  1. High-Speed Switching: Lower voltage excursions allow transistors to transition between binary states much faster, enabling high pixel clock rates without excessive slew-rate limitations.
  2. Reduced Electromagnetic Interference (EMI): The combination of a low voltage swing and balanced differential pairs minimizes radiated noise, as the electromagnetic fields generated by the complementary signals cancel each other out along the cable.