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ADS1299 Right Leg Drive Closed Loop Bandwidth: RC in Datasheet and EK

Alex von Luehmann
Prodigy 165 points
Other Parts Discussed in Thread: ADS1299

I am currently designing the RLD Loop for the ADS1299. For a selection of the components for loop-bandwidth/gain, I compared the ADS Datasheet recommendations, the ADS1299EEG-FE Board documents and the TI article Improving Common-Mode Rejection Using the Right-Leg Drive Amplifier ()

In both the datasheet and the TI article on RLD, a feedback RC network of Rf=1MegOhm and Cf=1.5nF is proposed. As a protection resistor value, Rp=1Meg-50kOhm are tested.

On the ADS1299EEG-FE Board however, Rf=392kOhm and Cf=10nF are used. Rp is 4.99kOhm, as with all other Input channels.


According to the TI article and common sense, the closed loop bandwidth of the RLD loop is approximately fBW = 1/(2 pi Rf Cf)

and thus for the 1Meg/1.5nF RC approximately 106Hz, for the 392k/10nF RC on the FE Board approximately 40Hz.


Now, as I am currently designing my own RLD loop with the ADS1299, I wonder, why on the FE Board the bandwidth is chosen such that it is smaller than the 50/60Hz mains hum, the frequency that -as far as I know - is of main interest for the RLD feedback to increase CMRR.


Any information is most appreciated!

Best regards
Alex

  • 0
    TI__Guru 66296 points
    Hi Alex,

    Your reasoning is definitely justified. As you alluded to, reducing the RLD bandwidth in the ADS1299 to only 40Hz would negate the added CMRR benefit obtained by driving the body with an inverted 50Hz/60Hz common-mode.

    I do not know the reason why these values were chosen for our EVM. The EEG signal bandwidth is typically less than ECG (< 50Hz without gamma waves). If you used an external CLK at 1.5MHz (the minimum), you could choose the highest OSR to achieve a data rate of 183SPS, which would have a -3dB passband of 48Hz. Theoretically, you might only care about cancelling common-mode signal < 50Hz at that point.

    However, even in this scenario, it may still be a good idea to widen the RLD bandwidth to > 60Hz to reduce the power line common-mode noise before it even reaches the digital filter, where it could alias into the passband.

    I hope that helps. :)

    Best Regards,