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ADS1299: Channel 1 with all inputs floating.

Nic Gough
Prodigy 615 points
Part Number: ADS1299

Hi Data Converters Team,

I had a customer who is working on an EEG setup and is using the ADS1299 and had a question.

If we setup ADS1299 channel 1 to output the AC lead-off detection signal at 7.8 Hz, what would you expect to see on channel 1 with all of the inputs floating?

Thank you for your help,

Nic Gough

  • 0
    TI__Genius 11550 points

    Hi Nic,

    When the input is floating with AC leadoff turn on, the channel input will saturate close to the ±VREF voltage. As a result, the FFT plot will show a signal at 7.8Hz with almost a full-scale input amplitude.

    Thanks

    -TC

  • 0 in reply to TCT
    TI__Prodigy 615 points

    Hi TC,

    The customer responded back as that was what they were expecting, but they are seeing the voltage drift toward the positive rail. After a few seconds the signal is railed and they see no signal (they have the gain set to 12).

    They will do some more testing, but is this expected behavior or is something off?

    Best regards,
    Nic Gough

  • 0 in reply to Nic Gough
    TI__Genius 11550 points

    Hi Nic,

    This is not the expected behavior for the AC lead-off when the channel input is floating. The ADC output will consistently rail to +VREF and -VREF at the fixed frequency of 7.8Hz. They can also verify the AC lead-off functionality with the EVM.

    Thanks

    -TC