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LMK01801: LCPECL emitter resistor calc for reduced output common-mode setup

Part Number: LMK01801

Hello,

We want to interface the LMK01801 clock buffer to a DC-coupled clock receiver with around 1V input common-mode voltage (self-biased input with integrated 100 ohm differential termination). LVDS can not be applied in this case since it gives us insufficient swing (min. 500mVpp differential swing required). LCPECL output format comes into play which seems to be a good fit to maintain the output to input common-mode voltage requirements. The emitter resistor for both LVPECL and LCPECL should be in range of 120 to 240 ohm. But how we can calculate the optimum resistor value to get roughly the same common-mode voltage at both sides, driver output and receiver input?

Who could help out here?

Best regards,
Joerg

  • Hello Joerg,

    Yes, the LCPECL output format helps us achieve relatively high swings with relatively low common mode voltages.

     * Can you confirm, when you say min 500 mVpp diff, that means each output, viewed as single ended is producing minimum 250 mVpp single ended?  Refer to page 13 for clarification of terminology.  Note that LVDS has 750 mVpp differential, and a Vod/id of 350 mV.

    Note that LCPECL with the datasheet termination of 50 ohms to 0.6 V termination has close to a 1 V Vcm, it's about Vcm = 1.175 V from the Vhigh = 1.6 V and Vlow = 0.75 V; this gives a Vod = 0.85 V which is a differential peak to peak voltage of 1.7 Vpp.

    In your case, needing a DC coupled connection to a 100 ohm termination, to achieve Vcm = 1 V, only a small reduction in swing would be necessary to take common mode from 1.175 V to 1 V.

    I suggest the following topology and solving for the desired results:

    73,
    Timothy