Specification

ardEEG shield

Hardware


Channels	8 biosignal inputs
Extra inputs	Reference (shared by all channels) and bias
Analog front end	Texas Instruments ADS1299
Resolution	24-bit, delta-sigma, all channels sampled at the same moment
Sample rate	250 SPS as set by the sketch (the ADS1299 supports 250 SPS to 16 kSPS)
Gain	1 as set by the sketch (the ADS1299 supports 1, 2, 4, 6, 8, 12, 24)
Reference voltage	4.5 V internal
Internal noise	About 0.5 µV RMS per channel (measured, Signals & Datasets)
Signals	EEG, EMG, ECG
Electrodes	Dry or wet Ag/AgCl, 2.54 mm header
Host	Arduino UNO R4 WiFi
Interface	SPI (D13, D12, D11, chip select D10) and DRDY on D7
Power	5 V from a small power bank: up to 1 A, up to 1,000 mAh
Isolation	None. Battery operation is required.
Licence	Software and SDK: MIT

SPI settings

Setting	Value
Clock	600 kHz
Mode	1 (CPOL 0, CPHA 1)
Bit order	MSB first
Chip select	D10

Data format

Each sample is 27 bytes, read from the ADS1299 when DRDY (D7) goes low:

Bytes	Content
1–3	ADS1299 status word
4–6	Channel 1
…	…
25–27	Channel 8

Each channel is a 24-bit, big-endian, two's-complement value.

Over Wi-Fi, the sketch sends UDP packets of 1,350 bytes = 50 samples to your computer on port 13900. At 250 SPS that is 5 packets, or 6,750 bytes, per second.

Converting to microvolts

The conversion used by the PiEEG scripts:

raw = (b[0] << 16) | (b[1] << 8) | b[2]   # 3 bytes of one channel
if raw & 0x800000:                       # negative value
    raw -= 16777214
uV = raw * 4.5 / 16777215 * 1_000_000    # about 0.268 µV per count

Minimal Python receiver

import socket
 
UDP_IP, UDP_PORT = "192.168.1.241", 13900   # this computer's IP
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind((UDP_IP, UDP_PORT))
 
def to_uv(b):
    raw = (b[0] << 16) | (b[1] << 8) | b[2]
    if raw & 0x800000:
        raw -= 16777214
    return raw * 4.5 / 16777215 * 1e6
 
while True:
    packet, _ = sock.recvfrom(1350)
    for s in range(0, len(packet), 27):                  # 50 samples per packet
        sample = packet[s:s + 27]
        ch = [to_uv(sample[3 + 3*i: 6 + 3*i]) for i in range(8)]
        print([round(v, 1) for v in ch])

ADS1299 settings used by the sketch

Start-up: WAKEUP (0x02), STOP (0x0A), RESET (0x06), SDATAC (0x11), write the registers below, then RDATAC (0x10) and START (0x08).

Address	Register	Value	Meaning
`0x01`	CONFIG1	`0x96`	250 SPS
`0x02`	CONFIG2	`0xD4`	Internal test-signal settings (not used in normal mode)
`0x03`	CONFIG3	`0xFF`	Internal reference buffer on, internal bias reference, bias buffer on
`0x04`	LOFF	`0x00`	Lead-off detection off
`0x05`–`0x0C`	CH1SET–CH8SET	`0x00`	Normal electrode input, gain 1
`0x0D`–`0x11`	BIAS_SENS, LOFF_SENS, LOFF_FLIP	`0x00`
`0x14`	GPIO	`0x80`
`0x15`	MISC1	`0x20`	Reference electrode shared by all channels (SRB1)
`0x17`	CONFIG4	`0x00`	Continuous conversion

Write a register with three bytes: [0x40 | address, 0x00, value].

Noise self-test: set every CHnSET to 0x01 (input shorted) in the sketch. The signal should stay within a few µV. See Noise elimination.

Signals & Datasets Troubleshooting