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CDCDB2000: Output Termination 100-ohm Need

Jacob Butler
Intellectual 1710 points
Part Number: CDCDB2000

Hello Team,

Can you help me with a question of a potential use case for the CDCDB2000 clock buffer?

A design is looking to use the 20 channel clock buffer, but needs 100-ohm impedance rather than the 85-ohm impedance integrated on the buffer for their specified endpoint.

Please advise how can best match the desired differential output impedance.

Thank You,

Jacob Butler

  • +1
    TI__Genius 16565 points

    Hi Jacob,

    CDCDB2000 does not support a 100 ohm impedance (per datasheet spec) because of the 85 ohm integrated termination.

    I suggest using 3 CDCDB800/CDCDB803 devices instead. These devices can be configured for 100 ohm impedance.

    Regards,

    Jennifer

  • 0 in reply to Jennifer Bernal
    TI__Intellectual 1710 points

    Hi Jennifer,

    Thanks for your response and pointing out the output impedance options present in the 8 channel buffers.

    Two follow up questions:

    1. Is the datasheet spec you pointed out a measure of the differential impedance seen at the output terminals of the device or what it can support on a transmission line?

    2. Does the device operate as a voltage source or current source?

    The reason I ask these is that I'm wondering if it would be possible to add series resistors to achieve proper differential impedance termination. The use case is such that ~18 channels will be required and using 3xDB800 buffers would be a significant footprint increase.

    Best Regards,

    Jacob Butler

  • 0 in reply to Jacob Butler
    TI__Genius 16565 points

    Hi Jacob,

    1. It is a measure seen at the output terminals and what we recommend on the transmission line.  

    2. The device operates as a voltage source.

    Using series resistors (Rs) to try to create a 100 ohm differential impedance with the CDCDB2000 may affect performance. This is because the use of an integrated 42 ohms with an external 7 ohms may result in discontinuity. It is possible that if the Rs is placed close enough to the output pad, then the discontinuity would be diminished.

    Regards,

    Jennifer