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Electrode impedance measurement with ADS1299

Mohammad Behfar
Prodigy 110 points

Other Parts Discussed in Thread: ADS1299

Hi, 

I am currently using ADS1299 in my project for EEG signal acquisition. My challenge is how to measure the electrodes' impedance through AC lead off detection. 

Is it possible to continuously measure the electrodes' impedance without using any external current source since in the data sheet (page 50) it is said that for continuous lead-off detection it is necessary to externally apply an out-of-band ac current source or sink to the input?

Another question is that in case of continuous impedance measurement, how it is possible to distinguish between the output data caused by impedance measurement (feeding exciting current) and output data originated from EEG signal?    

 Regards, 

Mohammad.

  • 0
    TI__Guru 69106 points

    Hi Mohammad,

    Welcome to the TI E2E forums!

    One of the ways that the ADS1299 can achieve Lead-Off Detection is by using an internal excitation signal. This signal is generated by settings you can control in the LOFF: Lead-Off Control Register. Such available settings include:

    Bits[1:0] FLEAD_OFF[1:0]: Lead-off frequency

    Bits[3:2] ILEAD_OFF[1:0]: Lead-off current magnitude

    However, I believe this only serves for the One-time/Periodic, in-band option.

    For continuous AC Lead-Off Detection, as you mentioned, the datasheet only mentions the use of an external AC current source at an out-of-band frequency. Please allow me to confirm this with a colleague and provide a more definite answer on that tomorrow.

     

    Regarding your other question, the user can distinguish between the output EEG data and the excitation signal in post-processing. Implementing these algorithms will filter out the unwanted frequency content. When this excitation signal is outside the bandwidth of the EEG (typically 0.5Hz - 50Hz), it can be easily filtered from the output conversion result of the ADC with minimal impact of the original signal integrity.

    I would also recommend taking a look at this article for an overview of Lead-Off Detection in ECG for more information: http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=sbaa196 .

     

    Thanks and regards,

  • 0 in reply to Ryan Andrews
    Prodigy 110 points

    Hello Ryan, 

    Thank you for your response and also the article.

    I am waiting for your answer about my second question.

    Regards,

  • 0 in reply to Mohammad Behfar
    TI__Guru 69106 points

    Hi Mohammad,

    After speaking with my colleague, I understand more clearly what the ADS1299 will be capable of affording you for AC lead-off detection.

    You can use AC Lead-off detection in a continuous fashion with the internal excitation signal source alone. In the LOFF Register section on Pg. 43 of the datasheet, you'll see that you are able to select different frequencies for the FLEAD_OFF signal. The 7.8Hz and 31.2Hz options are within the typical EEG bandwidth. While you could use these frequencies for continuous monitoring, I would not recommend doing so because they will interfere with the EEG data that shares the same frequency components, and you won't be able to distinguish the lead-off signal from the EEG signal in post-processing. Instead, these frequencies should be used for one-time or periodic monitoring.

    The 4th option allows you to select an AC lead-off frequency of f(DR)/4. As an example, if you are sampling at 500sps, your f_lead-off would be 125Hz and safely outside the EEG bandwidth. This will allow you to easily filter the lead-off signal in post-processing while continuously monitoring lead-off status. The only benefit to an external excitation source that I can see right now would be the ability to choose lead-off signal frequencies even further outside the bandwidth of the EEG signal.

     

    Let me know if you have any more questions!

    Regards,

     

  • 0 in reply to Ryan Andrews
    Prodigy 110 points

    Hello Ryan, 

    Thank you very much ! it helped me a lot so far.

    I will contact you if I have further questions in future.

    Regards, 

    Mohammad.