I want to know why in ganglion board, there's additional instrumental amplifier (AD8237) for each input channel not just PGA of the ADC like in Cyton board?
I can't speak for openbci designers & others knowledgeable about all the choices, but here's one pointer ....
The Ganglion ADC-etc-chip MCP3912, desirable for Ganglion partly because of lower-cost than Cyton's, has fairly low input impedance, which also varies with PGA gain. See the chip's spec page 4, Table 1-1 for Differential Input Impedance.
Other advantages offered by the instrumentation amp rather than straight-into-MCP3912 are the opportunity for gain (chosen 51 at mid-band) and the opportunity to include high-pass filtering, and it eases inclusion of low-pass filtering before the 3912 without affecting board-input-impedance. There may be other advantages I'm not seeing.
The Cyton ADC was designed specifically for direct-connect bioinstrumentation so it has much better features & performance for that. The Ganglion ADC seems to have initially been designed primarily for AC power analysis applications.
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