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ADS1299: using with external amplification?

Scott Brenneman
Genius 3295 points
Other Parts Discussed in Thread: ADS1299

We're contemplating an EEG application which we believe could benefit from amplification, or at least buffering (Gain = 1) of the electrode signals.  However, it's not clear to me if the ADS1299 would support this, or if features such as biasing or lead-off detection will be throw off by low-impedance and possibly boosted incoming signals.  I readily acknowledge my novice status in the EEG field, so apologies in advance for asking stupid questions!

Thanks,

Scott

  • 0
    TI__Mastermind 49956 points
    Hello Scott,

    I've moved this post from the Medical Forum to the Precision Data Converter Forum, where the ADS1299 is supported. I'm sure you'll find an answer here.
  • 0 in reply to Praveen Aroul
    Genius 3295 points

    Thank you!

    Scott

  • 0
    TI__Mastermind 38745 points
    Hey Scott,

    There are no stupid questions! The ADS1299 was designed to be the AFE for EEG data acquisition systems. It has a very low input bias current and great input-referred noise (sub-1uV!). However, it is not unheard of to want an additional gain stage.

    Like you mention, lead-off detection will be unavailable to buffered inputs. Lead-off detection works by injecting a low-magnitude excitation current onto the electrodes and reading the voltage at the channel with a simple comparator to measure impedance. When the impedance becomes too high, the voltage exceeds the threshold of the comparator and it trips the circuit. Adding a buffer in front of the ADS1299 will regulate the voltage at the input such that the comparator will never be allowed to switch.

    Using the integrated bias amplifier is still allowed, however, since this feature is simply an output which is intended to be connected to the patient.

    May I ask why you would choose to use a unity gain buffer in front of the inputs? This will increase the input-referred noise of the signal chain. Is there some other concern you have with the ADS1299?

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Genius 3295 points

    Hi Brian,

    I can't get into much detail, but we want to put the electrodes at a greater distance from the measurement equipment, i.e. longer wires, so we'd like to buffer the signals at the patient end.  That's why we're looking at doing this.

    We could also boost the signal level instead of just using unity gain.  Would this work better, and would the ADS1299 still be happy (other than lead-off detection) as long as we didn't saturate its inputs?

    Is lead-off detection the only function of the ADS1299 which will be adversely affected, other than the added noise you bring up?

    Thanks!

    Scott

  • 0 in reply to Scott Brenneman
    TI__Mastermind 38745 points
    Hey Scott,

    The ADS1299 will be agnostic to gain stages in front of it, so I do not see a problem with your proposal. Adding gain would give the benefit of reducing the input-referred noise level of the signal chain (assuming the amplifier generates less noise than the ADC). I believe lead-off detection is the only feature on this device that relies on a direct connection to the patient on the inputs.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Genius 3295 points
    Excellent, thank you for this helpful info!
    Scott