ADS1294: Recording Electrode Impedance with DC-Lead-Off circuit and pull-up/down resistors

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testcycle

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Part Number: ADS1294

Hi,

I need to record Electrode Impedance values, not only on or off state. I would like to implement the DC-Lead-off algorithm and calculate the impedance from the output value. I'm doing this by the following registers:

LOFF 0x13

LOO_SENSP 0x07
LOO_SENSN 0x07

This enabled lead off detection with DC-Lead-Off circuit and pull-up/down resistors on Channels 1,2,3. Now the differential voltage should correspond to the Impedance between two electrodes. The DC Offset will change. Therefore i calculate like given here: https://www.elektroniknet.de/elektronik/halbleiter/erkennen-von-kontaktfehlern-bei-ekg-geraeten-170157.html 

 R_in = (2*V_DC*R_Pullup) / (2*V_ref - V_DC)

Is this workflow correct? Is there another possibility to easily record the impedance value?

Thank you!

• ADS1294

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testcycle

• Jun 3, 2020 8:53 AM
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• Mastermind 46465 points

  

  Ryan Andrews

  

  • Jun 5, 2020 8:56 PM

  Hello,

  Thank you for your post.

  Yes, the DC offset voltage will change due to the differential impedance between two ADS1294 input pins. This impedance is the sum of passive components in the signal chain, the electrode cable, and the electrode-to-skin contact impedance. Please note that the absolute accuracy of the current sources and pull-up/down resistors is not very accurate. Some initial calibration can be done to improve your impedance measurement results. 

  The only alternative for you to determine the electrode impedance is the AC lead-off excitation current, but the principle is the same. Both must be analyzed in post-processing.

  You may find this external publication useful:

  

  Regards,

  ---

  Ryan Andrews

  Applications Engineer | Precision ADCs 

  Are you working on a bio-potential application? Check out these helpful resources: ADS129x BIOFAQ | ECG Online Training
  

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