Tuesday, August 19, 2014

Series termination layout

While reconsidering the trace size and termination topology for the address and control bus (see Fat, fat traces), it became apparent that I needed to keep my layout as tight as possible for the series resistors. The resistors needed to be packed tightly to allow them to be placed as close as possible to the driver (the FGPA) and to limit trace length differences to each resistor.

The first solution that came to mind was the termination part used on Lattice's ECP3 Versa development card. They used a CTS Corp RT2402B7-TR7 (Figures 1-3). They are designed to be used for common termination applications such as DDR3, VME64, PCIX, etc. I always try to limit the number of new components that I have to include in CES' database, but this part certainly fits the bill as a compact termination solution.

As I studied it a bit more, I made this list:

  • For series termination, the C Series looks like the right option. Figure 2 shows an RT2402 part used in a series termination application.
  • 18 series resistors are in the RT2402 (C Series), this is four short of the 22 resistors that I need.
  • In in smallest (1.0mm) package, the RT2402 is 4.0mm by 9.0mm. When I tried to replicate the arrangement and trace pattern in Figure 3 using discrete 0402 resistors, my pattern was approximately 4.0mm by 10.0mm (Figure 4).
While the RT2402 looks like a fantastic part for many applications, I think that well-placed discrete resistors will work well for me.

Figure 1. CTS Corp. RT2402 (from the RT2402 datasheet)

Figure 2. Series Termination with the RT2402 (from the RT2402 datasheet)

Figure 3. Escape Routing with the 1.0mm RT2402 (from the RT2402 datasheet)

Figure 4. My Series Resistor Routing Inspired by the RT2402.

Creative Commons License
The PCB layout shown in this post by Custom Embedded Solutions, LLC. is licensed under a Creative Commons Attribution 4.0 International License.

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