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Original question:

ADS1299: Electrode impedance measurement with ADS1299

ADS1299: Can I engage Bias loop which reduces common mode noise in Impedance measurement mode.

Swapniil Kanwar
Prodigy 160 points
Part Number: ADS1299

I am using ADS1299 for my experiments in EEG. In normal EEG function, I have engaged by corresponding PGA output to bias amplifier through BIAS_SENSP register so that all the common mode noise gets suppressed. Now when I have to do impedance measurement, I need to engage a current source at a channel and let it sink in the BIASOUT pin as described in the article here: http://eeghacker.blogspot.com/2014/04/openbci-measuring-electrode-impedance.html 

I have a doubt in whether to engage BIAS_SENSP here or not? What will happen if I do so? 

Thanks 

  • 0
    TI__Mastermind 19815 points

    Hi,

    Our company blocks the website you posted. Would you mind print/save the website as a PDF and share it here?

    Are you trying to do 9.3.2.4.3 Lead-Off Detection? If yes, may I ask which method you are thinking of implementing?

    9.3.2.4.3.1 DC Lead-Off  has 1. Pull up/down method or 2. DC current method

    or

    9.3.2.4.3.2 AC Lead-Off (One Time or Periodic)  3. AC current Lead Off method

    Are you also using the BIAS_SENSP and/or BIAS_SENSN registers and BIASOUT pin and the to drive the Unit Under Test?

    The patient bias signal is used for biasing the patient to set the common mode of the EEG signals (typically mid supply).

    Datasheet Figure 35. Lead-Off Detection suggests use BIASOUT pin to drive UUT, so it's suggested to use BIAS_SENSP and/or BIAS_SENSN registers and BIASOUT pin for appropriate sensitivity and/or specificity.

    Customer will need to try and test with different settings/combinations to meet the desired results for their products&systems.

    May I ask do you happen to have/use an evaluation board/kit(EVM)?  It's suggested to use EVM to do some tests and evaluation.

    Thanks.

  • 0 in reply to ChienChun Yang
    Prodigy 160 points

    Here is the link to the PDF of the website: https://drive.google.com/file/d/16sv6C5gZdbk_eggHlaSAwmFk00amN0W7/view?usp=share_link 

    I am using 9.3.2.4.3.2 AC Lead-Off (One Time or Periodic). I am using only BIAS_SENSP, it is set to 0x00 as I have ADS1299-4. But when I use EEG functionality I put Bias_sensp as 0x0F to have all the channel noise minimised. But I am not sure can we use bias in AC lead off detection. 

    I don't have evaluation board so I have made my own custom PCB based on datasheet. 

  • 0 in reply to Swapniil Kanwar
    TI__Mastermind 19815 points

    Hi,

    Based on the datasheet, it doesn't mention to avoid/leave out the BIASOUT when using 9.3.2.4.3.2 AC Lead-Off.

    So, this leaves to customers/developers to decide whether to use BIASOUT or not.

    In fact, it's suggested to use the BIASOUT as mentioned in the earlier/previous reply.

    So, customers could try/test how the AC Lead-off detection works(in terms of sensitivity and stability) as when you attach or detach the BIASOUT electrode/patch from UUT and/or turning On/Off the BIAS Amp.

    Ideally and theoretically, BIASOUT could help the sensitivity and/or stability.

    But, again, TI leaves above for customers to try, experiments, tests, analysis before making decisions as it depends on many various external factors.

    Thanks