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ADS1292: Do I need AC coupling in my inputs?

Ernesto Gonzales
Prodigy 225 points
Other Parts Discussed in Thread: ADS1292

Hello,

I designed a board with the ADS1292 low-pass filtering the 4 input electrodes. Also, I'm RLDxP and RLDxN bits to 1. When I used the chip in continuous mode, I noticed that there was a large varying offset in the order of mV (my input signal is between 25uV to 100uV, using it for EEG stuff).

Will putting a bypass capacitor before the input low pass filter help? if so, why? because I think that'd eliminate the RLD biasing on the patient.

Thanks for your help!

  • 0
    TI__Mastermind 38745 points
    Hey Ernesto,

    Are you using a RL electrode to bias the patient? If you place a capacitor in series there will be no need to bias the patient anymore since that will effectively block any DC content from entering the inputs.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 225 points
    Indeed, I'm using an RL electrode to bias the patient. But then, if I place the cap and get rid of the RL electrode, the signal will be biased at 0V, right? wouldn't that be a problem?
  • 0 in reply to Ernesto Gonzales
    TI__Mastermind 38745 points
    Hey Ernesto,

    You can bias it at some other DC voltage with a voltage divider. The result would be that your channel outputs would be centered at zero, but that is not a problem. I would also be careful to warn you that a DC blocking cap and a voltage divider form an RC HPF with a cutoff; below which signal content will be lost.

    All of that being said, you could also filter out the offset digitally. As long as your inputs are not drifting out of the common mode range for the device, you can always run your data through a high-pass filter in real time to center it at zero then you can run all of your algorithms.

    Brian