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CDCM7005 - How to decide pull down resistor for AC coupled LVPECL interface

Kawai
Guru 19785 points
Other Parts Discussed in Thread: CDCM7005

Hi Team,

In the CDCM7005 datasheet, I can see that the recommended pull down resistor is 150 Ohm for AC coupled LVPECL output.

Our customer asked us for the calculation how to decide the best pull down resistor value for the DC path.

I am also reading SLLA1220, which showed the resistor range of 140 to 220 Ohm.

Could you please advise us how to decide (calculate) the best value ?

Thanks in advance.

Best Regards,

Kawai

  • 0
    TI__Genius 10320 points

    Hi Kawai-san,

    The range of emitter resistor values provided have been selected to provide best performance under corner conditions while keeping the output structure biased properly. Note that the lower the ER value the higher the current draw will be, reducing the edge fall time. Rbias can be readily calculated by Rbias = (Vcc - 1.3)/Idc, where Idc is the DC current draw of the output structure. Here the worst case current draw is approx 14.3 mA (140 ohm ER) or best case is approx 9 mA (220 ohm ER).

    Gabe