DVI TMDS Differential Signaling Voltage Levels

Transition-Minimized Differential Signaling (TMDS) used in the Digital Visual Interface (DVI) standard relies on specific electrical specifications to transmit high-speed digital video data. This article outlines the exact DC and AC voltage levels employed by TMDS, detailing the supply voltage, single-ended logic levels, differential peak-to-peak swings, and common-mode characteristics defined by the DVI 1.0 specification.

Electrical Topology and Supply Voltage

TMDS uses Current Mode Logic (CML) topology. The transmitter consists of a switched current sink, and the receiver uses 50-ohm (\(\Omega\)) pull-up termination resistors tied to a nominal power supply rail.

  • Nominal Supply Voltage (\(AV_{cc}\)): \(+3.3\text{ V DC}\) (typically with a tolerance of \(\pm 5\%\), operating between \(3.135\text{ V}\) and \(3.465\text{ V}\)).

Single-Ended Voltage Levels

Because the receiver pulls the differential data lines up to the supply rail via internal \(50\ \Omega\) resistors, the single-ended levels on each conductor of a differential pair are:

  • Output High Voltage (\(V_{OH}\)): Equal to \(AV_{cc}\), nominally \(+3.3\text{ V}\). When the transmitter’s current sink is turned off, no current flows across the termination resistor, holding the line at the supply voltage.
  • Output Low Voltage (\(V_{OL}\)): Nominally between \(+2.7\text{ V}\) and \(+2.9\text{ V}\) (typically \(+2.8\text{ V}\)). When the transmitter switches on its nominal \(10\text{ mA}\) to \(12\text{ mA}\) current sink, the voltage drop across the \(50\ \Omega\) load pulls the line down by approximately \(500\text{ mV}\) to \(600\text{ mV}\).
  • Single-Ended Swing (\(\Delta V_{SE}\)): \(400\text{ mV}\) to \(600\text{ mV}\), with a nominal target of \(500\text{ mV}\).

Differential Voltage Levels

TMDS transmits data over complementary pairs (True and Inverting lines). The differential voltage (\(V_{ID}\)) is defined as the voltage of the non-inverting line minus the voltage of the inverting line:

  • Differential Peak-to-Peak Swing (\(V_{DIFF(p-p)}\)): \(800\text{ mV}\) to \(1200\text{ mV}\) (nominally \(1000\text{ mV}\) or \(1.0\text{ V}\)). Because the two complementary signals alternate between high (\(3.3\text{ V}\)) and low (\(2.8\text{ V}\)), the differential amplitude alternates between \(+500\text{ mV}\) and \(-500\text{ mV}\), resulting in a total peak-to-peak differential amplitude of \(1.0\text{ V}\).
  • Receiver Differential Threshold: A DVI-compliant receiver must reliably detect transitions with a differential voltage as low as \(150\text{ mV}\) to \(200\text{ mV}\), ensuring signal integrity over longer cable runs where attenuation occurs.

Common-Mode Voltage

The common-mode voltage (\(V_{CM}\)) represents the average voltage of the differential pair relative to ground:

  • Nominal Common-Mode Voltage (\(V_{CM}\)): Approximately \(+3.0\text{ V}\) to \(+3.05\text{ V}\). It sits halfway between the high state (\(3.3\text{ V}\)) and the low state (\(2.8\text{ V}\)), fluctuating slightly within the range of \(AV_{cc} - 0.3\text{ V}\) to \(AV_{cc} - 0.25\text{ V}\).