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Bias Shield Drive

Sean Grech
Prodigy 50 points
Other Parts Discussed in Thread: ADS1299

Hi,

I was hoping someone could help me regarding driving a bias signal for actively biasing my EEG electrodes using the RLD circuit. I am currently using the ADS1299EEG-FE Rev. B EVM, which doesn't have the components required to drive the shield. The following are my questions:

1) Is a buffer required in the first place, or can I simply connect the shield to the bias output directly? Why?

2) If I do require buffering, does the buffer have the be inverting? i.e. 180 degrees shift? Why?

Thanks,

Sean

  • TI__Mastermind 38745 points
    Hey Sean,

    1) The buffer is not required. The footprint was placed for convenience. You may want to buffer it in case you wish to source more current than what the ADS1299 is capable of. If you just plan on using it purely as a patient bias, you will not need to buffer it.

    2) If you do, you'd likely want a non-inverting configuration since you would just be using the buffer to supplement current.

    Regards,
    Brian Pisani
  • in reply to Brian Pisani
    Prodigy 50 points

    Hi Brian,

    Thanks for your reply. What I wish to know however is if a buffer is required to use the bias signal specifically for shielding the electrodes, not for providing bias to the subject via a dedicated electrode (which I am already doing).

    Regards,

    Sean Grech

  • in reply to Sean Grech
    TI__Mastermind 38745 points
    Hey Sean,

    It would depend what the equivalent capacitance of the shield is. If the shunt capacitance is too large then you could potentially see some stability issues with the bias amplifier since it's not meant to drive a large capacitive load. The only way to know for sure would be to test it. Alternatively, many designers tie the shield to ground to achieve a sort of Faraday cage effect.

    Regards,
    Brian Pisani