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Original question:

CDCLVC1108 output termination advice

CDCLVC1108: Unused outputs termination

Salvador Díaz Burgos
Prodigy 10 points
Part Number: CDCLVC1108

Hi,

I am currently designing with a CDCLVC1108 to distribute clock signals to other devices. Not all outputs are going to be used, so I need to know exactly how they must be terminated so that they don't affect the circuit.

The device is powered with 3.3V. As per the datasheet, I would think that a resistor divider from Vdd to GND of 100 Ohms + 100 Ohms would fit. 

Is this right? Please, provide the detailed info.

Also, for used outputs, do we need also this termination at receiver circuit side?

Best regards,

Salvador

  • +1
    TI__Genius 12120 points

    Hi Salvador, 

    Unused outputs from the CDCLVC1108 should be left floating to reduce their impact on the rest of the circuit. On the receiver side, most LVCMOS receivers do not require any termination unless you need to reduce the signal amplitude or change the common-mode voltage. Section 9 of the datasheet includes some information for typical applications. In Figure 10, the 100 Ohm to VCC+GND voltage divider is used to create a 50 Ohm line termination, which would set the amplitude at the receiver to around 0.5 * 3.3V.  

    Regards, 

    Connor 

  • 0 in reply to Connor Lewis
    Prodigy 10 points

    Hi Connor, 

    Thank you very much for your prompt response, that eliminates my doubts.

    Best regards,

    Salvador

  • 0 in reply to Connor Lewis
    Prodigy 10 points

    Hi again Connor,

    I have been discussing this with my colleagues, and there is a concern still remaining.

    The datasheet recommends to leave pins open. That is OK. But, what happens with EMI? The clocks signals have a frequency of about 90 MHz, is it possible that not terminated pins radiate to the rest of the circuit? Does a track used to terminate those pins with an impedance radiate as well? 

    How would you address this issue to avoid effects on the rest of the circuit from EMI point of view? Which specific termination is the optimal for non used outputs?

    Regards,

    Salvador

  • +1 in reply to Salvador Díaz Burgos
    TI__Genius 12120 points

    Hi Salvador, 

    We haven't done EMI testing on this part so we don't have data for different termination schemes. However, generally it's best to leave the pins floating since the copper trace itself can radiate noise. If the unused outputs are still going to be connected to a trace, you can terminate into an RC low pass filter (ex: 1K Ohms and 10 pF capacitor) to reduce the EMI effect from high slew rate.

    On the used outputs, you can also increase the series resistance value to dampen the output which would also help slow down the output slew rate and improve EMI performance. 

    Regards, 

    Connor 

  • 0 in reply to Connor Lewis
    Prodigy 10 points

    Hi Connor,

    Now it's clear to me, I think I can put this issue as resolved now.

    Thank you very much.

    Best regards,

    Salvador