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Original question:

LMX2572: Sub-Fractional Spurs variation

LMX2572: MASH_SEED setting

Koshi Ninomiya
Expert 4105 points
Part Number: LMX2572

Hi Team,

My customer is evaluating LMX2572 on their ES board, and has a question about MASH_SEED setting.

Q1: They currently use the MASH_SEED feature.
       If they do not use the MASH_SEED feature by MASH_SEED_EN=0h of Register R37, dose the LMX2572 set optimal value of MASH_SEED automatically?
       Or, dose the LMX2572 use any default value of MASH_SEED?

Q2: MASH_SEED feature is used for noise shaping, can it be set not to use this?
        Probably using MASH_SEED will suppress noise, but they want to know if it can be set.

Thank you.

Best Regards,

Koshi Ninomiya

  • 0
    TI__Guru* 81360 points

    Hi Ninomiya-san,

    You can write something to MASH_SEED but not using it. For example, MASH_SEED=100, MASH_SEED_EN=0. In this case, MASH_SEED is not used.

    Right, MASH_SEED can be used to reduce spurs due to the MASH engine. However, the same MASH_SEED value may not apply to another part. That is, part to part variation makes MASH_SEED inconsistent. 

    Silicon default MASH_SEED value is 0. After the part is powered up, if you do not write something to MASH_SEED, its value will remain 0.

  • 0 in reply to Noel Fung
    TI__Expert 4105 points

    Hi Noel-san,

    Could you let me confirm about the MASH_SEED value?
    You explained as follows,
       "Right, MASH_SEED can be used to reduce spurs due to the MASH engine. However, the same MASH_SEED value may not apply to another part. That is, part to part variation makes MASH_SEED inconsistent. "

    What kind of part to part variation makes MASH_SEED value inconsistent?

    Thank you.

    Best Regards,

    Koshi Ninomiya

  • 0 in reply to Koshi Ninomiya
    TI__Guru* 81360 points

    Hi Ninomiya-san,

    MASH_SEED uses the sigma-delta modulator to shift output signal phase with respect to the input reference, the phases of the spurs sources will also be affected. In cases where MASH_SEED makes the same-frequency spurs out-of-phase, the spurs amplitude is minimized. However, spurs amplitude varies from part to part because the degree of crosstalk and the phases of the spurs sources may be different. As such, the same MASH_SEED value may not be able to apply to every chip.