AD9852 and LTC6957


I am tried to feed REFCLOCK of AD9852 using LTC6957-3 as a source. It seems that AD9852 refuse to work with CMOS level signal coming from LTC6957-3. Does anybody tried to use CMOS leve REFLOCK on AD9852. The datasheet is confirm it should accept that.I am using single mode configuration (when ^REFCLOCK connected to the ground and DIFFCLKEN is connected to the ground as well.



  • 0
    •  Analog Employees 
    on Apr 25, 2014 7:20 PM

    My mistake.  I looked at the wrong logic levels for the LTC part.

    Message was edited by: Kevin.G

  • 0
    •  Analog Employees 
    on Apr 27, 2014 6:06 PM

    Hi doctor,

    I believe that the LTC6957 is an output buffer for your clock source. Could you check first if the clock source that you are feeding to the LTC6957 is within its input specifications.



  • Clock source (OCXO) is OK. The second output from LTC6957 is connected to MCU and working perfectly fine there.

    Actually I got the reply from AD Engeneering dept. and they confirm that LTC6957 and AD9852 has slightly different CMOS levels meanings. So, I was implement MC100LVEL16 in between of LTC6957 and AD9852 to correct that. Its working perfectly fine - I got the REFCLOCK. And I am able to re-program AD9852.

    Now I am fighting for another strange issue.For some uncertain reason I didn't get any output from its DAC's. I am thinking if analog power supply (separated regulator for 3.3V) is not good enough for the application. I am try to change the frequency but it didn't do anything with output signal. I am using SPI interface and 10Mhz Ref. signal. The sequence I am sending to AD9852:

    1.IO reset

    2. CS LOW

    3. 0x2 0x0 0x41 0x8A 0xE4 0xCA 0xF0

    4. IO Update

    5 CS HIGH

    For the step 3: 0x2 - Command to set frequency, 0x0 0x41 0x8A 0xE4 0xCA 0xF0  - FREQ = 10001 Hz

    FTW = (10001*2^48)/10^7 = 418AE4CAF0

    I would like to clarify if the following sequence is OK or I was need to send it in "reverse order"

  • 0
    •  Analog Employees 
    on Apr 28, 2014 9:08 PM

    Hi d0ct0r,

    Where did that reply from “AD Engeneering Dept.” come from?  I had posted a reply to your thread last Friday, but immediately after submitting it I realized that I was looking at the wrong specs in the LTC6957-X datasheet.  I deleted my message on the forum to try and eliminate any confusion, but our forum system would have already sent you en email notice with that information.  My apologies for any confusion I may have caused.  I was looking at the specs for the LTC6957-1 on page 4 (LVPECL outputs) when I should have been looking at the LTC6957-3 specs on page 8 (CMOS outputs).

    Unless I'm making a mistake again, the LTC6957-3 specs appear fine.  Voh min is Vdd - 0.2V (sourcing 3mA) and VOL max is 0.2V (sinking 3mA).

  • 0
    •  Analog Employees 
    on Aug 2, 2018 2:38 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin