Signal Integrity

Trend: Interface standards increasing data rates
Result: Protection being added to line requires lower capacitance to maintain signal integrity

Interface Standard High Speed Pairs Max Data Rate (Gb/s) Differential Impedance Max Capacitance Tolerance (pF)
HDMI1.2 4 1.65 100 Ω 0.9
HDMI1.3/1.4 4 3.4 100 Ω 0.6
Display Port 1.1 1, 2, 4 2.7 100 Ω 0.7
Display Port 1.2 1, 2, 4 5.4 100 Ω 0.4
USB2.0 1 0.48 90 Ω 1.2
USB3.0 3 4.8 90 Ω 0.4
eSATA 1.0 2 1.5 100 Ω 0.9
eSATA 2.0 2 3 100 Ω 0.65
eSATA 3.0 2 6 100 Ω 0.4
LCD Panel TCON: LVDS Up to 12 0.6 100 Ω 1.2
LCD Panel TCON: V by 1 8 3.75 100 Ω 0.5

Problem: Fastest interfaces utilize most voltage sensitive chipset technology
Solution: ESD protection that reduces clamping voltage while maintaining signal integrity

Key Test: Eye Diagram

Eye Diagrams are used to characterize a high speed signal source or transmitter (receiver testing usually requires bit error rate testing). A typical eye diagram test setup is shown below.

Typical Eye Diagram Test

Real world high speed digital signals suffer significant impairments including attenuation, noise, crosstalk, etc. The data eye diagram for a typical high speed digital signal is shown in the figure below. Notice how the diagram more resembles the shape of an eye.

Key Measurements

data eye diagram for a typical high speed digital signal

Typical High Speed Digital Signal with Eye Diagram
data eye diagram for a typical high speed digital signal

Typical Eye Diagram Measurements

Using eye diagrams for standards compliance

The quality of a high speed digital signal can be quickly determined by using a compliance mask overlay on the eye diagram display. A typical mask includes both time and amplitude limits. An eye diagram with compliance masks is shown below.

eye diagram with compliance masks

In the above diagram, the gray block areas represent "keep-out" areas. In order to pass the mask compliance test, the transmitter output must not have ANY samples fall within the "keep-out" regions.

Key Test: TDR

What is TDR?

A Time−Domain Reflectometer (TDR) is an electronic instrument for measuring the impedance of a signal carrying medium. TDR can show the impedance of the whole signal path. Any mismatch in the signal path can be observed on the TDR. Designers can use the information to improve the impedance matching and therefore signal quality of their products.

Test Results

Test Setup

TDR Test Results image
See More Detail
TDR Test Setup photo

For more information please refer to ON Semiconductor Application Note, Time−Domain Reflectomete (TDR)

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