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ADS1299: Invalid EEG Data from ADS1299

Pradeep Nalawade
Prodigy 20 points
Part Number: ADS1299

Hi,

We are using ADS1299 in one of our projects to capture EEG signals. We are not getting the proper data from the ADS1299. Normally the EEG data will be very low in uV, but we are getting the data in the range of 50 to 100mV when gain is set to 1. When gain is set 24 we are getting the data in the range of 1.5 to 2.2 V. We have connected the bias out signal to the bias electrode, SRB2 to the reference electrode and ADS1299 first four channels to the EEG electrodes.

Please find below the values of the configuration registers of ADS1299,
CONFIG1 = 0b10110110
CONFIG2 = 0b11010000
CONFIG3 = 0b11111100
CHnSET = 0b00001000
BIAS_SENSP = 0b11111111
BIAS_SENSN = 0b11111111
MISC1 = 0b00000000
LOFF = 0b00000010
Other registers kept in there default state.

Please help us to resolve the issue.

Please find below the schematic file.


 

Thanks,

Pradeep Nalawade,

Team Lead,

Funambolo Technologies Pvt. Ltd. Bangalore

  • 0
    TI__Guru**** 176671 points
    Hi Pradeep,

    Welcome to our e2e Forum! There is nothing in your ADS1299 schematic that jumps out to me as being 'wrong'. Can you tell us if everything works properly when you enable the internal test signals? If that test works properly, can you possibly provide details on your layout?
  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    Hi Tom,

    Thanks for your reply.

    I have tested the internal test signal. It works properly.

    Please find below the layout details.

    Thanks And Best Regards,

    Pradeep Nalawade

  • 0 in reply to Pradeep Nalawade
    TI__Guru 64316 points

    Hello Pradeep,

    Thanks for the update.

    If the internal test signal is measured with the correct amplitude (1.875 mVp or 3.75 mVp multiplied by the PGA gain), then the issue must be with the external signal itself. What are you using for a signal source? Have you tried using a function generator?

    A simple test would be to apply a 100 mVp sine wave to SRB2 with a mid-supply offset voltage. Be sure to share the function generator ground with the ground on your board.Tie the INxN pins to mid-supply as well. Each channel should then see a 100 mVp sine wave (times gain) at the output centered around 0 V.

    I have a few additional comments about your schematic:

    1. The BIASOUT signal is currently shorted to BIASIN and BIASINV is left floating. Was this intentional? BIASIN is used to route the BIAS amplifier output into the input MUX. From there, the BIAS output can either be measured on the one of the channels against BIASREF, or it can be routed to one of the external input pins. Are you doing either of these options?
    2. The 1 M || 1 nF feedback components should be connected between BIASOUT and BIASINV (the inverting BIAS amplifier input). Currently, the BIAS amplifier does not have a closed feedback loop.
    3. The START pin should either be tied to DVDD or DGND with a resistor. Are you using the START SPI command to control conversions? If yes, then the START pin should be tied low. Otherwise, tie START high.

    Regarding the register settings, you probably don't want to select the unused input pins from Channels 5 - 8 for BIAS common-mode derivation. The settings would then be BIAS_SENSP = 0x0F and BIAS_SENSN = 0x0F.

    FYI - if you only need 4-channels, there is an ADS1299-4 that we released last year with equivalent performance, features, and footprint.

    Best Regards,