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ADS1299 GND and BIAS electrodes

Florencia Grosso24
Prodigy 70 points
Other Parts Discussed in Thread: ADS1299

Hi,

We have built an EEG acquisition board using an ADS1299, based on the design you propose on the application notes of the ad converter + the kit. It works pretty good an we are able to acquire signals correctly.

We only have one issue about it: at the beginning of our testing stage, we noticed that our measurements had too much noise and we could't read the signals quite well. We decided to place the board inside a metal case connected to GND. Since this wasn't enough, we did some research and found out that having the subject under test making direct contact with the case helped a lot getting the correct signal. We've been doing this for some time but we really need to make our device independent of it. In our current configuration we are using two channels in differential mode and a GND electrode (besides from the hand).

What we suspect is that the size of the GND electrode is not being big enough to make its purpose, and therefore we need to use a bigger surface (the hand, in this case). We are not using BIAS electrode, which as I understand should help to increase the rejection to common mode signals. Can this be contributing to our high noise scenario? or is it just that a common electrode with 0.5cm diameter doesn't provide enough contact surface for a GND connection?

Note: My colleague Brian has already posted a question about using BIAS electrode and he was advised that it wasn't of much help when working with few electrodes.

Thanks for reading this!

Best,

Florencia Grosso

  • 0
    TI__Mastermind 47595 points
    Hello Florencia,

    Thanks for your post and welcome to the forum!

    I believe that using a BIAS electrode would certainly help to reject this noise.

    Where are the differential electrodes connected?
    Are they close to the GND electrode?
    Would the noise coupling onto the body have to go past/through the differential electrodes on it's way to the GND electrode?

    I agree with the theory that a larger GND electrode surface area should help, but I am not sure by how much - somewhere in between having a smaller GND electrode and the patient being in direct contact with the chassis GND. If using a BIAS electrode is out of the question then this would be a worthwhile experiment.