high impedance with cup electrodes? poor SSVEP results. [resolved]

HasniHasni Pakistan
edited July 19 in Electrodes

I have used silver and gold cup electrodes and also bought a fresh Ten20 Paste still my electrode impedence is 100k to 500k.. Why it is too high ???

What is the best way to check impedance ? (Maybe i am using wrong method to check)

Board Used: Cyton
Project : SSVEP Data Collection

Comments

  • wjcroftwjcroft Mount Shasta, CA

    Hello Hasni,

    You can check the validity of your impedance measurements, by using a glob of paste to connect one cup (say channel 1) with reference (SRB2), and include Bias as well. When you click on the 'ohm' [Ω] sign for that channel. Result should be close to zero ohms. This would verify your cabling and Cyton operation.

    William

  • retiututretiutut Louisiana, USA

    You can only check impedance on one channel at a time for Cyton due to a limitation of the ADS1299 chip, as it injects a small amount of current into the channel. Doing this for multiple channels will distort the impedance readings.

    Take Care,
    RW

  • HasniHasni Pakistan

    @retiutut said:
    You can only check impedance on one channel at a time for Cyton due to a limitation of the ADS1299 chip, as it injects a small amount of current into the channel. Doing this for multiple channels will distort the impedance readings.

    Take Care,
    RW

    Thankyou for response.. i did same as you told... still i am getting 30k to 50k ohms impedance... where could the possible error ??
    I use two earlobes (for SRB and BIAS... is this right ??/)
    i am not getting any prominent ssvep frequecy peaks in my spectrum

  • retiututretiutut Louisiana, USA

    @wjcroft I think @hasni is close. Can you recommend or rephrase a test that can be done to ensure the board is functional?

  • HasniHasni Pakistan

    @retiutut said:
    @wjcroft I think @hasni is close. Can you recommend or rephrase a test that can be done to ensure the board is functional?

    @Hasni said:
    I have used silver and gold cup electrodes and also bought a fresh Ten20 Paste still my electrode impedence is 100k to 500k.. Why it is too high ???

    What is the best way to check impedance ? (Maybe i am using wrong method to check)

    Board Used: Cyton
    Project : SSVEP Data Collection

    For basic tests .... i** always start with eye blink test and Jaw Clench test... they come out to have good results** in Open BCI GUI Plot
    But SSVEP is never recognized or never seen in my plots even after processing in MATLAB
    Electrodes I am using for SSVEP : O1 O2 Oz PO3 PO4
    BIAS and Referance are on Earlobes

  • retiututretiutut Louisiana, USA
    edited July 19

    @hansi Please, let's focus on getting good impedance signals first. Thank you for the additional information though.

    Try checking impedance on the forehead: Fp1 and Fp2 first. These sites do not normally have hair.

  • HasniHasni Pakistan

    @retiutut said:
    @hansi Please, let's focus on getting good impedance signals first. Thank you for the additional information though.

    Try checking impedance on the forehead: Fp1 and Fp2 first. These sites do not normally have hair.

    sure brother... i will check and let you know... so you mean FP1 and FP2 should show impedance less than 5k ? while hairy scalp has more than 5K ? (for my case 30k to 50k)

  • retiututretiutut Louisiana, USA

    Fp1 and Fp2 should have the lowest impedance. Also, can you do this in the OpenBCI GUI send a screenshot of the GUI before and during impedance check, so I can have a look at the raw signals.

  • HasniHasni Pakistan

    sure brother... i will share with you...

  • wjcroftwjcroft Mount Shasta, CA

    Did you try the test mentioned? This verifies that your wiring and connections are all good.

    You can check the validity of your impedance measurements, by using a glob of paste to connect one cup (say channel 1) with reference (SRB2), and include Bias as well. When you click on the 'ohm' [Ω] sign for that channel. Result should be close to zero ohms. This would verify your cabling and Cyton operation.

  • HasniHasni Pakistan

    @wjcroft said:
    Did you try the test mentioned? This verifies that your wiring and connections are all good.

    You can check the validity of your impedance measurements, by using a glob of paste to connect one cup (say channel 1) with reference (SRB2), and include Bias as well. When you click on the 'ohm' [Ω] sign for that channel. Result should be close to zero ohms. This would verify your cabling and Cyton operation.

    Yes i got exactly zero...

  • wjcroftwjcroft Mount Shasta, CA

    Read this regarding why you may be having difficulty with SSVEP. It's because your screen flashing is not 100% accurate. Some tests are mentioned that you can perform with a photodiode and provided program.

    https://openbci.com/forum/index.php?p=/discussion/3052/not-getting-expected-results-from-vep-visual-stimuli-eeg-data-resolved

  • HasniHasni Pakistan

    Thanku> @wjcroft said:

    Read this regarding why you may be having difficulty with SSVEP. It's because your screen flashing is not 100% accurate. Some tests are mentioned that you can perform with a photodiode and provided program.

    https://openbci.com/forum/index.php?p=/discussion/3052/not-getting-expected-results-from-vep-visual-stimuli-eeg-data-resolved

    Thankx

  • wjcroftwjcroft Mount Shasta, CA

    These videos are helpful, in showing how cups/paste are placed on scalp areas with hair. The hair in those areas should be pushed aside, so that the paste has optimal direct contact with skin. If this push-aside is not done, you will have higher impedance.

    https://openbci.com/forum/index.php?p=/discussion/176/placing-electrode-sensors-on-the-scalp

  • wjcroftwjcroft Mount Shasta, CA

    Read this regarding how the high amplifier INPUT impedance (1 gigaohm), means that skin impedances in the ranges you mentioned, are not actually an issue of any importance.

    https://openbci.com/forum/index.php?p=/discussion/comment/13328/#Comment_13328

  • HasniHasni Pakistan

    Thankx alot brother> @wjcroft said:

    Read this regarding how the high amplifier INPUT impedance (1 gigaohm), means that skin impedances in the ranges you mentioned, are not actually an issue of any importance.

    https://openbci.com/forum/index.php?p=/discussion/comment/13328/#Comment_13328

  • HasniHasni Pakistan
    edited July 20

    @retiutut said:
    Fp1 and Fp2 should have the lowest impedance. Also, can you do this in the OpenBCI GUI send a screenshot of the GUI before and during impedance check, so I can have a look at the raw signals.


    this is of impedence at FP1


    This is when sitting idle with electrode at FP1


    This is pic showing mulitple eye blinks at FP1

  • wjcroftwjcroft Mount Shasta, CA

    Why is your notch filter set at 60 Hz, when your mains power frequency is 50 Hz?

  • HasniHasni Pakistan

    I just needed impedence check..that why i didnot set the filter of 50hz

  • retiututretiutut Louisiana, USA
    edited July 20

    Signals look acceptable. Would have been better to look at the noise floor with 50Hz Notch filter.

  • wjcroftwjcroft Mount Shasta, CA

    The impedance sensing mode of the ADS1299 injects a small current at 31 Hz, which you can see on the FFT.

    But... I don't believe the algorithm / signal processing that displays the impedance on the Time Series graph, is bandpass filtering at 31 Hz. Hence, any other strong signals, such as your 50 Hz mains, will distort the actual calculated impedance value. Making it seem higher than it really is.

  • HasniHasni Pakistan

    Ok got you point thankyou

Sign In or Register to comment.