Project Specification — Wearable EEG BCI Acquisition Board
Purpose
Wearable, non-medical, OpenBCI-like 8-channel EEG/BCI prototype for human-head research/development use.
Core Architecture
- Analog front end: U1 ADS1299IPAGR, 8 EEG channels.
- Main MCU: U2 STM32L152RET6, SPI1 master to ADS1299, SWD programming header J1.
- Wireless/storage module: U3 ESP32-WROOM-32E, STM32 SPI2 bridge, USB-UART programming/debug via U8 CP2102N, microSD on ESP32 VSPI.
- Power: LiPo J5, MCP73831 charger U4, main switch SW4, TPS62840 buck to +3V3, TPS61023 boost to +5V5, TPS7A2050 LDO to +5V0A analog.
Named Rails and Grounds
- VBUS: USB-C input.
- +VBAT: LiPo/charger battery rail.
- +VSYS: switched system rail from +VBAT via SW4.
- +3V3: MCU, ESP32, CP2102N, microSD, ADS1299 DVDD.
- +5V5: boost output feeding analog LDO.
- +5V0A: ADS1299 analog supply.
- GND: digital/system ground.
- AGND: analog/electrode ground and ADS1299 AVSS.
Mechanical/Layout Targets
- Board outline target: 55 mm × 40 mm.
- Stackup: 4-layer; Top signal/components, Inner1 solid GND reference, Inner2 power distribution, Bottom signal.
- Keep analog electrode front end separated from ESP32 RF and switching power.
- ESP32 antenna must face an edge with no copper/ground/enclosure metal in the antenna keepout region.
Schematic Status
Captured major functional blocks: power tree, ADS1299, STM32, ESP32, USB-C/CP2102N, microSD, LiPo connector/switch, electrode connector/protection.
Known Design Decisions
- TPS7A2050PDBVR pin 4 is NC; no NR capacitor is used.
- ADS1299 uses internal oscillator: CLKSEL tied high; CLK pin no-connect.
- ADS1299 VREFP decoupled to AGND; VREFN tied to AGND.
- J3 pin 12 tied to AGND as spare/shield.
- BIASOUT goes through 1 kΩ to J3.BIAS.
- SW4 is a 5 A DPDT slide switch used as a high-current replacement for the unavailable ≥1 A SPDT SMD switch.
Open Engineering Items
- Confirm ADS1299 BIAS amplifier topology: BIASREF, BIASINV, and BIASIN are intentionally left unresolved pending the desired driven-bias/reference implementation.
- Replace C29 470uF/0402 with a physically realistic bulk capacitor package/MPN.
- Assign specific manufacturer part numbers to generic passives/inductors/LEDs for fabrication.
- Confirm exact electrode cable pinout and patient-safety requirements before human-connected testing.
Intended Use Limitation
This is a non-medical prototype. It is not designed or documented as a certified medical diagnostic, treatment, or patient-monitoring device.