Armbrain prototype #3: Introducing WEEGEE, the Wireless EEG recording device

I consider my Armbrain prototype #2 was a partial success. I get some improvements compare to the 1st prototype in term of circuit footprint. On the other hands, the analog system doesn't perform as I expected. In prototype #1, I use an ADS1299 Front end - Demo Kit from TI as an analog system with a STM32F407 Discovery board from ST as a digital system. I get everything working together to record ECG and alpha waves of the brain. I decided to layout a new circuit board which comprises essential components from those two evaluation boards. It's the prototype #2. 

Its digital system works like a charm. If I send Test Signal from the ADS1299 channels, I can plot the exact plot on the Matlab interface. Also, I demonstrated temperature recording with this hardware in the previous post. I felt great about myself. I moved to the next task, checking analog system performance which is the key feature of this board. It just didn't work. I always got offset value for DC signal and weird noise for AC signal. I tried every possible way to debug the system such as measuring large signal, measuring EKG signal from EKG simulator or using logic analyzer to monitor data transfer. I could locate the issue comes from the ADS chip, yet I couldn't pinpoint the exact problem. I was upset because of this failure. I put off the project for awhile after countless testing and debugging efforts. A year later, I found that I made mistakes in placing polarized capacitor components on negative voltage rail. The positive pole of these capacitors should be soldered to the ground, whereas negative poles should be soldered to Vss.  

Capacitor placement on negative rail

I design a better version 3 which is named WEEGEE. The name stands for Wireless EEG recording device. I built and tested the system. It performed very well. It can catch alpha waves, do some SSVEP experiments and measure electrode impedance. I will post more in upcoming blog posts. This new version has a charge circuit for a lithium battery. Any battery from 2.5V to 12V can power up the device thanks to the buck/boost converter TPS63060.

WEEGEE - Wireless EEG recording device (v3)

The basic spec of this board is listed as below

Item	Font Size
Number of channels	8 (expandable to 32)
Analog Input Option	Differential and single ended input for all channels
Resolution	24 bit
Min input voltage step	22.3nV
Input voltage full-scale	188mV
CMRR	-110db
Analog power supply	Dual ±2.5V
System power supply	Single Lithium polymer battery cell 3.7V
Steady state current	30mA

Also, I summarize the system architecture in the figure below