DVI Differential Voltage Swing Across 50-Ohm Termination

This article examines the differential voltage swing generated across the 50-ohm termination resistors in a Digital Visual Interface (DVI) connection. It details the technical specifications of Transition-Minimized Differential Signaling (TMDS), breaks down how the current-mode driver interacts with receiver terminations, and defines both the nominal single-ended and differential peak-to-peak voltage values required by the DVI standard.

TMDS Signaling Architecture

DVI relies on Transition-Minimized Differential Signaling (TMDS) to transmit high-speed digital video data. The physical layer uses Current Mode Logic (CML). In this architecture, the receiver terminates each line of a differential pair with a 50-ohm pull-up resistor tied to a 3.3-volt supply (\(AV_{CC}\)).

The transmitter contains a switched constant-current sink. Rather than driving a voltage directly, the transmitter steers current through the differential pair to pull the signal lines low relative to the 3.3 V reference level.

Current and Voltage Calculations

The TMDS transmitter typically sinks a nominal current between 10 mA and 12 mA:

  • Single-Ended Swing: When current flows through one of the 50-ohm termination resistors, the voltage drop across that resistor is calculated using Ohm's Law (\(V = I \times R\)). With a 10 mA to 12 mA drive current across a 50-ohm load, the single-ended voltage drop is nominally 500 mV to 600 mV (with an allowable specification range of 400 mV to 600 mV). This pulls the active line down from 3.3 V to roughly 2.8 V–2.7 V, while the inactive line remains at 3.3 V.
  • Differential Voltage Swing: Differential voltage (\(V_{ID}\)) represents the difference between the positive and negative conductors (\(V_+ - V_-\)). Because the current switches between the two lines, the differential voltage alternates polarity between \(+500\text{ mV}\) and \(-500\text{ mV}\) (nominally).

Final Differential Swing Values

Across the 50-ohm termination resistors, the resulting differential swings are:

  • Nominal Differential Voltage: \(\pm 500\text{ mV}\) to \(\pm 600\text{ mV}\) zero-to-peak.
  • Peak-to-Peak Differential Voltage Swing: 1000 mV to 1200 mV (1.0 V to 1.2 V) peak-to-peak differential (\(V_{DIFF(p-p)}\)).
  • Specification Limits: The DVI 1.0 specification defines the allowable differential input voltage range at the receiver between 800 mV and 1200 mV peak-to-peak (400 mV to 600 mV single-ended swing per line).