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DS25BR110: DS25BR110 output DC couple to CML input

Part Number: DS25BR110

Whether DS25BR110 output can DC couple to CML input? From datasheet below and Figure 10. Typical DS25BR110 Output DC-Coupled Interface to an LVDS, CML or LVPECL Receiver, it assumes that the receivers have high impedance inputs, but our chip CML input has 50ohm internal termination, the CML input has 50ohm resistor to bias supply 1.05V, whether it can work with DC couple? We may change to AC couple next phase, but currently the board has been fixed.

OUTPUT INTERFACING

The DS25BR110 outputs signals compliant to the LVDS standard. It can be DC-coupled to most common differential receivers. The following figure illustrates typical DC-coupled interface to common differential receivers and assumes that the receivers have high impedance inputs. While most differential receivers have a common mode input range that can accommodate LVDS compliant signals, it is recommended to check the respective receiver's datasheet prior to implementing the suggested interface implementation.

  • Hi,

    The DS25BR110 outputs will work with a CML common mode voltage bias to 1.05V.  The DS25BR110 will try to pull up this common mode voltage slightly to its nominal 1.2V value.  What will happen to the CML input if the input common model is slightly above 1.05V due to the DS25BR110 output connection?

    Regards,

    Lee

  • Hi Lee,

    Thanks for the feedback, it is OK CML input common mode is slightly above 1.05V, my concern is the 50ohm pull up resistor to 1.05V within CML input, the DS25BR110 output will sink the current, then the CML input 50ohm pull up resistor will also source or sink the current because of low impedance, it will have impact to the DS25BR110 output waveform, whether the impact is acceptable? Whether TI has tested DS25BR110 output DC couple to CML input with internal 50ohm pull up in CML input?

    Best regards
    Winson

  • Hi Winson,

    I have run them with a 50 Ohm to GND termination.  The output still works well.  There is some resistance in the common mode feedback path, with just ~ 0.15V between the two the added current will minimal.

    Regards,

    Lee 

  • Hi Lee,

    Thanks for the testing, two more questions:

    1.Since there is 50ohm pull up resistor to 1.05V within CML input, whether we still  need add the 100ohm resistor between P and N of the CML input?

    2.For next phase, do you prefer we change DS25BR110 output to CML input connection to AC couple or keep DC couple ?

    Best Regards
    Winson

  • Hi Winson,

    1. The 50 Ohms pull-ups are all that is required.  No need to insert the 100 Ohm differential termination resistor as well.

    2. I would prefer to see AC coupling as it is a bit cleaner, but the signal will need to be encoded or scrambled to make this a good option.  However, if the board is working I do not think this application requires a change.

    Regards,

    Lee