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ADS1294: How to suppress line frequency interference?

Cherry Zhou
Mastermind 22235 points
Part Number: ADS1294

Hi team,

Using the ADS1294 as a dual-channel electromyography acquisition, to increase the input impedance, an instrumentation amplifier, AD8222, was added to the front end, 100 times amplified and single ended to the ADS1294. The detailed circuit diagram is as follows:

Measurement result: In indoor office environments, line frequency interference is severe and the signal cannot be measured without the RLD electrode connected. The interference improves a lot with the RLD electrode connected, but it still exists, much like the one using the myoelectric differential-mode signal. However, when measuring outdoors, it basically meets the customer's requirements.

The customer is assuming that the RLD amplifier gain setting is not sufficient to completely cancel line frequency interference. The test circuit is battery powered and the MCU captures the ADS conversion data and sends it to the PC via the serial-to-USB cable. Formal products also include those sent to a PC via a wifi module, which means that there is no cable connection. 

(The Chinese text in the figure above represents the electromyography, the common-mode noise, the common-mode noise)

Could you help check this case? Thanks.

Best Regards,

Cherry

  • 0
    TI__Mastermind 19815 points

    Hi,

    May I ask which customer is this? If not allowed to disclose in public, I can reach out to you via email.

    -------------------------------------

    Some comments and suggestion.

    What was the injecting signal sources to the input pins?  in other words, are the electrodes floating or connect to an ECG simulator or a human subject?

    Can customer do some more tests?

    1. try ADS129x internal short test to observe whether the interference are there?

    2. try ADS129x internal test signal to observe whether the interference are there?

    3.

    "additional instrumentation amplifier, AD8222" is not suggested in a reference design-

    Has customer tested without the "additional instrumentation amplifier, AD8222"?  Did such interference present without the AD8222? 

    I see many capacitors around AD8222, a slightly mismatch or tolerance difference could degrade the CMRR. 

    4. how are the ECG cables and wires? are ECG cables and wires properly shielded?

    5. regarding to "The interference improves a lot with the RLD electrode connected", which makes sense. Customer could try toggle different combination of the RLD_SENSP and RLD_SENSN switches as well as the RC at the RLDOUT to see whether it helps improve.

    If the signal source are from human subject, is the skin condition good? any hair, fur, dead skin and/or too dry?

    Noise and interference are very design, peripherical parts and environment dependent(mentioned and not mentioned above), so customers need to tweak here and there, above and not limited to above based on the designs, parts and environments, to make the product less interfered/more immuned.

    -------------------

    By the way, How does customer know the noise/interference is Common Mode noise? and what is x-axis unit shown in the plot?

    Thanks