• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 20 subscribers
  • Views 1243 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

LMX2581 osc input levels vs. LMX2541

Jonathan Jank
Intellectual 1045 points
Other Parts Discussed in Thread: LMX2581, LMX2541, LMK04808

I have a customer switching to the LMX2581 for his new product.  He previously had used the LMX2541.     He has found in the datasheet that the input Osc levels are different between the two parts.  It would appear he needs 400mV pk-pk minimum on the LMX2581 input.  The LMX2541 only needed half of this.   The problem here is that he has an LVDS source which is typically 400mV, but can vary a bit below that by spec.    His input frequency is 368MHz.  We don't want to add a buffer since that will add phase noise which we can't afford.

Are we trying to say that this part can't take an LVDS input ??    That would seem strange to me...   I hope not.

I also see Figure 12 in the datasheet that suggests we don't really need 400mV pk-pk.   It shows -23dBm sensitivity at 368MHz.

Can he use an LVDS input ??

thanks,

  • 0
    TI__Expert 5370 points

    LMX2581 support LVDS, no question.

    your question is very good and i suspect the following:  there are details missing an possibly a factor missing due to single ended/differental or voltage peak-to-peak diffenretial versus peak-to-peak only. I can see LMX2541 specify single ended and differential but noe LMX2581. We will look into the matter and find the details.

    Regards.

  • 0 in reply to Simon Damphousse
    TI__Intellectual 1045 points

    Thanks!   looking forward to hearing the details.   It just seems unlikely that we would not allow LVDS to drive one of our new clock chips such as the LMX2581.

  • 0 in reply to Jonathan Jank
    TI__Intellectual 1045 points

    HI,

    customer is right in the middle of testing on the eval board and is doing his layout of the board.  So, any clarification would help.  

    We did try this on the EVM and performance looks good.   Here are the customer's exact words:

    I am testing the LMX2581 with a 368.64MHz SE LVDS output from a LMK04808 eval board and the performance looks pretty good.  Hoping we can get dispensation to use it that way.

    Thanks for your help.

  • 0 in reply to Jonathan Jank
    TI__Expert 5370 points

    We should get beack to you early next week. Apology for the delay, regards, Simon.

  • 0 in reply to Simon Damphousse
    TI__Intellectual 1045 points
    Customer needs to finalize his design by tomorrow. Hoping we can give him a good response today.

    thanks!
  • 0 in reply to Jonathan Jank
    TI__Mastermind 28001 points

    Johnathan,

    In truth, pushing the spec from 400 mV to 368 mV is not much of a push and should work fine, but the issue comes if you want a guarantee for this.  The datasheet says minimum of 0.4 V, and I can NOT GUARANTEE this operation.  To get the guarantee requires a special stamp off, which will require us to know the business opportunity to justify changing our test program.   This means that it will likely involve the customer, volume, and some business commitment to justify changing our test program. 

    You might wonder why we just didn't guarantee 350 mV up front, which would have been nice in your case, but the LMK048xx family LVDS actually can go down to a min of 250 mV, so that would not help operating the LMX2581 with those devices. For the LMK devices, you can change the output format to LVPECL or LVCMOS as well as LVDS. 

    So approaches that would get the guarantee would be:

    1.  Use buffer

    2.  Use balun

    3.  Use another format like LVCMOS or LVPECL.  We also have a formats of "BLVDS" and "HSDS"  on some parts that are basically a higher drive level LVDS.

     

    Regards,

    Dean