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ADS1298: Electrode Impedance Measurement

Tommy Cooper
Prodigy 50 points
Part Number: ADS1298

I would like to measure the electrode impedance for all 8 channels using the AC Lead Off function.  I have done some preliminary measurements and it looks feasible but there are accuracy and resolution concerns..  I would like to verify that the electrode impedance is below 5k ohm.  Reducing the series "Patient Protection Resistor"  would allow for more resolution/accuracy(?) in the measurements.  I am aware of the patient risk current requirements.  Is the need for this resistor related to a single fault failure in the ADS1298 that would result in AVDD or AVSS being connected to the patient input at the ADS1298?  If so, are there ways to limit the current without such a high series resistance?  Or, are there ways to limit the risk current without putting a high value resistor in series?

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    TI__Guru 57306 points

    Hi Tommy,

    Thanks for your post and welcome to our forum!

    The lead-off current sources in the ADS1298 are not intended for accurate impedance measurements. This is why we only spec their typical accuracy as +/-20%. Instead, their purpose is to allow you to detect relative changes in the electrode impedance throughout the measurement. There really is no way to improve that tolerance with external components. Also, keep in mind that the AC lead-off function will measure the impedance of each lead (INxP - INxN), not the individual electrodes themselves.

    You are correct about the patient protection resistors - I believe some standards even require double-fault protection. There are protection methods to clamp the voltage on those pins and prevent them from exceeding the supplies (i.e. in the event of a defibrillation pulse), but the protection resistors themselves should be designed to handle the current from a direct connection to AVDD and AVSS.

    Best Regards,