TI Eval Kit: isolation, RLD, Reference, and BIAS

DeepaksenDeepaksen
edited June 2015 in Other Platforms
a) I hope BIAS helps to avoid floating of the voltage level of the body. Then what about the EMI that the body is affected by? (As a large amount of EMI should be coupled to the patient's body.)

Is just a DC bias operating point enough to reduce the noise. Does EMI not make any differece as it account as the common mode voltage, and gets cancelled out in differential amplifier?
So my question is: Is Right Leg Drive required to provide 180 degree phase shift for EEG application, Or is RLD meant for only ECG and EMG purposes?

b) I have heard that Improving the isolation between the device ground and the patient ground helps to improve the system CMR. So my question is: while biasing the body to some mid- supply voltage of the system, shouldnt the body be also connected to the Ground to complete the BIAS loop? So how can the isolation be possible?

Will it work if I avoid BIAS and tie the analog ground to the body?

I will be very happy If you could explain with the schematics of 'ADS1299 EEG-FE development board from TI' in mind. I will be very grateful if you could give a sample list of register values of ADS1299 with an idea of the correspong wiring of the circuit. I am having these many troubles as I am using TI development Board for which so many bugs are being reported. At present I am in a hope that configuring the ADS1299 via arduino will help me sort out all the troubles.

Comments

  • wjcroftwjcroft Mount Shasta, CA
    Deepaksen,

    There are two levels of EMI filtering. RC filters on each channel pin, and a Sinc filter inside the ADS1299. See the TI data sheet. There are several other threads on the forum that discuss EMI issues, use the Search box at lower right.

    The Bias / RLD drive helps in all applications to cancel mains noise, not so much EMI.

    Isolation from the Mains is via our battery power supply. If you are talking about the TI Eval Kit, that is not intended for connection to subjects, only for eval testing. Do not connect to analog ground, this is what Bias / RLD is for.

    If you want isolation for the TI Eval Kit, use the search box with 'isolation'; medical rated power supplies and USB isolators are available. For example the models from CUI.

    William
  • biomurphbiomurph Brooklyn, NY
    Hi,

    I don't know how to answer your question about EMI, but I can say that the BIAS of the OpenBCI (ADS1299 IC) is equivalent to RLD.
    On the OpenBCI Board, we are using the ADS1299 internal reference (AVDD+AVSS)/2 and any channels that have their BIAS_SENSx bits set.
    Figure 46 on page 48 diagrams how these parts work.
    The BIAS_SENS connection includes those channel signals and they become included as inverted common mode in the BIAS output.

    It's super important that the subject and device share the same ground. Our BIAS pin does this, with the fancy addition of user-selectable inverted common mode.
    This is important, for example, if you are measuring EEG and ECG. You could remove the ECG electrodes from inclusion in the BIAS generation.

    Also, on all of our boards, we have connected the BIAS_IN pin to the BIAS_OUT pin, so you can measure it if you like.

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