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ADS1299: AC Lead Off / Impedance Measurement

Part Number: ADS1299

Hello,

We want to use the ADS1299 in our design for it's Impedance Measurement feature.


Actually, the design would have 2 differential channels and a reference electrode.

The Vin- of each channel must be kept separated.



2 requirements for the impedance measurement :

- We want to do impedance measurement on each individual electrode, before the acquisition begins;

- We want to do impedance measurement while the acquisition is running, minimally the combined impedance of both probes in a channel.



Can the ADS1299 do this without any external component ?

Any idea, what could be done with the reference electrode to measure it's impedance ?



See the pictures below...






  • Hi Stephane,

    Thanks for your post!

    You caught us at the end of the day so please give us until Monday to respond!

    In the mean time, please consider searching the forum, datasheet, and EVM user's guide! 

  • Good,

    I have checked but where seem to be some contradictions about this,  mostly with the single electrode measurement possibility, with internal current source.

    Most EEG discussions talk about grouping negative electrode, but I cannot in my case.


  • Hello Stephane,

    Thank you for your post, and please excuse the delay in our reply.

    There are quite a few posts on this subject already, but I’ll try to address a couple points in your question to start. First, in order to do any impedance measurements, the ADCs will have to be converting. The impedance calculations are done by converting the raw ADC output codes to volts and then algebraically solving for the electrode impedance. Only the DC lead-off comparator outputs can be read from the device registers without the ADCs converting.

    It is possible to do both single-ended and differential impedance measurements in the manners you described. If only one side of the lead-off current is enabled, the return current will flow through the BIAS amplifier output.

    A couple things to keep in mind that affect the accuracy of these impedance measurements:

    1. The tolerance of the current sources is only given as +/- 20%
    2. The DC offset voltage of each channel may account for a large percentage of the impedance measurement error. It is recommended to calibrate this offset error first using an internal short measurement (especially for nA of lead-off current).


    Best Regards,
  • Hello,

    What about the possibility to measure the reference electrode, Z5,  in my drawing ?


    Thanks

    Stephane

  • Hello Stephane,

    Measuring the BIAS electrode impedance (Z5) is also possible, but it's not straightforward with the available options in the ADS1299.

    I've been in contact with your local TI FAE Dmitry via email. Would you mind if I added you to the conversation using your E2E profile email address?

    Best Regards,
  • No problem to add me !