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ADS1292R Noise

Tacito Almeida
Prodigy 40 points
Other Parts Discussed in Thread: ADS1292R, ADS1292RECG-FE

I am testing the ADS1292R Demonstration Kit and I need help.
I am using the RA, LA and RL electrodes on my chest and the AFE registers are with the default configuration.
The problem is that the ECG signal is noisy, and when I touch the DB9 connector shield it gets perfectly clean.
Is there a mistake in the RLD configuration? Any ideas?
See the attached images.
Thanks.


 

  • 0
    TI__Guru 66166 points

    Hello Tacito - Welcome to our forum!

    I assume that the DB9 shield is connected to ground? I'm not sure why it makes sense to see a cleaner signal when you touch GND, to be honest. Perhaps your are not biasing the body correctly, as you mentioned.

    By default, the RLD amplifier is powered down. Also, none of the channels are selected for RLD derivation (RLD_SENSx = 0x00). RLD is needed to set the DC bias point of the body relative to the ECG subsystem. This ensures that your input signals are within the proper common-mode range for the PGAs. By default, the non-inverting input of the RLD amplifier (RLDREF) is internally set to mid-supply.

    To improve common-mode noise rejection, try selecting the channels connected to RA and LA by setting the appropriate RLDSENSx bits. This will connect the corresponding PGA output to the inverting input of the RLD amplifier.

    Do you have an RC feedback loop for the RLD amplifier (connected between RLDOUT and RLDINV)? If so, what is the closed-loop bandwidth? It should be greater than 50Hz/60Hz to help reject power line noise.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Hello Ryan,

    Thank you for your reply.

    I'm using the board from TI kit ADS1292RECG-FE without any modification.

    The DB9 shield is connected to ground through resistor R4.

    The default configuration for the ADS1292R version has the RLD amplifier powered on and also has both the positive and negative inputs from channel 2 selected for RLD derivation. It seems to be correct since for the ADS1292R, the channel 1 is used for respiration and channel 2 for ECG.

    The RC feedback loop for the RLD amplifier has R36 and C23.

    The situation is simple: I connect the LA, RA and RL electrodes (Covidien) to my chest, connect the board to the PC, open the GUI and check the signals. They are noisy. When I touch the DB9 shield or any ground point, the signal gets clean.

    See more images below: in the Scope Tab, the channel 1 is noisy. When I touch the ground it turns flat.

    What is wrong ?

    Best regards,
    Tacito

  • 0 in reply to Tacito Almeida
    TI__Guru 66166 points

    Thanks for the added info, Tacito.

    Can you run an FFT of the results and share them as well?

    Remember that Channel 1 is configured for Respiration measurements on the ADS1292REVM. The signal path includes multiple DC-blocking capacitors and a 10M resistor divider to set the DC common-mode. I would disable the respiration modulation/demodulation signals in the RESP1 register and focus on reading data only from Channel 2 for now. You can use Pin 1 of the DB9 for RA and Pin 3 of the DB9 for LA. This will connect your inputs to IN2N and IN2P, respectively.
     
    I still believe the noise you are seeing is likely 50/60Hz noise. The ADS1292R EVM does not automatically filter this noise out. You can apply a high-order notch filter in the GUI to a set of data points after the collection is finished to view the effects of the filter. If possible, you can try running the PC off the battery supply.

    Let me know if this helps.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Ryan,

    After removing the DC plug from the laptop everything was resolved.
    Thank you for the support.

    Best regards,
    Tacito