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DS250DF810: Typical VOD and FIR Values

Guru 29690 points
Part Number: DS250DF810

Hi Team,

The datasheet Table 2 shows Typical VOD and FIR Values.
Do you have a table that shows the following?

1) PRE-CURSOR=0, MAIN-CURSOR=0, POST-CURSOR=-1, -2, -3, ..., -31
2) MAIN-CURSOR=0, POST-CURSOR=0, PRE-CURSOR=-1, -2, -3, ..., -31

My customer wants to know the table for their evaluation with DS250DF810.

Best Regards,
Yaita

  • Hi,

    We do not have a table with these VOD vs FIR values, as these FIR settings per items 1 and 2 in your post are not valid. To obtain a valid waveform the retimer FIR main tap weight must exceed both the pre-cursor and post-cursor tap weights.

    Regards,

    Rodrigo Natal

    HSSC Applications Engineer

  • Hi Rodrigo-san,

    Thank you for your reply. I understand.
    Please give us further support on FIR filter coefficients.

    The datasheet shows the following about RpstdB.
    RpstdB = 20 * log10 (v1 ⁄v2 )

    If PRE CURSOR is 0, I believe the relationship between output amplitudes (v1, v2) and FIR filter coefficients (MAIN CURSOR, POST CURSOR) are as the following.

    v1 = MAIN CURSOR - POST CURSOR
    v2 = MAIN CURSOR + POST CURSOR

    Is this correct?
    If the answer is yes, there is mismatch between the calculation result and RpstdB value in Table 2.
    ----------------------------------------------------
    - PRE CURSOR=0, MAIN CURSOR=18, POST CURSOR=-1
    calculation: 0.97 (dB)
    Table 2 in datasheet: 2.1 (dB)

    - PRE CURSOR=0, MAIN CURSOR=17, POST CURSOR=-2
    calculation: 2.05 (dB)
    Table 2 in datasheet: 2.5 (dB)

    - PRE CURSOR=0, MAIN CURSOR=16, POST CURSOR=-3
    estimation: 3.30 (dB)
    Table 2 in calculation: 3.1 (dB)
    ----------------------------------------------------

    Best Regards,
    Yaita

  • Hi Yaita-san,

    I don't understand your calculation. How are you determining the v1 and v2 values in milli-volts?

    Regards,

    Rodrigo Natal

    HSSC Applications Engineer

  • Hi Rodorigo-san,

    Thank you for your reply. I may have confused myself..

    Is it possible to calculate v1~v8 from FIR filter coefficient (PRE CURSOR, MAIN CURSOR, POST CURSOR)?
    If the answer is yes, could you share the calculation formula?

    Best Regards,
    Yaita

  • Hi Yaita-san,

    You may roughly estimate the voltage levels resulting from FIR coefficients by applying a linear approximation. But the accuracy will vary considerably depending on the VOD level. I would recommend to use the datasheet VOD table values (which come from measurements) as your main reference.

    • 35mV is the approximate LSB step size for the FIR taps i.e. approximate Pre-, main-, post-cursor LSB in Volts when taking the max VOD  and dividing by number of steps
    • The VOD for the case of C(0)=C(-1)=C(+1) is 205mV
    • With the assumptions above you obtain the equation below
      • v = 35*(coefficient sum) + 205mV
    • Let's apply this equation to a couple of datasheet examples, to see how the estimated values compare with the measured values

    Datasheet cases

    FIR SETTINGS

    Peak-to Peak

    VOD(V)

    RPRE(dB)

    RPST(dB)

    PRE-CURSOR: REG_0x3E[6:0]

    MAIN-CURSOR: REG_0x3D[6:0]

    POST-CURSOR: REG_0x3F[6:0]

    0

    +13

    -6

    0.960

    NA

    5.8

    0

    +23

    -4

    1.165

    NA

    2.2

    Estimated values

    • C(0)=13, C(+1)=-6, C(-1)=0
      • v1 = 35*(19) +205 = 870mV
      • v2 = 35*(13) +205 = 660mV
      • Rpost = 20* log(v1/v2) = 20*log(927/699)~ 2.45dB
    • C(0)=23, C(+1)=-4, C(-1)=0
    • v1 = 35(27) + 205 = 1,150mV
    • v2 = 35(23)+ 205 = 1,010mV
    • Rpost ~ 1.12dB

    As per above the estimated VOD value are close, but the Rpost is being under-estimated relative to the measurement.

    Regards,

    Rodrigo Natal

    HSSC Applications Engineer

  • Hi Rodrigo-san,

    I appreciate for your kind support!
    I would like to discuss further.

    1)
    I believe coefficient sum of v1 and v2 is the following. Is it correct?
    coefficient sum of v1: C(-1) + C(0) - C(+1)
    coefficient sum of v2: C(-1) + C(0) + C(+1)



    If the answer is yes, your v1 is correct however v2 isn't correct.

    2)
    I would like to confirm about "Peak to Peak VOD(v)".
    Regarding datasheet, Peak to Peak VOD(v) is v7-v8 and not v1, is it correct?



    Best Regards,
    Yaita

  • Hi Yaita,

    In the conceptual waveform v1 corresponds to the post-cursor amplitude. So it corresponds to C(0) + C(+1).

    In the waveform example v7 - v8 corresponds to VOD. VOD is determined by the full FIR coefficient sum = C(0) + C(-1) + C(+1)

    Cordially,

    Rodrigo Natal

  • Hi Rodrigo-san,

    >In the conceptual waveform v1 corresponds to the post-cursor amplitude. So it corresponds to C(0) + C(+1).
    Does it mean the following your calculation is incorrect?
    *C(0) + C(+1) = 13 + (-6) = 7, but the following shows 19.
    ----------------------------
    ・C(0)=13, C(+1)=-6, C(-1)=0
     ・v1 = 35*(19) +205 = 870mV
     
    ・v2 = 35*(13) +205 = 660mV
    ----------------------------

    I believe calculation of coefficient sum for each v1~v8 is the following.

    v1:  C(-1) + C(0) - C(+1)
    v2:  C(-1) + C(0) + C(+1)
    v3: -C(-1) + C(0) + C(+1)
    v4: -C(-1) - C(0) + C(+1)
    v5: -C(-1) - C(0) - C(+1)
    v6:  C(-1) - C(0) - C(+1)
    v7: -C(-1) + C(0) - C(+1)
    v8:  C(-1) - C(0) + C(+1)




    Best Regards,
    Yaita