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I'm analyzing the OpenBCI Ganglion board and I reasoned that switches SW1 through SW4 (part number CJS1200) might in fact be useless. The negative input for each channel (X_IN-) is connected via a 330K resistor to a buffer (INPUT CURRENT BIAS, bottom left of the schematic), the switches let the user choose if they want to send the amplifier the real X_IN- or REF, which is connected to the same buffer via a 330K resistor.

Couldn't the exact same thing be accomplished by leaving the negative input disconnected?

If the negative inputs really need to be connected together (for reasons I have yet to understand), could the job be done by simple ON/OFF switches connecting them to a common rail itself connected to the buffer via a 330K resistor? In this way we would avoid the signal degradation caused by the switch if we are interested in using the differential input. Complete schematic

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We're missing some context here since you've only showed us one page of the schematic. I would like to see where REF and IN_X- are coming from. But if I had to guess, I would say that the SW1-4 are NOT "useless" as they could be changing the mode of the AD8237 operational amplifiers and could greatly influence the gains of the amplifiers.

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  • \$\begingroup\$ REF is connected to the op-amp buffer named INPUT CURRENT BIAS in the bottom left corner, X_IN- refers to an electrode connected to the patient. \$\endgroup\$ Dec 6, 2023 at 6:20

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