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LMK00308: CLKin termination

Expert 1120 points

Replies: 5

Views: 860

Part Number: LMK00308

Hi,

We would like to use si570 to drive LMK00308 with LVDS (Vcm=1.20V) signal and we want to use as few components as possible due to space limit which means DC coupling, direct connection is the best. 

In LMK00308 datasheet it only mentioned "The CLKin input has an internal bias voltage of about 1.4 V", but no info on whether the CLKin has internal 100-Ω termination. A naive guess would be there is no internal 100-Ω  termination since the CLKin input is a universal receiver. So we should do it as illustrated in Fig 33 in page 26 of the datasheet, DC termination with AC coupling at load, right?

I am also a bit confused about the position of the 100-Ω termination resistor, should it be placed between the DC-blocking capacitors and the load (which is more commonly seen) or between the transmission line and the DC-blocking capacitors as in Fig33?

Thanks & Best,

HP

  • For minimal components, just use DC-coupling from LVDS driver to LMK00308 CLKinX_P/N inputs, and place 100-ohm termination resistor across the P/N inputs. There is no need for AC coupling since LVDS levels are within the common mode input and differential input swing voltage ranges of LMK00308. There is no internal termination.

    Instead of Si570, I recommend using LMK61E2 I2C-programmable differential oscillator with EEPROM which has 100 fs RMS jitter performance.

    Regards,
    Alan
  • In reply to Alan O:

    Thank you Alan for your support.

  • In reply to Alan O:

    Hi Alan, I checked LMK61E2, seems it has a better performance. How do you recommend to interface LMK61E2 with LMK00308 with LVPECL signal? Should we follow Fig3 in scaa059c ?

    Thanks!

    Best,
    HP

  • In reply to haopeng han:

    DC coupled LVDS is the simplest way (only needs 100ohm differential termination at CLKIN input pins).

    Alan

  • In reply to Alan O:

    But we need LVPECL for a 1GHz clock. LVDS of  LMK61E2 supports only up to 900MHz. Thanks.

    Best,

    HP

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