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ADS127L11EVM-PDK: Unexpected Aliasing frequency

Part Number: ADS127L11EVM-PDK

Hi team,

My customer is evaluating ADS127L11EVM and got unexpected aliasing frequency with hundreds Hz error.

What kind of cause can you think of for this phenomenon?

ex.)

OSR:32

Vin: 2.5 Vp-p

Input Frequency: 300kHz

expected aliasing frequency: 75kHz

->Displayed in FFT with Analog calculator: 75.348 kHz

 

Best regards,

Yuto

  • Hello Yuto-san,

    I assume the clock frequency is 24MHz and you are using the wideband filter?  Can you provide a copy of the exported data?

    I cannot think of a reason for this, unless the signal is actually noise and not the aliased version of the input.  Looking at Figure 8-11 in the datasheet suggests the aliased signal should be attenuated by about 88dB.  Since the input is one half of full scale (FS=5Vpp), this would translate to an amplitude of about -94dBFS.

    Regards,
    Keith

  • HI Keith,

    Thanks for the support.

    They are using 24MHz for clock and the filter type is wide.

    I will send yo the raw data of time scale and FFT data exported by analog calculator via email.

    Please refer that and answer here.

    Best regards,

    Yuto

  • Hello Yuto-san,

    I ran the data through the Analog Engineer's calculator, and the aliased signal is about 75.35kHz.  Is it possible the customer's signal source is not set to exactly 300kHz?  I will research this a little more to see if I can reproduce these results in the lab.  Please give me until the end of the week to reply to this post.

    Regards,
    Keith

  • Hello Yuto-san,

    I plan to take additional measurements early next week.  However,  I did take some initial data using an input frequency of 305kHz and 301kHz, with Vin=2.5Vpp and the same EVM settings. There are many intermodulation terms, as well as multiple harmonics from the signal generator, but I was able to find and measure the alias (70kHz at 305kHz input and 74kHz at 301kHz). In both cases, the frequency measured was higher than expected by 2Hz, which is within the tolerance of the ADS127L11EVM clock accuracy.  Also, in order to measure these lower levels, I had to take 1 million samples to lower the noise floor enough to see the alias signal.

    At this point, I do not think the 73.35kHz tone in the customer data is an alias signal of the input, but some other noise term, possibly from the signal generator.

    Regards,
    Keith