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LMK01801 INPUT BUFFER CLARIFICATION

Rajesh
Expert 1010 points
Other Parts Discussed in Thread: LMK01801

Hi,

I am using LMK01801 in my design, For the clock distribution purpose .with the following operating conditions:

Input: RF sine wave single ended or LVTTL single ended

Output: LVCMOS, LVPECL

Mode of controlling: PIN mode

Based on my operating conditions which input buffer(CLKinX_BUF_TYPE) i should choose whether Bipolor or CMOS,. Which type will be better

Regardds,

Rajesh

  • 0
    TI__Mastermind 28550 points

    Hello Rajesh,

    With respect to the sine wave input.  Please be sure to meet the minimum slew rate requirements on CLKin.  (0.15 V/ns minimum, 0.5 V/ns typ).

    When using pin control mode, you will be forced to use the default buffer type, which is Bipolar.

    I recommend Bipolar, hence the default value of Bipolar.

    • Bipolar is in general a good solution which will work when AC coupled with any signal.
    • CMOS allows for DC coupling an LVCMOS signal.
    • CMOS has a 55 mV offset on the input buffer which will prevent false clocks when no signal is present.
    • Bipolar doesn't have a voltage offset on the input buffer (VCLKinX-offset, bipolar) which allows for best performance.  But, when no signal present it is possible for oscillations due to noise or self-oscillation.  [This varies from datasheet to datasheet among LMK devices.  Always check the electrical specifications.  Some bipolar inputs have ~20 mV offset.
    • If an offset is required in Bipolar mode, it is technically possible to add a pull-up and/or pull-down on the input to pull the inputs apart.  This may or may not slightly impact performance.

    73,

    Timothy

  • 0 in reply to Timothy T
    Expert 1010 points

    HI,

    Thanks for the immediate response.

    Regards,

    Rajesh