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CDCE18005 reference input circuit

RF
Intellectual 930 points
Other Parts Discussed in Thread: CDCE18005

Hi,

I am using the CDCE18005 and I have an LVDS driver connected to the secondary reference clock input. I assumed that if I set that input to LVDS mode I could drive it directly, dc coupled, from an LVDS driver. It does not seem to work and I am wondering if I have damaged the part by doing this. I have compared to the EVM and I have all the register settings the same but I am not getting any output from my circuit. The only difference that I can see is that the EVM has coupling caps on the ref inputs.

Could someone tell me if I should be using coupling caps between my LVDS driver and the CDCE18005 reference inputs?

Russ

 

  • 0
    TI__Expert 4585 points

     

    Hi Rus,

    You should not be using the coupling caps if you want to drive it directly ( assumed 100 ohm termination is also external).  Otherwise, we need to provide biasing right after ac coupling.

     

    Thanks,

    Firoj

     

  • 0 in reply to Firoj Kabir
    Intellectual 930 points

    Hi Firoj,

    I attached part of our schematic showing how I connected a SN65LVDS1DBV to the clock input of the CDCE18005. I could not get this to work and it seems that it caused some damage to the CDCE18005, although it could have been caused by something else. I ended up adding coupling capacitors on all four REF inputs and I got that to work. Do you think that it should work properly the way my schematic is drawn?

    Russ

     

  • 0 in reply to RF
    TI__Expert 4585 points

     

    Hi Russ,

    I like to know your register settings of the input  bias/ termination.  It looks like you chose internal termination (as well as biasing). I don't see any external termination. In generally LVDS driver should work fine with DC connection. I am concerned if common mode voltage goes below 1 V for any reasons. When you added coupling capacitors, CDCE18005 is using internal biasing alone and termination and it is 1.2V for LVDS. This works fine.

    Can I get your phone number so that we can discuss?

     

    Thanks,

    Firoj

     

     

     

  • 0 in reply to Firoj Kabir
    Intellectual 930 points

    Hi Firoj,

    I attached a file with the register settings I am using now for the secondary ref input. These are the settings I am using now with coupling caps on the inputs and it works fine. I do not remember the exact settings I was using before I added coupling caps - it was not clear to me from the datasheet what settings I should use and I did a lot of experimenting with different settings and never got it to work properly. I concluded that the IC was damaged so I replaced it and added coupling caps and then I was able to get it working.

    If you want to talk you can call me next week at 604-421-1311 ext. 44.

    Russ

     

  • 0 in reply to RF
    TI__Expert 4585 points

     

    Hi Russ,

     

    I called you and left a voice mail.

    With the attached settings, you are using internal termination and biasing.  This just works fine.

    I don't expect IC was damaged. It can be common mode issue.

    If you want to drive directly,  you turn off internal terminations and  add 100 ohm externally . Even with internal terminations and biasing (1.2V), it should work without coupling caps.

     

    Thanks,

    Firoj