• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 46 subscribers
  • Views 1590 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.

ADS1299 EVM CMRR measurements.

Jason Kim
Prodigy 175 points
Other Parts Discussed in Thread: ADS1299

Hi, 

  For ADS1299 EVM, how does CMRR is measured exactly ?  Referring to formulation of CMRR in dB 20log(A_d/A_cm), how does common mode gain is measured for computation ?

 I can only think of measuring twice, one with differential mode and another with common input mode, than combining them to compute CMRR. But, on ADS1299 EVM, it picks up CMRR from FFT analysis window. Then, does that mean CMRR is only computed based on FFT from raw data ? 

 I would appreciated if anyone can give me any reference to read.

Thank you.

  

  • 0
    TI__Mastermind 38745 points
    Hey Jason,

    It is computed correctly only if you apply a specific test. Namely, short the inputs and swing them together with a magnitude you specify and the FFT analysis will be computed as 20*log(magnitude you specified/highest magnitude in the FFT). The assumption would be that the common mode would peak over the noise a little and show up as the highest component in the FFT. You have to specify the magnitude since otherwise the EVM would have no way of knowing what the input common mode magnitude was. Hopefully I explained it clearly enough. The CMRR calculation will not really mean anything on a normal acquisition where the inputs are not shorted.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 175 points
    Hi, Brian, thank you as always :)

    So, then, when I'm measuring SNR with single-ended mode, CMRR number on the FFT analysis window is meaningless.. ? (neither inputs are shorted nor magnitude is not specified..)

    Am I getting it right ? Then, to measure CMRR of each channel, I would need to drive known signal meets input common-mode range(page.20 of ADS1299 data sheet) , using waveform generator. ?
  • 0 in reply to Jason Kim
    TI__Mastermind 38745 points
    Jason,

    You've got it. If you think about it, there's really no other way to measure CMRR with the current setup. When designing the EVM software, it didn't make sense to make another window just for CMRR when we had an entire window devoted so performance measurements in the first place. I apologize since that is a bit confusing. Your feedback is good for helping us when we design new EVMs!

    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 175 points
    Thanks Brian.

    Then, on ADS1299 Data sheet page 3, it states that "CMRR is measured with a common-mode signal of AVSS + 0.3 V to AVDD – 0.3 V. "

    What does that mean ? Is that value for AVSS and AVDD purely just set for the chip verification only ?

    (I'm use +-2.5 V as AVDD and AVSS respectively.)


    Thank you.
  • 0 in reply to Jason Kim
    TI__Mastermind 38745 points
    Hay Jason,

    The datasheet does not specify a supply configuration for that measurement. What that line is indicating is that the peak-to-peak voltage of the signal applied to shorted channel inputs used for the test measured from AVSS + 0.3 V to AVDD - 0.3 V. In the case of a +/-2.5V supply configuration, that means the common mode signal spanned from -2.2 V to +2.2 V pk-pk. If the supply configuration was 0-5 V, the signal would have spanned 0.3 V to 4.7 V pk-pk.

    Regards,
    Brian Pisani