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ADS1299EEGFE-PDK: collision between the acquisition function and the electrode-off detection function

Part Number: ADS1299EEGFE-PDK
Other Parts Discussed in Thread: ADS1299

Hello,

One customer used the ADS1299 for his application, when debugging the ADS1299, there was a collision between the acquisition function and the electrode-off detection function, which showed that when he  turned on the electrode-off detection function, the CMRR fell from 120dB to around 90dB. What is the problem?

Note: 500Hz sampling rate, 2.048Mhz  external clock.

Best regards

Kailyn 

  • Hi Kailyn,

    Is the customer using the ADS1299EVM?

    It would be nice if the customer could provide some screen shots or waveform to clarify what they mean by " collision between the acquisition function and the electrode-off detection function, which showed that when he  turned on the electrode-off detection function"?

    And, how did they measure the CMRR? Do they 1. inject any known signal, or 2. use the internal test signal, or 3. short the channel inputs externally, or 4. short the channel inputs internally?

    Are they using 1. DC resistor lead off detection, or 2. DC current lead off detection, or 3. AC current lead off detection? Suggest them try different lead detection methods and observe whether such or similar "collision" still exist?

    Please note ADS1299 datasheet section 

    9.3.2.4.3.2 AC Lead-Off (One Time or Periodic)
    In this method, an in-band ac signal is used for excitation. The ac signal is generated by alternatively providing a
    current source and sink at the input with a fixed frequency. The frequency can be chosen by the
    FLEAD_OFF[1:0] bits in the LOFF register. The excitation frequency is chosen to be one of the two in-band
    frequency selections (7.8 Hz or 31.2 Hz). This in-band excitation signal is passed through the channel and
    measured at the output.
    Sensing of the ac signal is done by passing the signal through the channel to be digitized and then measured at
    the output. The ac excitation signals are introduced at a frequency that is in the band of interest. The signal can
    be filtered out separately and processed. By measuring the magnitude of the output at the excitation signal
    frequency, the electrode impedance can be calculated.

    Thanks

  • Hi,
    Since I did not hear back from you, I believe my suggestions answered your questions.
    I will close this post and if you have any pending questions, feel free to post them here or open a new thread.
    Thanks and have a great day!