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ADS1299: Internal Bias not at 2.5v

JAGAN KB
Prodigy 50 points
Part Number: ADS1299

Hi,

I'm designing an EEG acquisition system with ADS1299 with unipolar power supply. I'm using the internal bias with internal reference. I have connected the bias electrode to the patient. When I tried getting EEG signals with wet electrode, the waveform seem to get clipped. upon measuring the potential at bias_out pin I found it out to 5.04v instead of the mid supply 2.5v. I have also tried using ADS1299 FE kit and configured the device to internal bias with internal reference and there too i'm getting the same 5.04v. my question is whys is this happening?

  • 0
    TI__Mastermind 47595 points

    Hi Jagan,

    Can you provide the register settings that are being used? 

  • 0 in reply to Alexander Smith
    Prodigy 50 points

    Hi,

    Theses are the register settings I have used.

    Bias Signal Source = BIASREF = (AVDD+AVSS)/2

    BIAS BUFFER = ENABLED

    BIAS MEASUREMENT = OPEN

    I have tried enabling and disabling all the bias sense bits. I Have measured  for both the cases and still the bias remains to be 5.04v.

     

  • 0 in reply to JAGAN KB
    TI__Mastermind 47595 points

    Hi Jagan,

    Please provide the complete table of register settings and a schematic. 

    There are a couple things that can be happening:

    1. Lead off detection is enabled on disconnected channels; this will pull the inputs to rail, and when these inputs are added to the Vcm derivation, it will cause the RLD buffer to rail as well.

    2. There is an imbalance in the lead-off currents. Make sure that for however many channels have lead-off detection enabled, their currents are balanced - otherwise the RLD buffer is forced to sink/source the remaining current.

    3. There is a large resistance on the output of the RLD buffer. This would lead to an offset. 

    To start, try disabling lead-off if it's enabled.