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Noise on ADS1292

viney chaddha1
Expert 1280 points
Other Parts Discussed in Thread: ADS1292

Brian,

Off late I noticed a lot of noise on the output of the ADS1292 and tried everything possible to overcome that. I changed the setting in the RLD_SENS register to 0x2D to give theRLD drive from all the three electrodes hoping that the increase in the RLDOUT would compensate for the noise. Everything in terms of ambient was same as compared to the time I was getting clean output. Then I checked the resistance of the lead which had marginal increase of 1 Ohm but I did change the lead and got the clean output again. Would the 1 Ohm increase in resistance of the lead make so much of a difference, given the fact that there is a 100K Rp in series with it.

While searching for the solution I also came across a post of yours which said that one should have the a cap of 25.5 nF put in parallel to the Rp to get a 60 Hz cutoff. Can"t i change the cap between the RLDOUT and RLDINV to get the 60 /50 Hz cutoff rather than having it configured as a LPF of 106 Hz? Could you please explain that too.

Regards

Viney Chaddha

  • 0
    TI__Mastermind 38745 points
    Hey Viney,

    The cutoff for the amplifier should exceed 50/60 Hz since those are among some of the frequencies you would want the amplifier to actively cancel. Limiting its bandwidth beneath that would reduce the amplifier's ability to produce active cancellation. In any case, is the noise you are seeing clearly 50/60 Hz or does it look like broadband noise?

    What do you mean when you say the lead's resistance increased? Are you referring to the actual electrode?

    Regards,
    Brian
  • 0 in reply to Brian Pisani
    Expert 1280 points
    Brian,
    The peak in FFT is at 50 to 51.5 Hz. By lead resistance I mean the ECG leads connecting the electrode to the ADS using DB9 connector.

    Coming to the RLD amplifier. The 1M Ohm and 1.5 nF cap in parallel across the RLDOUT and RLDINV would give the frequency bandwidth of 106 Hz, correct me if I am wrong here. Between the RL electrode and the RLDOUT there is a 100K resistor. If I connect a 47 nF cap across that the pole/zero is at 50 Hz, Can I not do that at the amp level itself? I have not understood that part well.
    Regards
    Viney Chaddha
  • 0 in reply to viney chaddha1
    TI__Mastermind 38745 points
    Hey Vinney,

    Is the capacitor shunting the output of the RLD amplifier?

    Regards,
    Brian Pisani
  • 0 in reply to viney chaddha1
    TI__Guru 71526 points
    viney,
    Did you get this resolved?
    ~Leonard