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Original question:

ADS1299-4: Regarding measurement of EEG using ADS1299 AFE

ADS1299-4: Noise in all channels

Swapniil Kanwar
Prodigy 20 points
Part Number: ADS1299-4
Other Parts Discussed in Thread: ADS1299

Hi, 

I have a conceptual doubt regarding the EEG circuit .

I have made circuit using ADS1299, same as given in datasheet Figure 73. I have made BIASREF_INT to (AVSS+AVDD)/2 and connected all BIASP, BIASN open. I am using SRB1 as the reference electrode for all channel. BIAS circuit was enabled with Rf=1Meg Ohm and Rp=47k ohm. 

The thing I noticed is that when all channels are kept open in air, touching one channel is showing noise in all channels (with the touched channel having maximum noise). Shouldn't it be like the one which is touched should have noise rest other remain unaffected. 

To be sure if one channel introducing noise to all, I shorted channel 2 to 8 with SRB1 and BIAS which was again shorted with circuit ground 0V. Now only channel 1 IN1P is open in air. This time I can only see noise in channel 1 and rest other channel were close to 0V with negligible noise as compared to channel 1. 

So overall, why noise in any one channel in air causing noise in other channels too when all channel in air, but not when other channels shorted to some voltage. 

Thanks

  • 0
    TI__Mastermind 19815 points

    Hi,

    Sorry for the delay. I will need more time to look at this and will get back to you on 3/9.

  • 0 in reply to ChienChun Yang
    TI__Mastermind 19815 points

    Hi

    Do you happen to have/use a ADS1299 evaluation kit/board(EVM)?

    It's suggested to use EVM with an ECG or EEG simulator to do some basic experiments&tests&evaluation.

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

    For, "connected all BIASP, BIASN open."

    There is no definite way to connect or disconnect BIASPx or BIASNx and the Rf and Rp as these depend on the product designs, electrodes, patches, cables, etc and various factors. i.e. developers&designers will need to tune those based on experiments&tests, data collection&analysis and do some comparison and V&V before making decisions.

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

    The way to observe the noise is to do internal short tests.

    Not sure what you mean by "touching one channel "

    Touching by hand(s) or body contact? EMI picked by body may propagate/affect the chip no matter what channel and could cause the chip to be unstable.

    And, when open-in-the air the voltage is unknown/floating(i.e. could be high, could be low, could be changing), so it could be any unexpected noises. 

    " I noticed is that when all channels are kept open in air, touching one channel "   what are the noises when you just keep them floating and not touching?

    Do you have any ESD strap and/or wrist band?  Maybe you can do the same experiment and compare between wearing it and not wearing it?

    further, you can try with and without using BIASOUT to contact the Unit Under Measurement.

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

    "I shorted channel 2 to 8 with SRB1 and BIAS which was again shorted with circuit ground 0V. Now only channel 1 IN1P is open in air. This time I can only see noise in channel 1 and rest other channel were close to 0V with negligible noise as compared to channel 1. "

    When doing internal or external short, the voltage now is known to be bound or have some bound around some expected voltage/potential, and differential/instrumentation amplifier help to subtract the common mode noise out to reduce the noises.

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

    NOTE: TI DOESN'T RECOMMEND USE EVM TO CONTACT or INTERACT WITH ANY ANIMAL SUBJECT(S)

    Thanks

  • 0 in reply to ChienChun Yang
    Prodigy 20 points

    Hi, 

    Thanks for the reply. 
    "Touching by hand(s) or body contact? EMI picked by body may propagate/affect the chip no matter what channel and could cause the chip to be unstable." This statement makes sense. But shouldn't it make only that particular channel unstable and rest channel to behave as they were behaving before touching by hand/ any body contact. 

    The thing I am noticing is that touching my finger in one channel is causing a huge noise in all channels. 

    Noise when all channel kept open in air: 

    Noise when touching only channel 1 with my finger:

     
    Notice that all channels getting the 50Hz noise whereas I was expecting this in channel 1 only as rest channel to be as in previous image. 

  • +1 in reply to Swapniil Kanwar
    TI__Mastermind 19815 points

    "

     But shouldn't it make only that particular channel unstable and rest channel to behave as they were behaving before touching by hand/ any body contact. 

    The thing I am noticing is that touching my finger in one channel is causing a huge noise in all channels. 

    "

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

    IC could be very complex and sensitive, large noise or any excessive unknown voltage in one channel could affect other channel(s).

    And, be aware human body may carry many charges that could potentially damage the IC.

    Does the body probably grounded and/or bias?

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

    And, again,

    The normal/proper way to observe the noise is to do inputs internal and/or external short tests.

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

    Directly body contact/touching on any part of the chip is not recommended as a normal operation/usage.

    NOTE: TI DOESN'T RECOMMEND USE EVM TO CONTACT or INTERACT WITH ANY ANIMAL SUBJECT(S)

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

    Thanks