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electrical grid noise (50 Hz) in ECG signal

Moran Shoeg
Prodigy 165 points
Other Parts Discussed in Thread: ADS1292, ENERGIA, ADS1298, MSP430F5529

Hi experts, 

I'm using ADS1292 with 1 lead and no RLD circuit connected. I noticed that lately when I preform an ECG check the signal is messed up, and doesn't look like an ECG at all. I also notices that when my body was connected to VCC and GROUND, the signal returned to be noise free.

Here is a screenshot of the signal samples coming directly from the ADS1292 without any filters. In the first half of the signal, my finger touched the VCC and the GROUND, and in the other half they did not.

I preformed FFT on the clean ECG signal and the noised ECG signal and  got these results:

My guess is that its the electrical grid noise, but why does it have harmonics?

Why is the noise canceled when my body is connected to VCC and GROUND, and is it a good solution?

If i'll connect the RLD, would it improve the results?

I'l appreciate any help.

Thanks,

Moran Shoeg 

  • 0
    TI__Guru**** 168605 points
    Hi Moran,

    Did you ever figure out where those 'glitches' were coming from? On this ADS1292 query, when you have essentially 'open' inputs to the ADS1292, it's not that surprising to see that you get a large noise content in your sample set. Picking up 60 Hz is also not that surprising as you can pick that up from equipment in the room. Have you taken a look at this application note yet?

    www.ti.com/.../sbaa188.pdf
  • 0 in reply to Tom Hendrick
    Prodigy 240 points
    Hi Tom,

    I have been following some of your previous posts on how to interface an ads1298 using SPI.I am having some problems with the same and I have posted a question in the forum with the title "Interfacing the MSP430f5529 MCU with ads1298 using energia".Could you please have a look and suggest where I may be going wrong?

    Thanks so much.

    Regards,
    Harsh
  • 0 in reply to Tom Hendrick
    Prodigy 165 points
    Hi Tom,
    Well, these 'glitches' happen, as you said, because of large delta in magnitude. When it get into the filter, its mess it all. The root of the large magnitude is error int the SPI reading which reflected in the status bits, but not always. Some time the status bits are not affected, but the sample does. To solve the problem, I ignored bad samples before filtering. Thank for the help in this issue! :)

    Anyway, I read the application note and connected the RLD circuit in my board, and I'm happy with the results.
    But still I would like to understand why the 50 Hz noise has harmonics?
    And why is the noise canceled when my body is connected to VCC and GROUND, and is it a good solution also?

    Thanks Tom,

    Moran Shoeg
  • 0 in reply to Moran Shoeg
    TI__Guru**** 168605 points
    Hi Moran,

    I'm happy to hear that you resolved the spike problem! I suspect the 50Hz is coming from your surroundings - it's most like source is the AC line. The harmonics show up because it's not a pure 50Hz source. When you have a proper signal applied to the leads, the pick up from ambient noise is reduced.