The Right Answer Is A: The Left Eye Is Superior

While the eye on the left may look a bit jagged with some ringing on it, it actually has a wider opening than the right eye.

To see why, let’s zoom in on the worst section of the eyes. The 2 horizontal lines represent VOLmax (maximum logic low spec) and VOHmin (minimum logic high spec).

The left eye shows that when the voltage dips, there is still plenty of margin left on the Vcc sag. The transient noise does not have any impact on the eye opening.

Even though the right eye may look smooth, there is not enough margin left to work with. This is caused by:

• Initial smaller opening
• Larger random jitter, which may cause sampling failures in the middle of the eye

A high Eye-Q (Eye Quality) is important in system level to:

1. Achieve large timing margin for ease of board design flexibility
2. Maintain reliability of operation

Learn about Stratix II signal integrity:

• Check out the Stratix^® II FPGA's LVDS eye diagram
• Check out the Stratix II FPGA's DDR2 eye diagram
• Learn more about the importance of performance margin
• Find out more about Stratix II pin capacitance
• Check out Stratix II vs. Virtex-4 FPGA eye diagram comparison