(seeking help) Questions about ADS 1299 for EEG project

[Alex]

Hello Opendous,

I am working on a portable EEG device as part of my university project. I am totally new to EEG acquisition, and glad to see that people like you have created open projects that helps newbies like me to get familiarized with the EEG acquisition methods.

To develop the sensor front end, my primary idea was using passive EEG electrodes> pre amplification using instru. amp>Notch-LP-HP filtering stages> then to ADC>to uC. This approach requires multiple amps and filtering stages if I use an 8 channel measurement and takes up much space in PCB.

Few days before i came to know about the ADS1299 and 1298 ICs designed specifically for EEG or biosignals and happened to find that you are also working on a project using 1299. For proceeding further on my project, I am stuck with a couple of doubts, and couldnt find the solution anywhere. It would be great if you share your knowledge on some of my questions.

1. Need of pre amplifiers and filters ? (Question priority: High)

Before feeding the weak biopotentials (acquired using passive wet electrodes) to ADS1299, should i use preamplifiers and filters ?

ie-- considering the input biosignals are in the range of microvolts(+noise) and as the maximum programmable gain for the IC is only 24, the input for the ADC will still be in microvolts. So any external amplifier/filter stages are needed ?

2. ADS 1299 or 1298 (Question priority: low)

I am also curious to know that why you switched from 1298 to 1299 for the project, though a 5mW power consumption is required per channel ?

thanks in advance.

Alex.

[opendous]

  First, please don't waste your time on a custom design until you have proved your application using a known-working EVM.  This will save you time, money, and effort you can devote to developing the sensor electrodes which are more critical to the function of an EEG system.  This was my mistake.  Budget for many electrode revisions or go with gel-based electrodes from good suppliers.
http://www.ti.com/tool/ads1299eegfe-pdk
https://estore.ti.com/ADS1299EEGFE-PDK-ADS1299EEG-FE-Performance-Demonstration-Kit-P3103.aspx
http://www.digikey.com/product-detail/en/ADS1299EEGFE-PDK/296-34948-ND
http://us.mouser.com/Search/Refine.aspx?Keyword=ADS1299EEGFE-PDK

  Figure out some sort of USB isolation as well since having a path to mains is not worth the risk.  Using a laptop running only on battery power is an option.
http://www.digikey.com/product-detail/en/USB-ISO/1188-1019-ND
http://www.digikey.com/product-detail/en/EVAL-ADUM4160EBZ/EVAL-ADUM4160EBZ-ND

  I decided to revise for the ADS1299 vs. ADS1298 to simplify the design of the electrodes. 

  I believe pre-amps and possibly filters are the best option to get the cleanest signals to the ADCs.  I should have spent more of my hardware effort on electrode research.

  I gave up on the project after dumping way too much money and time tracing bugs in the power supply and chasing noise.

[Alex]

Thank you for the quick response and the information. For electrodes, I am planning to use Ag-AgCl gel based electrodes, though there are many discomforts and disadvantages connected to wet electrodes.

wishing you much success for your projects.

-Alex.