DVI Cable Differential Impedance Specifications

Digital Visual Interface (DVI) cables rely on tightly controlled electrical characteristics to transmit high-speed digital video signals reliably. This article explains the exact characteristic impedance required for the differential pairs within a DVI cable, the standard governing these specifications, and the technical reasons why maintaining impedance matching is critical for video display integrity.

According to the official DVI 1.0 specification defined by the Digital Display Working Group (DDWG), the required differential characteristic impedance for the signal pairs is 100 ohms (\(\Omega\)) nominal, with an allowable tolerance of \(\pm 15\%\) (ranging between 85 \(\Omega\) and 115 \(\Omega\)).

DVI relies on Transition Minimized Differential Signaling (TMDS) to transmit digital video data. In this signaling scheme, video data and clock signals are transmitted across four shielded twisted pairs (in a single-link connection) or seven pairs (in a dual-link connection). Each differential pair consists of two complementary conductors:

• Differential Impedance: The target impedance measured between the two balanced conductors of the pair is 100 \(\Omega\).
• Single-Ended Impedance: Each individual conductor within the pair typically exhibits an odd-mode characteristic impedance of approximately 50 \(\Omega\) relative to ground.

Maintaining this 100 \(\Omega\) differential impedance across the cable, connectors, and PCB traces is essential for high-frequency signal integrity. DVI transmits data at rates up to 1.65 Gbps per TMDS channel on a single link (equating to a 165 MHz pixel clock). At these frequencies, any deviation or mismatch in impedance causes signal reflections at interface junctions.

These reflections distort the signal's eye pattern, leading to high bit error rates, increased jitter, and signal attenuation. In real-world operation, improper differential impedance results in visible artifacts such as digital "sparkles," flickering pixels, frame drops, or total loss of display synchronization. Consequently, compliant DVI cables utilize carefully manufactured shielded twisted pairs (STP) with uniform dielectric spacing and controlled twist rates to ensure constant 100 \(\Omega\) impedance throughout the entire length of the assembly.