Two-PCB low-noise EEG system: Digital board (ESP32-S3-MINI-1U + ADS1299 + CP2102N + USB-C + 1S Li-ion charger/power-path + DF40 mezzanine) and Analog board (3x ADS1299 + REF/BIAS/DRL + driven-shield electrode interfaces + local low-noise regulation + test points). DF40 mezzanine carries only power + digital SPI/control with shared SPI, shared START/RESET, and separate CS/DRDY per AFE. 4-layer stacks with strong analog/digital partitioning and interleaved GND pins on DF40.