Electrode compatibility, amplification and filtering

guestguest
edited August 2014 in Electrodes
Hello,
Can I use any passive gel electrodes to use with the system ?
Is there any need for pre-amplification and filtering of the biosignals as they are in the order of microvolts?

I had a look at the <a href="http://connectedcatmedia.com/phpBBdemo/files/ADS_V2-2-Sch.pdf">schematics</a>; and found that only one RC filter is used before giving the input to ADS1299. So is there any amplification done before giving the brainwaves to the ADS1299 ?

Thanks in advance.

Comments

  • chipaudettechipaudette
    Hi,

    As one who's been working with the early prototypes, I believe that you can use any EEG/ECG gel with OpenBCI. I happen to use Ten20 paste (for paste) and Electro-Gel (for gel). I haven't tried any others, but as long as they are conductive, they should work.

    As for additional amplification, no, none is needed. The ADS1299 has a built-in amplifier and a built-in 24-bit ADC so that it can acquire microvolt EEG signals without any additional hardware.

    Thanks for checking us out!

    Chip
  • guestguest
    Hello Chip,
    Thanks for the response. This is interesting. I was checking the other EEG projects and all of them were using many filter stages (to avoid GSR, HF interference and supply line noise) and multiple amplification stages. Is there any software filtering used for openBCI other than the <code>LPF(fc=6.8KHz!)</code> in the input stage ?
    thanks
  • wjcroftwjcroft Mount Shasta, CA
    Neuron, hi. The ADS1299 uses a Delta Sigma analog to digital converter, which has the big advantage of not requiring a lot of external components such as preamps or filtering. It does have a good onboard Bias generation, which cancels out most 60 Hz line interference, as well as other common mode signals.

    The RC LPF you see is covered in the TI data sheet, it performs EMI and "anti-aliasing" functions. The 'sinc' digital filter inside the chip has 'holes' that it cannot cover starting at 1 MHz, and then going up at 1 MHz intervals. The RC LPF blocks all of that, so is essentially an EMI filter. Sinc filter takes care of the rest.
  • guestguest
    William Croft,
    Thanks for the detailed response. Now it is clear that the external amplification and filter stages were eliminated because of the benefits from Delta sigma ADC.
  • chipaudettechipaudette
    Great (and truthful) reply. You guys are awesome. OpenBCI participants are the best!

    Chip
  • abheekboseabheekbose Singapore
    edited September 2018
    Hi William

    Regarding the Anti-Aliasing functions in the ADS1299. I wanted to know if the following is possible without any additional hardware:

    1. The signals we want to record are of max freq. of 500Hz.
    2. Accordingly, we are using Wifi Module to Sample at 1000Hz (Niquist Freq)
    3. Say there is a noise at 600Hz. This will be then be folded to form an alias of 300Hz

    In the case above - will the ADS1299 anti aliasing functions prevent this from happening?
    If not what would be the suggested solution?

    Appreciate your inputs in this.

    Thanks & Cheers
    Abheek
  • wjcroftwjcroft Mount Shasta, CA
    Abheek, why not just experiment to see what is the highest sample rate you can support. So 1K Hz is working for you? DId you try 2K Hz? If you reach a sample rate that the wifi cannot support, you could also try comparing results, by recording to the SDcard.

    There are no other external components that are appropriate to add before the analog to digital converter.

    I gave you several other related EMG sample rate links to look at, did you examine those? One link compared various high sample rates for EMG and found them unnecessary. 

    Regards,

    William

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