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ADS129x - how to interface to eeg caps?

Bruce McMillan
Intellectual 310 points
Other Parts Discussed in Thread: ADS1298, ADS1299

Hi. Maybe I am missing something.. EEG caps have 19+ "active" electrodes, a "reference" electrode (which can be earlobe), and a "ground" electrode (which can be fed with a DRL signal. In my experience, the 19 channels are arranged with differential inputs, with the N input of all 19 channels commoned together with the Reference electrode as input. Note that this arrangement is not single-ended inputs.

So I was dismayed when I find that the multiplexer does not allow "reference" (or "N") inputs to be linked together internally  (whilst still having the P input active. The internal switches are there - just not provided as one of the configuration alternatives. Dismayed because this is the way almost all our clients use our devices!

Happy to be corrected if I have misunderstood, else please include this config option in future releases of the device.

tks bruce 

  • 0
    TI__Guru**** 180751 points

    Hi Barry,

    You did not miss anything, the ADS1298 does not provide the ability to 'short inputs' between channels (your common "N" input).  You would need to rout the reference channel to each INxN pin.  We implemented this on our ADS1298EVM by means of a shorting block at the board input terminal.

  • 0 in reply to Tom Hendrick
    Intellectual 435 points

    Hi,

    you said that ADS1299 don't have ability for shorting all N inputs (INxN) in one point. What is with SRB1? In data sheet there is multiplexer scheme for connecting all INxN inputs to SRB1 and then you can use SRB1 as reference electrode. I'm I right or wrong?

    Thanks and regards,

    Vojin Ilic

  • 0 in reply to Vojin Ilic
    TI__Mastermind 43915 points

    Hi Vojin,

    The SRBx lines are used to allow you to route a common signal to all the inputs. One thing to keep in mind is that when you do this, you are going to have mismatching between your (+) and (-) inputs which may cause a higher 50/60Hz component to show. However, 50/60Hz is going to be present in the EEG application regardless of how you configure the inputs, so a notch filter in post processing is going to be required.

    Regards,

    Tony Calabria