• State Resolved
  • Locked Locked
  • Replies 10 replies
  • Answers 1 answer
  • Subscribers 22 subscribers
  • Views 433 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.

LMX2594: DC Coupled Input Possibility?

Gozkul
Prodigy 105 points
Part Number: LMX2594
Other Parts Discussed in Thread: LMK04832

Dear TI Family, 

We are planning to use LMX2594 in our project. However, we would like to use reference signal as a DC Coupled. We think that the capacitors we will put to be AC Coupled will create phase uncertainty. So, we should enter the reference signal directly to the LMX2594 OSCin port. Would we input the reference signal directly as DC Coupled?

Thanks in advance.

Gündüz. 

  • 0
    TI__Guru** 107280 points

    Hi Gunduz,

    OSCin pins have internal bias voltages, that's why we need AC-couple. 

    What is the OSCin frequency you are going to use? What kind of phase uncertainty you are foreseeing? 

  • 0 in reply to Noel Fung
    Prodigy 105 points

    Dear Noel,

    Thanks for your return. 

    We are planning to use 30.72 MHz Differential Signal (LVPECL or LVDS) signal for OSCin. We will make an certain delay adjustments around picoseconds. However, capacitors may cause phase uncertainty and in our opinion this uncertainty will vary from capacitor to capacitor due to tolerances. Therefore, we will not able to make specific phase adjustment. Do you have any idea about removing the capacitors?  

    Thanks in advance.

    Gündüz.    

  • 0 in reply to Gozkul
    TI__Guru** 107280 points

    Hi Gozkul,

    Let me check with the designer if DC-couple to these signal formats is acceptable or not, stay tuned.

  • 0 in reply to Noel Fung
    TI__Guru** 107280 points

    Hi Gozkul,

    I just aware that because of Diwali, the designers are taking time off, I am afraid I may not have the answer by this week. I will let you know once I've got the answer.

  • 0 in reply to Gozkul
    TI__Genius 12560 points

    Hi Gündüz,

    What is the DC common mode level expecting at OSCin input of LMX2594?

    Regards,

    Ajeet Pal

  • 0 in reply to Ajeet Pal
    TI__Guru** 107280 points

    Hi Gozkul,

    Sorry to keep you waiting, I have got the OSCin pins information as follows:

    The pins are biased to 1.65V via 5kΩ resistors.

  • 0 in reply to Ajeet Pal
    Prodigy 105 points

    Dear Ajeet,

    Thanks for your return.

    We will use LMK04832 1600 mV LVPECL output for LMX2594 OSCin Input. Therefore, we can assume that 800 mV Differential Output Voltage.  

    Best Regards,

    Gündüz

  • 0 in reply to Noel Fung
    Prodigy 105 points

    Dear Noel,

    Thanks for the information. As we said to Mr. Ajeet, we are planning to use LMK04832 Clock output for LMX2594 OSCin input. We suppose, this 1.65V biased could damage the LMK04832 Clock Output unless we use DC block capacitor. What do you think about this assumption?

    Best Regards,

    Gündüz

  • +1 in reply to Gozkul
    TI__Genius 12560 points

    Hi Gündüz,

    LMK04832 support multiple output formats and those are having different common mode voltage for each format. If receiver and transmitter support the same signal format, that can be used in DC coupled. Otherwise, to avoid bias mismatch or excessive loading of the bias circuitry, TI recommends connecting LVPECL outputs to the load through AC coupling capacitors. Same will be applicable for interface between LMK04832 and LMX2594 as they both have different biasing and suggest to keep DC blocking capacitor.

    Thanks!

    Regards,

    Ajeet Pal

  • 0 in reply to Ajeet Pal
    Prodigy 105 points

    Dear Ajeet,

    Thanks for your detailed explanation. We suppose, we have to use DC Blocking Capacitor for LMX2594 OSCin inputs. 

    Best Regards,

    Gündüz.