EEG part 2: Replacing the ADC motherboard

The motherboard which came with the ADS1263 provides power (3.3V digital, 5V analog and ground) and an ability to communicate over USB to ADC board. The ADC itself transfers data using SPI.

ADC daughterboard, raspberry pi and voltage reference with electrodes.

To replace it, I used a raspi to power (as it has both 3.3V and 5V pins) and to get data off of the daughterboard. While the ADC worked, there was a problem with this. The raspi power pins are too noisy for accurate reference. I solved this by moving to some low noise linear power supplies. An alternative might have been to ensure constant power usage for the raspi by disabling features such as SD card use or WIFI. While testing this, it was actually possible to see the power usage change by watching the rail voltage change while typing over SSH. Reading data off with SPI also added noise, so the spi program only reads when triggered on the data out pin of the ADC

You can actually see the bits for SPI on both the 5V and 3.3V rails

(see https://github.com/garthwhelan/ads1263 for the raspi and analysis programs. The raspi transfers data over netcat to the graphing computer.)

Which produces scrolling graphs like the following:

(trace from girlfriend) There’s no 60Hz EMF if you sample at 60Hz. In all of these, it should be noted that these are the best traces, there are often terrible drifts and artifacts from electrodes moving. It would be ideal to have flexible lines to the electrodes.
At 1200 Hz, coax electrodes
400Hz, coax
After applying low and high pass filters. Exact settings forgotten

To be further continued…

Author: Garth Whelan

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