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ADS1298 - Actually wiring up leads for EKG reading

Eric Forkosh
Intellectual 275 points
Other Parts Discussed in Thread: ADS1298

Hi,

I've successfully used the ADS1298 to read an EKG, but I seem to be having a grounding issue. I hook up the positive and negative to EKG electrodes on my chest and then take a wire connected to my circuit's ground and connect it to the correct position on my chest. Here's the weird thing though - I have to connect the ground wire directly to my chest, meaning metal contact directly to my body. I can't connect the ground wire to an EKG electrode which is stuck to my chest. If I do connect the EKG electrode to the electrode ( not direct metal contact with my body), then I get a reading of all zeros on that analog channel. Direct metal contact with chest - good EKG. Electrode ground contact - all zeros. 

Measuring the electrode's resistance, it looks like each has a resistance in the single digit kiloohm range. The wires connecting to the positive and negative points on the analog channel have no series resistor on them.

Is this a normal grounding thing? Whats the correct way to do it?

  • 0
    TI__Guru**** 180931 points

    Hi Eric,

    We're on break for the Thanksgiving Holiday here in the US through Monday next week.  We'll get back to you on this question as soon as possible.

  • 0
    TI__Mastermind 43915 points

    Hi Eric,

    Assuming you have the proper patient protection circuitry, I would look into the connection of the RLD electrode. All of our testing and everything we recommend is with a patient simulator. If you are not using a patient simulator, I am limited where I can help. Hopefully other people on the forum can comment.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Intellectual 275 points

    Hi Tony,

    Thank you for the very quick reply!

    For patient protection circuitry, I just have a 100k resistor between the channel input and the electrode.

    I didn't actually do anything with RLD in my circuit, can you go into further detail on what I need to do with it? 

    On page 79 of the ADS1298 datasheet, it gives an example of what registers to set. Do I just connect RLDOUT to the body ( Right Leg) ?  What do I connect to RLDINV and RLDREF - do I just leave them floating? Is it recommended to use some sort of voltage divider on RLDREF or instead to just use RLDREF internal reference? Is the difference significant? 

    Right Leg Drive

    Sample code to choose RLD as an average of the first three channels.

    WREG RLD_SENSP 0x07 // Select channel 1␣3 P-side for RLD sensing

    WREG RLD_SENSN 0x07 // Select channel 1␣3 N-side for RLD sensing

    WREG CONFIG3 b␣x1xx 1100 // Turn on RLD amplifier, set internal RLDREF voltage

    As of right now all the registers are at their defaults (except that Internal Clock is enabled).

    Sorry for all the seemingly basic questions, I've read the datasheet over and over again but I'm still not 100% clear. Thanks again for the meaningful, clear replies!