Why does my Cyton board record a different voltage than that displayed on my signal generator?

Emma · July 2019

Hi all!

I am fairly new to using the Cyton, so forgive me if my questions seem simple but I could do with some help. I am attempting to emit a signal with a 1V peak using a signal generator. However, when connected to the Cyton it seems that I am recording peaks in the low mV range. Some signal loss was expected, but not as much as this. I'm not sure if this is due to an impedance mismatch, but I was wondering if anyone has experienced anything similar to this or has any advice?

On another note, I am somewhat perplexed regarding the data format of the txt files. I've read the page regarding the OpenBCI cyton data format which states that it only applies to v1 (2014-2016) and v2 (Fall 2016). I purchased my Cyton later than this. I just wanted to ask, are the values already converted into microvolts or are they the ADC values which require adjusting with the scale factor?

Cheers

Emma

wjcroft · July 2019

Emma, hi.

The CSV text files produced by OpenBCI_GUI are in microvolts. This is assuming you are using the default scale factors. The commented header of the file also states this. So these values are not raw ADC 24 bit samples.

You may want to try using a pair of resistors as a voltage divider to create a signal more in a range of millivolts or microvolts.

https://www.google.com/search?q=voltage+divider

It's possible there could be impedance issues as well.

William

Emma · July 2019

Hi William,

Thanks for your speedy reply.

I have already incorporated a resistor divider to reduce the signal to millivolts and checked the signal with an oscilloscope but sadly, I am still seeing a large degree of signal loss.

Thanks again

Emma

wjcroft · July 2019

Are you changing the scale factor with the GUI? What happens with the GUI default scale factor (x24), and say a 100 microvolt signal?

https://docs.openbci.com/Tutorials/01-Cyton_Getting Started_Guide#cyton-getting-started-guide-v-connect-yourself-to-openbci-4-launch-the-gui-and-adjust-your-channel-settings

Another test, would be to see how much eye-closed Alpha is produced. Are you seeing about 5 to 10 microvolts of alpha?

https://docs.openbci.com/Tutorials/01-Cyton_Getting Started_Guide#cyton-getting-started-guide-vi-check-out-your-bodys-electrical-signals-4-alpha-brain-waves-eeg

Why does my Cyton board record a different voltage than that displayed on my signal generator?

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