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CDCP1803: Output differential voltage swing - Maximum value

sudarsan S R
Prodigy 30 points
Part Number: CDCP1803

Hi, 

I'm designing a clock fanout circuit (125MHz) for an ASIC which accepts LVDS or LVPECL levels but with a stringent differential voltage swing requirement of 500 - 900mV at its pins. In the datasheet (Page 7) of CDCP1803, the field "Output voltage swing between Y and Y" has a minimum value of 500mV but there is no typical or maximum value given. Power supply is 3.3V.

Is there a reason for this? Or is there a way to calculate the typical and maximum values from the datasheet? 

Fig. 3 shows the output voltage swing over frequency but this is only at 25°C (ambient). At 125MHz, this amounts to approx. 825mV. Is this the maximum value? 

Thanks!

  • 0
    TI__Mastermind 32575 points

    Per Fig 3, there is a frequency dependency and small dependency on Vcc voltage.  At 125 MHz, there is minimal attenuation of the nominal output swing from steady state / DC output levels.  While this graph does not capture temperature or process variation, we could look at the IBIS file to see how these parameters affect the output swing.  The output waveform in the IBIS model indicates the Vo can be up to ~0.93 V worst case.  While this may be higher than your spec, you could use a small series resistor near the driver output pins to form a voltage divider with the load termination and reduce the swing.  For example, using Rs = 10 ohms and Rt = 50 ohms (single-ended load termination), the Vo swing would be reduced by ~83% at the load, which would ensure the voltage swing falls within your ASIC input swing spec.  You could also use the CDCP1803 IBIS file to simulate the clock signal integrity with your channel and receiver models.

    Regards,
    Alan

  • 0
    TI__Expert 5370 points
    Dear Sudarsan,

    The author of the D/S in this case share a minimum swing capability which is quite good with the assumption that a max is not needed or a concern. at . Leave yourself flexibility in your design to be able to get the proper voltage swing for the receiver over temperature. I suggest to use this app note to see some options you may use including space for attenuation as needed.

    www.ti.com/.../slla120.pdf

    Remember the 500 mV minimum is over the free air temperature range.

    Hope this helps,

    Regards, Simon.
  • 0 in reply to Alan O
    Prodigy 30 points
    Thanks for the suggestion, I'll try to implement it in the simulator. I need to find the correct values so that the swing doesn't fall short of the lower limit when I try to reduce the upper limit. I'm marking this issue as resolved in the meantime.