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ADS1299 Measure CMRR from Raw Data

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

Hi, 

 I'm trying to measure CMRR of our application with ADS1299. (from raw data)

  Based on the data sheet of ADS1299, page 20, I'm driving input signal of sine-wave amplitude in CM range (500mV) with frequency 50Hz.

 Then, my question is where should I put the ground of waveform generator ? 

  For our application, we use single-ended, P- as reference and N- as channel-input. Firstly, I shorted out the P- and N- drive signal with waveform generator putting ground in BIASIN (same application as Figure 47 of ADS1299 data sheet). Then, I'm getting pretty bad CMRR, as well as power line noise and harmonics. 

 If I'm not using the bias-drive, then where should I put ground of waveform generator ?

  I believe bias drive further improves CMRR by rejecting remaining common-input of Bias Op-Amp (output of PGA)

  • 0
    TI__Mastermind 38745 points
    Hey Jason,

    For a CMRR measurement, you should connect the ground of the waveform generator to AVSS and then offset your waveform to mid-supply for the ADS1299. The positive output of the waveform generator should then be connected to the shorted inputs to the device. Then the CMRR can be measured as the peak of the output referred to the input divided by the magnitude of the input waveform.

    I should also tell you that the BIASIN pin on the ADS1299 is used to measure the voltage of the bias drive output. If the setting in the CHnSET register is for normal electrode input, the BIASIN pin will be internally disconnected from the channel.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 175 points
    Thanks Brian, I'm on it. I will see if it works..

    By the way, as I am trying to plot "THD FFT" and "FFT" as Figure 12, 13 of ADS1299 Data sheet (page 12), and in order for having strong back-data on operation of developed device,

    can I have information about the setting parameters used for those plots ? For example, what kind of programming language ? Is it in MATLAB or Python ? what kind of windowing is used ? window size ? FFT size ?

    Thank you.
  • 0 in reply to Jason Kim
    TI__Guru 66466 points

    Hi Jason,

    The raw data was acquired and processed in a LabVIEW-based characterization program that we have. For the 500SPS plot, we acquired 8,192 points. For the 16kSPS plot, we acquired 16,384 points. Both plots apply a low sidelobe window.

    Best Regards,