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ADS1299: Simultaneous EEG and ECG bias

Alwyn DSouza
Intellectual 360 points
Part Number: ADS1299
Other Parts Discussed in Thread: ADS1298

So lets say I have a device based on the ADS1299 that works as a 8 channel EEG.

I have another independent device based on ADS1298 that works as a 12 lead ECG.

Both of these devices may be independent or may be not so these can lead to 2 scenarios.

Scenario 1

Both devices working on independent power supplies powered by batteries and are in no way linked to each other.

Scenario 2

Both devices working on common 12VDC supply but their grounds may not be common.

___________________

My question is since both devices are providing bias electrodes respectively. How do you prevent a conflict of these bias. Will the EEG bias affect the ECG and vice versa. How do commercial products prevent this? do they have a provision where devices or products can share.

The EEG bias would generally be attached to the earlobe or somewhere thereabouts whereas the ECG bias is attached to the right leg? These are quite distant locations. So is it a cause for concern fo the biases causing jumbled up signals would this cause a degradation of 50Hz signals not being correctly removed due to 2 simultaneous biases being injected into the human body?

  • 0
    TI__Guru 63946 points

    Hi Alwyn,

    Thank you for your post.

    I don't have a definitive answer regarding how other products achieve this, but I do have a couple theories for approaching this problem.

    First, we definitely do not want multiple sources of a BIAS/RLD signal driving the body at the same time. The differences in their respective DC voltages, such as that due to amplifier offset, would create an additional DC current through the body's resistance. This might be small and potentially within safe limits, but it seems unnecessary and potentially conflicting to have multiple independent BIAS/RLD sources.

    Second, it may be beneficial to use one common BIAS/RLD signal with multiple points of contact on the body. To me, this sounds similar to designing a PCB with a thorough top/bottom ground copper and/or an internal ground plane with plenty of vias. The easier it is for return currents to get back to the source, the better the system generally performs. As you pointed out, the sensing electrodes for ECG are placed relatively remotely from the sensing electrodes for EEG. The RLD electrode would normally be placed much further away from the head than the BIAS electrode, so it makes some sense to think the ECG channels might see better CMRR than the EEG electrodes. I'm wondering if splitting one common BIAS/RLD signal to multiple points on the body would provide a more consistent impedance path for 50 Hz / 60 Hz noise to the BIAS/RLD amplifier output.

    Sensing the common-mode noise itself (i.e. enabling the RLD_SENSx or BIAS_SENSx bits on the ADS129x device) can be done as you normally would for either application. This means that you should select a few electrodes with decent coverage on the body to pick up the common-mode signal - not sure if it would matter if they're all from the ECG system, the EEG system, or a combination of the two. Again - this is really just a theory.

    Best regards,

  • 0 in reply to Ryan Andrews
    Intellectual 360 points

    Hi,

    Thanks for the extended reply. Is there any way to cascade a ADS1299 with a ADS1298 that would probably help to getting a common bias signal for both the products. The question that would remain is from which channels to derive the bias signals from the EEG channels or the ECG channels?

  • 0 in reply to Alwyn DSouza
    TI__Guru 63946 points

    Hi Alwyn,

    As long as the two devices are running off the same analog supplies (AVDD and AVSS), you can share the connections to the RLD/BIAS amplifier inverting input (RLDINV and BIASINV) similar to what is shown in the figure below. Only one amplifier would be enabled, as we previously discussed.

    I really can't tell you which signals to use to derive the common-mode and how to balance the selection between EEG and ECG electrodes. My guess would be that you only need a few and that which electrodes you select will not have that much of an impact. The more that you select, the larger the gain of the RLD/BIAS amplifier will be as each electrode is connected to the inverting input through 220k. Eventually, the amplifier will saturate, so 3-4 electrodes is probably enough with a 1-M external feedback resistor around the amplifier.

    Best regards,