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LMK00105: Recommended connection (schematic) between LMK03328 1.8V LVCMOS output and LMK00105 input

Kawai
Guru 19785 points

Part Number: LMK00105
Other Parts Discussed in Thread: LMK03328,

Hi Team,

Our customer is planning to buffer LMK03328 1.8V LVCMOS output with LMK00105.

Could you please draw a recommended schematic interfacing between LMK03328 output and LMK00105 input ?

I am thinking the recommendation is the following, based on Figure 6 in LMK00105 data sheet.
Since LMK03328 has output impedance of 50 Ohm, I believe it does not require dumping resistor "Rs".

What is your opinion and please give us your advice.

Best Regards,

Kawai

  • 0
    TI__Mastermind 32575 points
    Kawai,

    For 1.8V LVCMOS driver, I recommend Figure 7 with the following values:
    * Rs = 0 ohms
    * Do not install 50 ohms load termination (voltage divider) since the 1.8 V output swing is compatible with the maximum single-ended input swing of 2 V.
    * Rb1 = 10 k (pull-up to 3.3 V) and Rb2 = 3.9 k to set CLKIN- to ~0.9 V.

    Alan
  • 0
    TI__Expert 5370 points
    Dear kawai,

    Your assertion seems correct.

    howver, do you really need to match impedance? what frequency and edge rate will you have. How long will be that trace There is a good rule to use Z matching when the oneway propagation delay is equal or great than the Min(Rt,Ft).

    Regards, Simon.
  • 0 in reply to Alan O
    Guru 19785 points

    Hi Alan-san,

    Thank you for your advice. I understood your recommendation is DC coupling as below.
    Even with my schematic above, I think it is still able to work since having enough amplitude for VIH/VIL.
    Would there be any concerns using AC coupling connection ?

    Best Regards,

    Kawai

  • 0 in reply to Simon Damphousse
    Guru 19785 points
    Hi Simon-san,

    Thank you for your comments. I understand that you are talking about "Lumped constant circuit" or "Distributed constant circuit".

    Please allow me to confirm my understanding.

    Regarding LMK03328 datasheet , LVCMOS output tr/tf = 250ps (TYP).
    3 x 10^8 [m/s] x 250 [ps] = 0.075 [m]

    If the trace length "L" between LMK03328 and LMK00105 is...
    - L < 0.075m, termination is not required.
    - L >= 0.075m, 50 Ohm termination should be connected.

    Am I correct ?

    Best Regards,
    Kawai
  • 0 in reply to Kawai
    TI__Mastermind 32575 points
    Kawai-san,

    Yes, AC coupling approach would also work. I just chose DC-coupled interface it is a more common interface for LVCMOS driver.

    Regards,
    Alan
  • 0 in reply to Alan O
    Guru 19785 points

    Hi Alan-san,

    I have one more question to double check.

    Can LMK03328 and LMK00105 drive 50 Ohm load ?

    [LMK03328]
    It seems LMK03328 cannot regarding the output current for 1.8V LVCMOS output.

    [LMK00105]
    I couldn't find the output current specification in the datasheet.

    Best Regards,

    Kawai

  • 0 in reply to Kawai
    TI__Mastermind 32575 points
    Yes, both can drive 50 ohms through DC blocking capacitor, but the AC voltage swing at the load could be close to Vddo/2 due to the voltage divider formed by the driver output resistance and load resistance (both 50 ohms).

    Alan
  • 0 in reply to Alan O
    Guru 19785 points
    Hi Alan-san,

    I understood that both device CANNOT drive 50 ohms directly from the pin. DC blocking cap is the requirement.

    Best Regards,
    Kawai
  • 0 in reply to Kawai
    TI__Expert 5370 points

    Dear Kawai,

    You assertion is correct, impedance matching should be considered when the length is longer than 7.5 cm based on your calculation. This includes also the line trace. 

    Regards, Simon.

  • 0 in reply to Kawai
    Guru 19785 points
    Hi Alan-san,

    I would like to double check about the drive capability in another thread.

    Best Regards,
    Kawai
  • 0 in reply to Simon Damphousse
    Guru 19785 points
    Hi Simon-san,

    Thanks for the confirmation.

    Best Regards,
    Kawai