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LMK03328: Regarding the connection of AC-LVPECL output

user4658448
Prodigy 50 points
Part Number: LMK03328


Referencing the data sheet states that the LVDS output port requires an external AC coupling capacitor.

Isn't an external AC coupling capacitor unnecessary if it meets the 8.16 AC-LVDS Output Characteristics in the data sheet?

Please tell me an external AC coupling capacitor is the reason you need.

  • 0
    TI__Genius 11900 points

    Hello,

    Please see the paragraph above "Figure 62. Structure of AC-LVDS, AC-CML, and AC-LVPECL Output Stage" of the datasheet. The LMK03328 output driver isn't a traditional LVDS output driver, it is  a low power dissipation driver that is designed to require an AC-coupling capacitor at the output. Figure 62 describes high level implementation of the driver. For this reason an AC-coupling capacitor is required for AC-LVDS, AC-CML, and AC-LVPECL use cases.

    There is a footnote in Section "8.16 AC-LVDS Output Characteristics" indicating that the data is taken under the condition shown in "Figure 28. AC-LVPECL, AC-LVDS, AC-CML Output AC Configuration During Device Test". AC coupling capacitors are used in this implementation to take the characterization data.

    Thanks,

    Vibhu

  • 0 in reply to Vibhu Vanjari
    Prodigy 50 points

    Thank you for your reply.

    I understand that I need an AC-coupling capacitor.

    Why do I need an AC-coupling capacitor when the driver impedance is high?

    kakeru

  • 0 in reply to user4658448
    TI__Genius 11900 points

    Hello Kakeru,

    Let me look into this further and get back to you by Wednesday.

    Thanks,

    Vibhu

  • 0 in reply to Vibhu Vanjari
    TI__Genius 11900 points

    Hello Kakeru,

    You could directly couple the outputs to a differential resistive load. However, the VOS of the output is not guaranteed, and will vary with PVT and amplitude. HCSL mode with 50 ohm terminations to ground has better VOS and is directly related to amount of HCSL current (amplitude) at the output.

    Thanks,

    Vibhu