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Original question:

CDCVF2505: About the calculation for optimizing the propagation delay

CDCVF2505: about delay calculation

Ryoma Ohashi
Expert 5655 points
Part Number: CDCVF2505


Hi team,

When using Y0, Y1, Y2, Y3,

Should we calculate the delay for each of the Yn since thier capacitive load alters?

Also, just to confirm, the delay of Yn is determined by delta load (delta between Yn Load and CKLOUT Load) by using figure 10, right?

Figure 10. tpd, Propagation Delay Time vs Delta Load

Regards,

Ohashi

  • +1
    TI__Genius 9260 points

    Hi Ryoma,

    For calculating the delay. We usually average the capacitive load across all the outputs including the device output driver capacitance.  For more detail on calculating it you can refer this thread Calculating capacitance.

    Figure 10. shows the leading and lagging of of CLKIN based on CLKOUT and Yn capacitance relationship relationship.  If CLKOUT is capacitance is higher than average of Yn then then CLKIN will lag the outputs of device and vice versa.

    Figure 10. shows the propagation delay of how the output is coming out based on CLKOUT and Yn delta as mentioned above. You can reference this for expected delay lead or lag of CLKIN and outputs of device (Yn).

    For example, looking at the right side of plot Delta load is a positive value based on CLKOUT value (variable) and Yn = 3pF (average value). So when you have a positive delta then you expect the outputs of the device to come before the CLKIN in terms of phase based on feedback path. 

    I hope this clarifies it.

    Best,

    Asim

  • 0 in reply to Asim Khan
    TI__Expert 5655 points

    Thanks for the reply.

    So you mean that all the delay for each Y0, Y1, Y2, Y3 will be the same value. And that will be determined by the Yn average. Right?

    Regards,

    Ohashi

  • +1 in reply to Ryoma Ohashi
    TI__Genius 9260 points

    Hi Ryoma, 

    That is correct.

    Best,

    Asim

  • 0 in reply to Asim Khan
    TI__Expert 5655 points

    Hi Asim

    My customer has additional question.

     When Y3 connection destination is unused, if the Y3 load changes, it will lead the Yn average to change and all the phase for Y0~Y2 will change, right?

    From the 9.2 Functional Block Diagram, it doesn't look like that the load change in Y3 affect the phase for Y0~Y2.

    Could you explain why the average of Yn affects the delays and phases in Y0~Y3?

    Regards,

    Ohashi

  • 0 in reply to Ryoma Ohashi
    TI__Genius 10995 points

    Hello Ryoma,

    Asim will get back to you by this week.

    Best,

    Andrea

  • 0 in reply to Ryoma Ohashi
    TI__Genius 9260 points

    Hi Ryoma,

    Well this average is used to compensate for the phase or delay variation when there are different load on different outputs. 

    In a ideal condition, lets say you have same load on all the outputs, for example: 1pF on all outputs then the average values of these outputs would be 1pF as well 1+ 1 + 1+ 1 = 4pF / 4 = 1pF. So using this value on the feedback path which is CLKOUT load would result in CLKin and CLKOUT phase to be aligned or I would say zero delay. you can now chose or bigger or smaller value to adjust the Y0 output to come ahead CLKin or lag it.

    But if the load now varies between outputs where Y0, Y1 = 1pF and Y2, Y3 = 4pF. Now if you would select the extreme case (either 1pF or 4pF) then you would have extreme phase difference between Y0, Y1 and Y2, Y3 relative to CLKIN input. So we do an average to find a common ground to compensate for the extreme cases. Average helps us bring the phase difference between Y0, Y1 and Y2, Y3 down by some value.

    So the functional diagram is right based on external feedback for delay adjustment but if you have different loading then average would minimize delay difference or phase variation between all outputs as explained above. Its really difficult to do phase alignments when you have different loading conditions on all outputs. 

    I hope this clarify the average term used in datasheet.

    Best,

    Asim

  • 0 in reply to Asim Khan
    TI__Expert 5655 points

    Hi Asim,

    Thanks for the clarification. I got the point.

    So, with my understanding, I thought that in the case of Y0=Y1=Y2=1pF and Y3=0 (not used), we should use the average of Yn=1pF, not (1+1+1+0)/4=0.75pF. Is this correct?

    I thought that if all the Yn for used output is the same, we don't have to average it since all the delay/phase are the same. Is this correct?

    Regards,

    Ohashi 

  • +1 in reply to Ryoma Ohashi
    TI__Genius 9260 points

    Hi Ryoma, 

    Y3 still has internal output driver capacitance which needs to be considered. So that's why I suggested adding that when there is no load externally.

    Best,

    Asim