• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 32 subscribers
  • Views 990 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMH0318 OUT0 Return Loss Network

Mark Heh
Intellectual 270 points
Other Parts Discussed in Thread: LMH0318, LMH0324, LMH1218, LMH1218EVM

In LMH0324-18EVM_SNLU182.pdf (LMH0324 Evaluation Module User's Guide), a return loss network (3.3nH inductor in parallel with a 75 ohm resistor) is in on the output, OUT0+/- (pins 20 & 19) of the LMH0318. However, it is not on the other output, OUT1+/-. Why is a return loss network on one output but not the other?

  

 

 

 

Also, there is no mention of the return loss network in the LMH0318 datasheet. It is not shown in the sample configuration drawing either.

 

Furthermore, the return loss network is not on OUT0+/- of LMH1218 on the LMH1218EVM (snlu173a).

 

 

Is the return loss network needed for the LMH0318?  If so, why is it needed for OUT0 but not OUT1 on the LMH0318 but not the LMH1218, which is footprint compatible.

 

Thanks!

Mark

  • 0
    TI__Mastermind 29635 points
    Hi Mark,

    Good observation. The LMH0318 and LMH1218 both have an integrated return-loss network, so it is not required to implement a return-loss network if you are hard-pressed for space on the board.

    However, with that having been said, we have observed improvements to return-loss margin by adding in the external return-loss network, so the LMH0324-18EVM highlights this, with the resulting return-loss performance shown in Figure 7 of the LMH0324-18EVM User's Guide. If you want to ensure sizeable margin with the LMH0318 output and can afford the extra BOM and board space, we recommend adding the external return-loss network.

    To answer the other question, the other output (OUT1) is an SFF-8431 output to support data transport over differential optical fiber (or in the case of the EVM, SMA connectors). As this output is not intended for single-ended 75-ohm output, it does not have a return-loss network associated with it.

    Thanks,

    Michael