I am trying to understand how the OpenBCI Ganglion board works, but I can't figure out how these two sub-circuits work. I'm sorry I'm not too familiar with op-amps.
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1\$\begingroup\$ Plea embed the full circuits into your question as images in case the links are lost or become changed thus rendering your question invalid. \$\endgroup\$– Andy akaCommented Nov 29, 2023 at 9:49
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\$\begingroup\$ Google: "Right leg drive". It does the same for both ECG and EEG applications. Easier to understand once you have a physical circuit in your hands to play with. \$\endgroup\$– Turbo JCommented Nov 29, 2023 at 11:29
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1 Answer
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The driven ground part is simply an AC ground voltage driver which outputs 1/2 of the supply voltage which is used as the "ground" for AC signals. This is the baseline level around which ac signals will oscillate in the circuit (imagine a sine wave with DC offset). This makes sure that the AC signals are in the center of your supply range and thus maximizes the input/output dynamic range. The capacitor C46 provieds a low impedance path bypassing the output resistor for high frequency noise attenuation.
The input bias block is more or less the same thing, except for not having the bypass capacitor C46.
The important part is to realize how the whole system interacts with the body.
The input bias block connects to the electrodes through a 1/1 voltage divider which serves the purpose of ensuring that the common voltage of the electrodes is close to the AC ground. You want the common voltage to be as close to the AC ground as possible, because otherwise it will distort the results as differential amplifiers also amplify common mode voltages slightly.
The driven ground would connect to a grounding electrode (typically ear for EEG) which is outside the zone of measurement and thus unaffected by the signals you are measuring.
So why's there two driver amplifiers blocks that do essentially a similar thing (driven ground and input bias)? The biopotential at the grounding electrode and your measurement electrodes will likely be just slightly different due to interference from the environment, electrodes and the measured biopotentials too. This provides a path for currents to flow between the measurement electrodes through the body to the grounding electrode, essentially from one driver amplifier through the body to the other amp.
In short, it's a part of the interference reduction and biasing system of the biopotential amplifier.
