Advances within the JESD204B converter protocol: Page 7 of 9

December 16, 2013 //By Jonathan Harris & Ian Beavers, Analog Devices
Advances within the JESD204B converter protocol
A new converter interface is steadily picking up steam and looks to become the protocol of choice for future converters. This new interface, JESD204, was originally rolled out several years ago, but it has undergone revisions that are making it a much more attractive and efficient converter interface.
far-end signal. An improved 2D data eye can therefore be seen internally after these circuit blocks, as compared to the input pin ahead of these circuits. By accurately sweeping a comparator voltage against small time delays, a complete recovered eye scan of the signal after these blocks can be realized. This provides an internal electrical probe point to the signal that the JESD204B receiver inside the FPGA actually uses to processes the serial bit stream.

JESD204B physical layer evaluation.

Each JESD204B differential lane should be matched in length intra-pair between the +/- signals. While lane-to-lane, or inter-pair, length matching is not critical, matched lengths within a pair are still important. Any intra-pair trace mismatch will close down the available data eye seen at the receiver, effectively limiting the bandwidth of the link.

The JESD204B PHY of an ADC or DAC can be modeled using what is known as an IBIS-AMI (I/O Buffer Information Specification-Algorithmic Modeling Interface) model. IBIS-AMI not only provides the I/O characteristics of the pin, it also offers a way to simulate the behavioral algorithm of the PHY layer. With this capability, system designers can measure the impact of jitter, rise/fall times, pre-emphasis, and equalization within the signal path of their systems.

This type of model provides several advantages for system designers. It's interoperable with many advanced simulation packages, so models from different semiconductor vendors can be run together to simulate both ends of a link. Users can set the silicon control parameters in the IBIS-AMI model as needed for their system and even simulate a matrix of options to find the best-case profile. And with the model's efficient performance, multimillion-point simulations can be run in a matter of minutes.

JESD204B calls for some mandatory test patterns to be made available on the link by the transmitter. These patterns are necessary to be able to validate the data integrity of the serial interface. One set of patterns

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