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Original question:

LMK00105: AC coupling with LVDS input

LMK00105: Differential clock input termination

Naoyuki H
Prodigy 20 points
Part Number: LMK00105
Other Parts Discussed in Thread: CDCM6208,

Hi!

I have a question as title.
As I read past thread;

https://e2e.ti.com/support/clock-timing-group/clock-and-timing/f/clock-timing-forum/214501/lmk00105-differential-input-termination?tisearch=e2e-sitesearch&keymatch=LMK00105%25252525252525252520differential#

There is description about application report (https://www.ti.com/lit/an/scaa059c/scaa059c.pdf),

but I'm still not clear about differential input termination.

I consider about which circuit attached image below is better AC coupled LVDS with CDCM6208 for 2 types.

Type (1) is 2 AC coupling capacitors and 50 ohm termination for both positive and negative line.

Type (2) is 1 AC coupling capacitor for both positive and negative line and 100 ohm termination across the line.

Mother board is already mounted, and I want to use type (2) to decrease the parts.

I think type(2) will work, but I wonder there is any problem;

As CLKin and CLKin* is pre-biased about 1.4V, something wrong occur because of some difference of pre-biased voltage between CLKin & CLKin*.

If there are no problem, I want use type(2).

Best Regards,

Naoyuki

  • 0
    TI__Genius 12280 points

    Hi Naoyuki,

    Type 1) is recommended in the CDCM6208 datasheet for a situation "without receiver input termination and self biasing" but I am not familiar with Type 2) or how pre-biasing may affect the situation. I have paged a team member more familiar with termination.

    Thanks,

    Evan Su

  • 0 in reply to Evan Su
    Prodigy 20 points

    Thank you for your reply.
    I hope your team member will bring me further information.

    Best Regards,

    Naoyuki

  • 0 in reply to Naoyuki H
    TI__Genius 9250 points

    Hi Naoyuki,

    For Type 2 termination, if the output of CDCM6208 is not Hi-Z or tri stated, then this termination is fine for LMK00105 input. Other wise it would cause chatter when you have a 100 ohm resistor between two inputs in that mode. Because it would overrides the input offset voltage between inputs. 

    DC couple or 100 ohm standard termination before the AC coupling caps would be an option in that case.

    If output of CDCM6208 is always running, then type2 is perfectly fine.


    Best,

    Asim

  • 0 in reply to Asim Khan
    Prodigy 20 points

    Hi Asim,
    Thank you for your reply.

    I understand when CDCM6208 is Hi-Z, the offset voltage  would be overridden and causes chatter.

    As I guessed to use CDCM6208 with Hi-Z mode when PLL is not locked, your further information is very helpful.

    Best regards,

    Naoyuki