Other Parts Discussed in Thread: ADS1298
I have a doubt in use of BIASOUT pin for AC impedance and EEG measurement.
Everywhere on the internet I have read that BIAS pin act as a 'Driven Ground' and is used to cancel out the noise and set the common mode inputs within limits. In AC impedance calculation, this pin acts as a current sink/source to the internal current source attached.
But when I perform raw EEG data collection or AC impedance testing, I can see that the noise becomes very less when I short ADS1299 ground pin (0V) to the Bias pin. In AC impedance testing my values are under 10% error after shorting BIAS and ground pin together, otherwise the error is >60% when no ground pin is used.
For information: I am using internal reference (AVSS+AVDD)/2 (i.e. BIAS_REF_INT=1) and using SRB1 to route all the negative input of amplifier to one pin. And since I am using only channnel 8 as of now, I am setting BIAS8P and BIAS8N to 1 (these are set in BIAS_SENSP and BIAS_SENSN)
The doubt arises here that in my electronics circuit classes I have been taught that to measure voltage accross a foreign load one must need to set a common reference by connecting a ground pin to it. Then only one can be certain about the voltage readings accross it otherwise it will be arbitrary. I think similar thing is happening here. But there was no anecdote about connecting ground wire to human body/ load in EEG measurement and AC impedance testing as all is done by BIAS pin itself.
Please help me get the concepts right as I am facing problem with the BIAS pin itself and suggest a way to connect the pins.
(I have referring an OpenBCI and EEG Hacker blog for AC impedance: http://eeghacker.blogspot.com/2014/04/openbci-measuring-electrode-impedance.html where they are not using ground pin but still getting minimum errors )
