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ADS1299 Evaluation Board Impedance Measurements

Benjamin Madison
Prodigy 40 points
Other Parts Discussed in Thread: ADS1299

We are developing long term EEG electrodes and are looking to test them for impedance characteristics. We have an ADS1299 Eval Kit, and I believe I read just about everything I can find on using the device for testing but can not get results that seem logical.

I am currently connecting the INP & INM connections of a single channel thru a resistor. I enable the LOFFP & LOFFN for that channel and then acquire data. I'm using 7Hz & 6nA current. But I am not seeing the correct voltage. 

Can anyone point me in the right direction here?

  • 0
    TI__Mastermind 38745 points
    Hey Benjamin,

    What resistance value are you using? Also what does the output data look like (i.e. frequency, amplitude)?

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 40 points

    Brian,

    Using 7Hz and 6nA with both the LOFFP & LOFFN on for a single channel, and a 5k resistor between the channel inputs we are seeing Vpp ranges of 15-30 millivolts.

  • 0 in reply to Benjamin Madison
    TI__Mastermind 38745 points
    Hey Benjamin,

    The current source used for ac lead-off detection is not very accurate. On page 4, it specifies the accuracy as +/- 20%. This feature is really only designed for detection rather than accurate impedance measurement. If you're having trouble discerning between lead-on and lead-off states, I'd recommend increasing the magnitude of the current source by configuring bits [3:2] in the LOFF register.

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 40 points
    The current source isn't accurate and does have variance but you can work backwards from what you have connected in order to determine the actual current being driven from the LOFF source. From here you can then find decently accurate impedance measurements.

    The issue we're having is getting the configuration correct in order to measure this impedance. The manual discusses the topic but has no real detail on how it is actually performed.

    The plan is to connect an electrode to the + side of a channel & enable LOFF current here, a second electrode will be placed on the - channel without LOFF current enabled. Then the reference (RLD) electrode will then be connected to the ref input on the ADS1299 dev board. From here though I can not seem to discern what other settings I must make to properly get a measurement of voltage at the - channel input for impedance calculations. Do I need the ref buffer on? Off? Any board jumper settings I am missing?
  • 0 in reply to Benjamin Madison
    TI__Mastermind 38745 points
    Hey Benjamin,

    Measuring the impedance like you are suggesting can only be performed on a per channel basis. That means you will have to enable the LOFF current source on the negative input to create a loop. From there, you collect voltage data for that channel like you would in a typical acquisition, but to discern impedance information, you will have to band pass filter the data for that channel at the lead-off current source frequency and divide the peak measured voltage by the current source magnitude.

    Regards,
    Brian Pisani