​

Hardware

​

PCB Overview

1. Amplifier Stage: AD8422 instrumentation amplifier
2. ADC Stage: ADS1220 24-bit analog-to-digital converter
3. Power Stage: TPAP2112K-3.3 LDO regulator

​

Components

​

AD8422 Instrumentation Amplifier (U12)

• Gain: 100x (set by 100Ω resistor at R6)
• Input bias current: 25 pA typical
• Common-mode rejection: 100 dB
• Bandwidth: DC to 1 MHz
• Supply voltage: 3.3V

Gain = 1 + (9.9kΩ / RG)
Gain = 1 + (9900 / 100) = 100x

• IN+: Positive electrode input
• IN-: Negative electrode input
• OUT: Connected to ADS1220 AIN0
• RG: 100Ω resistor (R6)

​

ADS1220 ADC (U19)

• Resolution: 24-bit
• Sample rate: 20 to 2000 SPS (660 SPS in firmware)
• Reference voltage: 3.3V external
• Interface: SPI
• Supply voltage: 3.3V

• AIN0: Amplified signal from AD8422
• AIN1: Reference ground
• REFP0/REFN0: External 3.3V reference
• CS: Chip select (GPIO 34)
• DRDY: Data ready (GPIO 33)
• SCLK, MISO, MOSI: SPI bus

• Mode: Continuous conversion
• Gain: 1x (PGA bypassed)
• Data rate: 660 SPS
• Filter: 50/60Hz rejection enabled

​

TPAP2112K-3.3 LDO Regulator

• Input: 3.6V to 6.5V
• Output: 3.3V
• Current: Up to 300mA
• Dropout: 200mV typical

• VIN: From USB 5V or battery
• VOUT: 3.3V to all components
• GND: Ground plane

​

Seeed Xiao ESP32-S3

• CPU: Dual-core Xtensa LX7, 240 MHz
• RAM: 512 KB SRAM
• Flash: 8 MB
• Wireless: WiFi and BLE
• USB: Native USB-C

• GPIO 34: ADS1220 CS
• GPIO 33: ADS1220 DRDY
• GPIO 36: SPI SCLK
• GPIO 37: SPI MISO
• GPIO 35: SPI MOSI
• GPIO 21: Status LED

​

Signal Chain

​

Input Stage

Electrode → AD8422 → ADS1220 → ESP32-S3

​

Voltage References

• Source: 3.3V power supply
• Used for: ADC full-scale range
• Accuracy: ±50mV typical

• Better stability than internal 2.048V
• Matches power supply rail
• Simpler circuit design

​

Power Supply

​

Power Sources

• Input: 5V from USB-C
• Regulated to: 3.3V by LDO
• Current available: Up to 300mA

• Input: 3.7V LiPo battery
• Regulated to: 3.3V by LDO
• Current available: Limited by battery

​

Power Consumption

​

PCB Layout

​

Layer Stack

• Top layer: Signal traces and components
• Bottom layer: Ground plane

​

Design Considerations

• Large continuous ground pour
• Reduces noise and EMI
• Provides low-impedance return path

• Analog and digital separated
• Short traces for high-speed signals
• SPI traces kept under 5cm

• Star grounding from regulator
• Decoupling caps near ICs
• Wide power traces (>0.5mm)

​

Electrode Interface

​

Connectors

1. CH+ (Channel Positive): Main signal input
2. CH- (Channel Negative): Reference input
3. GND (Ground): Common ground

​

Recommended Electrodes

• Low impedance
• Stable over time
• Low noise

• Gold-plated: Higher impedance but reusable
• Dry electrodes: Convenient but noisier

​

Electrode Preparation

1. Clean skin with alcohol wipe
2. Apply conductive gel to electrode
3. Press firmly and secure with tape
4. Check impedance (should be < 10kΩ)

​

Hardware Versions

​

V4-0

• Initial design
• Basic functionality working
• No battery charging circuit

​

V4-1

• Improved ground plane
• Better component placement
• Still no battery charging

​

V4-2

• Added battery charging (TP4056)
• USB-C connector improvements
• Better EMI shielding

​

V4-3

• Final component optimization
• Production-ready design
• Full documentation

​

V4-4

• Latest iteration
• Minor layout improvements

​

CAD Files

• Schematic (.kicad_sch)
• PCB layout (.kicad_pcb)
• Component libraries
• Gerber files for manufacturing

​

Signal Quality

​

Expected Performance

• RMS noise: < 2 µV (electrode input)
• Peak-to-peak: < 10 µV

• Low cutoff: 0.5 Hz (hardware)
• High cutoff: 100 Hz (hardware)
• Usable range: 1-40 Hz (typical EEG)

• 80 dB at 60 Hz

• 70 dB at 50 Hz

​

Noise Sources

• 50/60Hz powerline interference
• WiFi and BLE signals
• Fluorescent lights

• FIR filter in ADS1220
• Shielded cables
• Good electrode contact

​

Testing

​

Basic Tests

• Measure 3.3V at test points
• Check current draw (< 200mA)

• Upload firmware
• Check Serial output for “ADC init success”

• Connect test signal (10mV, 10Hz)
• Verify ADC readings change appropriately

​

Advanced Tests

• Sweep input from 0.1 Hz to 100 Hz
• Verify -3dB points match specs

• Short inputs together
• Measure RMS noise over 1 minute
• Should be < 2 µV

• Apply known current (1 µA)
• Measure voltage change
• Calculate input impedance (should be >1 MΩ)

​

Troubleshooting

​

No Power

• USB cable connected
• 5V present at LDO input
• 3.3V present at LDO output

• Dead USB port
• Faulty regulator
• Short circuit on board

​

ADC Not Responding

• SPI connections to ESP32
• Chip select line toggling
• DRDY line going low when data ready

• Loose connection
• Wrong pin assignments
• SPI bus conflict

​

Noisy Signal

• Electrode impedance (< 10kΩ)
• Ground connection
• Power supply ripple

• Poor electrode contact
• 50/60Hz interference
• Digital noise from ESP32

​

Modifications

​

Changing Gain

Gain = 1 + (9900 / R6)

For 50x gain: R6 = 200Ω
For 200x gain: R6 = 50Ω

​

Adding Channels

1. Add another AD8422 amplifier
2. Connect output to ADS1220 AIN2 or AIN3
3. Modify firmware to read multiple channels

​

Battery Charging

1. Connect BAT+ to battery positive
2. Connect BAT- to battery negative
3. Connect USB 5V to TP4056 input

​

Safety

​

Electrical Isolation

• Personal research
• Development and testing
• Educational purposes

• Medical diagnosis
• Clinical research
• High-voltage environments

​

Precautions

• Do not use near AC power
• Keep away from water
• Use only battery power for safety-critical applications
• Never connect to mains-powered equipment