What is the impedance of dual screen HDMI to MIPI DSI adapter?
The impedance of a dual screen hdmi to mipi dsi adapter is typically 50 ohms for the HDMI input side and 100 ohms differential for the MIPI DSI output lanes, but this varies based on the specific PCB layout, signal integrity requirements, and the chipset used. For example, the LT8918 or LT8912B bridge chips commonly found in these adapters demand controlled impedance traces of 50 ohms single-ended on HDMI lines and 100 ohms differential on MIPI DSI data pairs to maintain signal quality at speeds up to 1.5 Gbps per lane. In practice, the actual impedance you measure on a physical board can deviate by ±10% due to manufacturing tolerances, dielectric material (like FR4 with a dielectric constant of 4.2 to 4.6), and trace width variations. For a dual screen configuration, the impedance matching becomes even more critical because the adapter must drive two separate MIPI DSI interfaces simultaneously, often with independent clock and data lanes, each requiring precise differential impedance control to avoid reflections and crosstalk.
Let’s break down the numbers. The HDMI specification mandates a single-ended impedance of 50 ohms ±15% for TMDS lines, which is standard for most adapters. On the MIPI DSI side, the D-PHY specification requires a differential impedance of 100 ohms ±10% for data and clock lanes, with a common mode impedance of 50 ohms to ground. In a dual screen hdmi to mipi dsi adapter, you’re essentially doubling the output channels, so the PCB must route two sets of MIPI DSI traces—each with four data lanes and one clock lane (for a total of 10 differential pairs per screen, or 20 pairs for dual screens). This increases the complexity of impedance control because the traces must be kept at equal lengths (within 0.5 mm tolerance) and isolated from each other by at least 3 times the trace width to minimize coupling. For instance, if the trace width is 0.15 mm for a 100 ohm differential pair on a standard 4-layer PCB with a 0.2 mm prepreg thickness, the spacing between the pairs should be at least 0.45 mm. Failure to maintain this can result in impedance mismatches that cause signal degradation, leading to screen flickering or no display at all.
Real-world measurements from tested adapters show that the HDMI input impedance often sits between 48 and 52 ohms, while the MIPI DSI output impedance ranges from 95 to 105 ohms differential. For example, a common design using the LT8912B chip with a 6-layer PCB (with dedicated ground and power planes) achieves a measured impedance of 49.8 ohms on HDMI and 99.2 ohms on MIPI DSI, according to TDR (Time Domain Reflectometry) tests. However, cheaper adapters using 2-layer boards often struggle, with impedance values dropping to 40 ohms on HDMI or rising to 110 ohms on MIPI, which can cause data errors at higher resolutions like 1920x1080 at 60 Hz. The impedance is also affected by the connector type—for HDMI, the standard Type A connector has a characteristic impedance of 50 ohms, while the FPC connector for MIPI DSI (typically 0.5 mm pitch) must be designed for 100 ohms differential, but many cheap connectors have a tolerance of ±20%, adding another layer of variability.
To get a clearer picture, here’s a table summarizing typical impedance values for a dual screen hdmi to mipi dsi adapter based on different PCB layers and chipset designs:
| Component | Impedance (Ohms) | Tolerance | PCB Layers | Typical Chipset |
|---|---|---|---|---|
| HDMI TMDS lines | 50 | ±15% | 4-6 | LT8918, LT8912B |
| MIPI DSI data lanes (differential) | 100 | ±10% | 4-6 | LT8912B, TC358775 |
| MIPI DSI clock lane (differential) | 100 | ±10% | 4-6 | LT8912B, TC358775 |
| Single-ended MIPI DSI (to ground) | 50 | ±10% | 4-6 | LT8912B, TC358775 |
| HDMI connector (Type A) | 50 | ±15% | N/A | Standard |
| FPC connector (MIPI) | 100 | ±20% | N/A | 0.5 mm pitch |
This table is based on datasheets from manufacturers like Lontium and Toshiba, as well as teardown reports of commercial adapters. The impedance values are critical because they directly affect the signal integrity at high frequencies. For a dual screen setup, the adapter must handle data rates up to 1.5 Gbps per MIPI lane (for 1080p at 60 Hz with 24-bit color), which requires a rise time of less than 200 ps. If the impedance is off by even 10 ohms, the reflection coefficient (Γ) can exceed 0.1, causing a voltage drop of 10% or more, which can push the signal below the receiver’s threshold (typically 150 mV for MIPI DSI). This is why many high-quality adapters use impedance-controlled vias and microstrip or stripline routing, with the dielectric material chosen for consistent permittivity. For instance, a 4-layer board using FR4 with a 0.2 mm core thickness can achieve a differential impedance of 100 ohms with a trace width of 0.15 mm and a spacing of 0.15 mm, but this requires tight manufacturing tolerances, which many budget adapters skip.
Another factor is the power delivery network, which also affects impedance indirectly. The adapter’s voltage regulator modules (VRMs) for the MIPI DSI interface must provide clean 1.2V or 1.8V rails with low ripple (under 50 mV peak-to-peak), because any noise on the power plane can couple into the signal traces and alter the effective impedance. In dual screen adapters, the current draw can be up to 500 mA per screen (for backlight and logic), so the PCB must have adequate copper thickness (1 oz or 2 oz) to handle the current without voltage drops that shift the impedance. For example, a 0.5 mm voltage drop on a 1.2V line can change the impedance by 2-3 ohms due to the change in the transistor bias points in the bridge chip.
If you’re designing or selecting a dual screen hdmi to mipi dsi adapter, you should also consider the termination resistors. On the HDMI side, the standard requires 50 ohm termination to ground at the receiver, which is usually built into the bridge chip. On the MIPI DSI side, the termination is 100 ohms differential between the positive and negative lines of each pair, which is also integrated into the display driver IC. But if the adapter uses external termination, like 49.9 ohm resistors to ground, the impedance must match exactly to avoid reflections. In practice, many adapters use 0402 or 0603 resistors with 1% tolerance, but the PCB trace impedance can still vary due to etching tolerances, which is why you’ll see impedance values ranging from 95 to 105 ohms in production units.
For a deeper dive into the technical specs and to see actual impedance test results, check out the dual screen hdmi to mipi dsi adapter product page, which includes detailed electrical characteristics and PCB stackup information. The page also provides data on how the impedance is maintained across different resolutions, from 480p to 4K, and how the dual screen mode affects the overall signal integrity. For instance, at 4K resolution (3840x2160 at 30 Hz), the MIPI DSI lanes must run at 1.2 Gbps per lane, and the impedance tolerance tightens to ±5% to maintain a bit error rate below 10^-12, which is achievable with a 6-layer board using low-loss materials like Rogers 4350B, but most adapters use FR4 and accept a higher error rate.
The impedance also interacts with the cable length and connector quality. For HDMI input, a standard cable up to 5 meters can maintain 50 ohms, but longer cables or cheap cables with 75 ohm impedance (common in some HDMI cables) can cause mismatches. For MIPI DSI output, the FPC cable length is typically limited to 100 mm to keep the impedance under control, because longer cables introduce parasitic inductance and capacitance that shift the impedance. In dual screen adapters, the two FPC cables must be routed symmetrically to avoid skew, which can alter the differential impedance by up to 5 ohms if one cable is 10 mm longer than the other. This is why many adapters use equal-length routing on the PCB and specify a maximum cable length of 50 mm in the datasheet.
Finally, the impedance is not static—it changes with temperature and frequency. At 25°C, the dielectric constant of FR4 is around 4.4, but at 85°C, it can drop to 4.2, which increases the impedance by 2-3 ohms. Similarly, at higher frequencies (like 1.5 GHz), the skin effect increases the resistance of the traces, which can reduce the impedance by 1-2 ohms. For a dual screen adapter operating in a hot environment (like inside a car or industrial enclosure), the impedance drift can cause intermittent display issues, so some adapters use temperature-compensated materials or active equalization circuits to maintain the impedance within spec. These details are often buried in the application notes, but they’re crucial for reliable operation.