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Massive mains noise on ads1294 reading

Lukeiy
Prodigy 190 points
Other Parts Discussed in Thread: ADS1294

I'm having trouble with the ADS1294 on my board measuring large amounts of mains hum in the signal. As you can see in the image below, the ECG is superimposed on top.

My configuration is as follows:

Two lead set up according to this:

with Rpullup = 3Mohms, Cblock = 1uF.

  • LP mode, 250 SPS
  • Internal reference buffer enabled, VrefP = 2.4 V, RLDREF = 1.2V and enabled, RLD sense disabled
  • PGA gain = 1

I have another possibly related issue where the RLD does not pull the leads to 1.2 V. I'm quite sure commands are going through because I can adjust the sample rate, but I'm probing it now and the voltage on the leads is basically still 0.

Cheers for your help

  • 0
    TI__Mastermind 38745 points
    Hello Lukeiy,

    Are you providing an external RLDREF = 1.2V? Otherwise, it should be equal to (AVDD+AVSS)/2. For example, if your power supplies are set to +/- 2.5 V, 0V would be an expected voltage for RLDOUT.

    Is the interference you are seeing at the utility frequency? Power supply coupling through electrodes is a major source of interference for ECG applications. If C_Block and R_Pull-up are not matched very well between the inputs, it could result in very poor CMRR at the inputs to the device. What tolerance is on those components?

    Regards,
    Brian
  • 0 in reply to Brian Pisani
    Prodigy 190 points

    The RLD problem is solved, I was sending the command too soon after powering on the device so adding a delay fixed that. The noise is at utility frequency. The capacitors have 10% tolerance and the resistors 1%, I'll keep that in mind to go for higher tolerances on revision 2. Thanks for your help.