• State TI Thinks Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 1 answer
  • Subscribers 47 subscribers
  • Views 284 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS5474: Harmonics query

ArTzy
Genius 15349 points
Part Number: ADS5474

Hi Exeprts,

We have a question about ADS5474 from customer:

Scheme consist of signal generator -> ADS5474 -> FPGA kintex7 xc7k325t.

I drive AIN_p and AIN_n signal with power -26 dbm, freq 100 khz, plot fft from ila data and where is no 2 - 3 harmonic, snr -80 dbm. I increased power to -25 dbm, and after fft i saw level of 2 - 3 harmonic -50 dbm. Level of 2 harmonic from generator - 75 dbm, 3 harmonic -80 dbm. In datasheet level of 2 harmonic is -89 dbm, 3 harmonic -93 dbm.

So from what source harmonic appears and how can fix them?

Thank you.

Regards,
Archie A.

  • 0
    TI__Guru* 91947 points

    Archie,

    Is the input a differential sine wave at 100KHz? What is "no 2 - 3 harmonic"?  Is this HD2 and HD3?

    Regards,

    Jim

  • 0 in reply to jim s
    TI__Guru* 91947 points

    Archie,

    Most likely as they are sweeping the input power up in small steps they will encounter the boundary between the pipeline stages of the ADC and this will result in a degradation in SFDR if the part has been trimmed with a residual interstage gain error.  

    One solution for this part would be to apply some dither to the input.

    Regards,

    Jim    

  • 0 in reply to jim s
    TI__Mastermind 25980 points

    Some other thoughts, is the customer is using our EVM?

    is the customer filtering the signal generator that is providing the 100kHz signal?

    Regards,

    Rob 

  • 0 in reply to jim s
    TI__Genius 15349 points

    Thank you, Jim and Rob.

    Customer responded:

    jim s said:
    Is the input a differential sine wave at 100KHz?

    Yes, generator have two outputs, one generates just a simple signal, another generates same signal, shifted 180 degrees. Сable length is the same. 

    jim s said:
    What is "no 2 - 3 harmonic"?  Is this HD2 and HD3?

    Yes, i mean HD2 and HD3.

    Rob Reeder said:
    Some other thoughts, is the customer is using our EVM?

    No, i am using custom board.

    Rob Reeder said:
    is the customer filtering the signal generator that is providing the 100kHz signal?

    No, there is no additional signal filtering.

    Attaching the schematics and graphs with spectrum. The red line is signal with power -26 dbm, the blue line is signal with power -25 dbm. What is remarkable the form of harmonics like sinc function.

    Regards,
    Andris/Archie A.

    power.png

    input and clock.png

    harmonics.png

  • 0 in reply to ArTzy
    TI__Mastermind 25980 points

    Hi Archie,

    The harmonics are coming from the signal generator if they are not filtering.

    Can you please ask what is the model number of the sig gen? or is this a function generator...?

    Thanks,

    Rob

  • 0 in reply to Rob Reeder
    TI__Genius 15349 points

    Hello Rob,

    The customer is monitoring this post, below is his response:

    Rob Reeder said:
    Can you please ask what is the model number of the sig gen? or is this a function generator...?

    This is function generator AM300 R&S. I measured HD2 and HD3 on FSW R&S.

    Regards,
    Andris/Archie A.

  • 0 in reply to ArTzy
    TI__Mastermind 25980 points

    As I suspected, unfortunately, this is a poor choice to use a function generator for a 14b ADC evaluation.

    Please use a filter on the output of the signal generator and rent/but/borrow a better signal generator like the SMA100A with option B22 in order to evaluate the converter properly.

    Regards,

    Rob