JESD204B: Is it for you?

If you have anything to do with high-speed data capture designs using an FPGA, you’ve probably heard the new buzz word, ‘JESD204B’. 

In my role here at TI, I’ve seen a lot of engineers request information on the JESD204B interface and how it works with an FPGA. They’re especially curious about how it will make their design life easier. 

The JESD204B serial interface standard for data converters provides some significant benefits over LVDS and CMOS interfaces. This includes easier layout and reduced pin-count. It’s no wonder it’s gained popularity and the attention of engineers with system-level benefits like these:

  • Reduced package size and cost: JESD204B serially packs data with 8b10b encoding and helps support data rates up to 12.5Gbps. This drastically reduces the number of pins needed on the data converter and FPGA, which helps reduce package size and package cost.
  • Simplified PCB layout and routing: Reduced pin count significantly simplifies PCB layout and routing, since there are fewer lanes on the board.  The layout and routing is further simplified, since there is a reduced need for skew management. This is because the data clock is embedded in the data stream, combined with the elastic buffer in the receiver, eliminates the need for ‘squiggles’ to match lengths.  Figure 1 shows an example of how considerably the JESD204B interface helps simplify PCB layout.
  • Flexible layout: JESD204B allows longer transmission distances because of relaxed skew requirements. This enables logic devices to be placed much farther from the data converter to avoid impact on sensitive analog parts. 
  • Future proof: The interface is adaptable to different data converter resolutions, removing the need for physical redesign of TX/RX boards (logic devices) for future analog-to-digital converters (ADCs) and digital-to-analog converters (DACs).

Figure 1:  PCB Layout with LVDS DAC (left);  PCB Layout of the same DAC with JESD204B (right)

Does this mean you should switch to the JESD204B interface?

Not necessarily. The drawback of JESD204B over the LVDS interface is that the longer absolute latency might be unacceptable for some applications.

While JESD204B provides many benefits, there are applications that require minimal latency – in an ideal world, no latency. A great example is a signal jammer used in in electronic warfare. This equipment requires absolute latency and a need to minimize any possible delay. 

For applications like this, you should consider the LVDS interface, since the delay in serializing the data on JESD204B is omitted. 

If you’d like to learn more about JESD204B, here are some additional resources:

  • JESD204B definitely makes it easy for the chip designers and the FPGA designers in many applications.   System designers should take note that there is a penalty to be paid in power/cost and some customers do not like the tradeoffs.   We have a few Radar customers that use a large number of High Speed Converters in a system.   Some of these systems have 96 channels of high speed ADC's.    The JESD204B would simplify the interface but the increase in overall power consuption was too great to use in a system.  

    We have a customer that does handheld Software Defined Radios.   They also stay away from the JESD204B due to power.   Our TI Medical X-ray AFE team recently considered JESD204B for a new generation of products.   Since the X-ray detectors are often battery powered, the JESD204B did not make sense from a power perspective either.   We also found that the JESD204B interface requires a more expensive class of FPGA to support the interface.  

    TI Product Definers, Designers, and Marketing teams please beware.   JESD204B will increase system power and cost in many cases.   If your target market  or application requires low power and or low cost, you may want to provide a non-JESD204B option.   The interface is a great option for some applications and markets but should not be adopted for all products.   More application notes and articles should include the tradeoffs of JESD204B so that designers understand when it should be adopted.