Project EOGee: EOGee2 and Noise, Part 2

This is a bit fragmented because I mostly described the differences between EOGee1 and EOGee2 from a circuit perspective in the previous article about DC coupling. Here I will give a quick overview of the intention behind EOGee2 as well as the physical differences

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Screenshot 2020-05-24 at 19.04.57

EOGee1 PCB Render

Screenshot 2020-05-24 at 19.00.10

EOGee2 PCB Render

EOGee1 was built as a proof-of-concept, based on the Backyard Brains Spiker Shield design. However it had a number of problems including noise issues and AC coupling as well as being generally hard to work with due to the weird shape and compressed design. All of these issues were resolved in this iteration as well as a number of other features, including:

  • Addition of mono-audio jacks to allow individual cables for each electrode
  • Jumper to reverse polarity of signal measurement
  • Signal reference is variable with a DAC rather than fixed at mid-rail which enables DC coupling
  • Variable gain stage using MCP41010
  • STM32F072CBTx has significantly more FLASH for more complex firmware
  • Corrected voltage regulator footprint

Much of the concept revolved around making a platform that was easy to use and debug when developing algorithms and collecting data. The design is single sided for easy access to all components and form factor is intentionally simple to make designing a case easy.

IMG_6604

3D printed case covered in grounded copper tape for shielding

As mentioned earlier, there was a noise issue with EOGee1 which went away when data was streamed over SPI instead of USB, suggesting the noise is due to the shared digital ground and analog ground pins on the microcontroller.

Screenshot 2020-04-12 at 00.22.43

EOGee1 noise on data captured via USB or SPI

On EOGee2 we are using a larger microcontroller with separate analog ground and digital ground pins. Furthermore much more care was taken to have smaller ground loops which should help with signal integrity.

noise_measurement_no_probe

EOGee2 noise

As expected we see a reduced peak-to-peak noise of about 7 counts which is better than EOGee1’s peak-to-peak noise of about 30 counts when streamed over USB however slightly worse than when data was streamed over SPI from EOGee1. This could be because I didn’t short the inputs for the EOGee2 measurements whereas I did for the EOGee1 measurement, or it could be some remaining noise from the USB. Nonetheless it is sufficiently low that I am not actively searching for a solution at this time.

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