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PGA970: pga970 primary dac

Cor
Intellectual 375 points
Part Number: PGA970

Hi E2E guys,

The PGA970 primary wafeform Dac is 14 bit. If the dac value is greater than about 0x170a the output, even without any load,

is distorted. with 14 bit I would expect 0x3fff as maximal value for the DAC. Is the DAC not truly 14 bit?

Kind regards Cor

  • 0
    TI__Mastermind 31655 points

    Hi Cor,

    How are the waveform gain settings set up? Where is this scope capture taken? Is this directly at the waveform DAC output, or after the waveform gain at the P1/P2 outputs?

    Regards,

  • 0 in reply to Scott Cummins
    Intellectual 375 points

    Hello Scott,

    Herewith I send you the settings. I could not see any difference in the signal shape if a change any of the settings except that the signal is smaller or larger.

    You van also see the end of the table with 0x170A being the highest value. With 14 bits I would expect 0x3FFFbeing the highest value.

    I also have added a new screenshot from the scope : ch1: PI, Ch2: P1, Ch3: P2

    So PI and P1 are still clipping. Another observation is that in the evm software it is not possible to generate a table with a higher WFM Amplitute than 365mV (which is 0x12B0 or 4784dec. With this limited table there is no distortion or clipping on the signal. But the values are then just about a quarter of the maximal resolution (2^14 =16384).

    Kind regards,

    Cor

  • 0 in reply to Cor
    Intellectual 375 points

    Hello Scott,

    Is there something I could measure or should I use another setting to improve the signal?

    Kind regards,

    Cor

  • 0 in reply to Cor
    TI__Mastermind 31655 points

    Hello Cor,

    Please check the DAC offset value in the WAVEFORM_DAC_OFFSETx registers. If the offset is not 0 than the output will clip below 0x3FFF accordingly. If you haven't already, I also recommend looking at the datasheet section 7.5.6, Turning on the Waveform Generator. This provides a more systematic approach to configuring the waveform generator including the specific registers, which are a bit obfuscated in the GUI labeling.


    Regards,