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ADS1293: AC Lead off at 267 samples per second ECG sampling.

Gary Holder
Prodigy 70 points
Part Number: ADS1293

Tool/software:

Hello,

I am using the ADS1293 as a two channel ECG with RL drive. I am currently sampling at 267 samples per second and the digital filters 3db cutoff are at 55Hz. 

I am exploring using the AC lead off detection. I am wondering if there is any way to set the AC frequency above my Nyquist frequency and be able to filter out the AC Lead off drive from the ECG signal. 

Or do I need to set the AC frequency below my Nyquist and filter it out with my own digital filter post sampling of the ECG? 

I was thinking if the AC drive clock and the sampling clock are in sync, It would be possible to filter the AC by sampling at a multiple of 267 Hz. 

Do you have any recommendations for what type and what settings to try for detecting lead off. 
What have you found works best for gel type electrodes on the chest. 

  • 0
    TI__Prodigy 640 points

    Hi Gary,

    Thanks for reaching out! Expect a delay in response as Ryan is currently out of office.

    Regards,

    Shane

  • 0
    Prodigy 70 points

    I have been testing more with AC lead off. It does seem to interfere with the ECG signal. Since my sample rate is 267 I tried a AC LOD frequency of 78HZ and a drive current 0f 209na and the noise was greater than my ECG signal. The LEAD off function worked great at these settings when applied to me,
    I am just wondering if AC lead off is only intended as a spot check for lead connection or is there a way to do AC lead off and not interfere with the ECG signal. 

    Back in the old days I used to design ECG circuits and the best lead-off detection scheme was using a high frequency like 32 KHz for respiration and lead off. At higher frequencies you are allowed to drive the patient with more than 10ua. If I remember correctly, I used about 30 or 40 ua at 32kHz and my loose lead was set down to 2Kohms. 

    With the ADS1293 AC LOD set to 78 HZ and 209na drive, it seems my LOD is around 390K ohms. 

    Just looking for some guidance on what is the best option for continuous LOD.