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Please note: Although this question involves EEG, I don't expect anyone here to be knowledgeable in this area! This is a simple electronics question!


I'm trying to understand the circuitry devised in this DIY EEG Instructable. The main thing that jumps out to me is that - at no discernable point - do the analog signals (generated by your brain and then subsequently amplified) get transformed into digital signals via some ADC/converter. But obviously, by the time they reach the Processing program, they must be digital.

It appears that the author is recommending feeding the signals into a laptop via RCA jacks. Is this where the analog-to-digital conversion is taking place, or am I missing something?!

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  • \$\begingroup\$ He's feeding the signals into his PC's microphone socket, so the PC's sound card is doing the ADC job. \$\endgroup\$
    – brhans
    Feb 16, 2016 at 16:08
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    \$\begingroup\$ DO NOT USE THIS CIRCUIT! It relies strictly on the PC's isolation, which is tested to IT standards, not medical. \$\endgroup\$
    – Matt Young
    Feb 16, 2016 at 16:25
  • \$\begingroup\$ Good to know @MattYoung (+1) - but can you explain what you mean by "it relies on the PC's isolation"? \$\endgroup\$
    – smeeb
    Feb 16, 2016 at 16:57
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    \$\begingroup\$ It means that if your PC is/becomes ungrounded, the power supply EMC filter capacitors will act as a voltage divider causing the PC ground (and the electrodes on your head) to float at half of mains voltage. The current would still be limited by the capacitance to nonlethal levels, but it wouldn't be enjoyable and the instrumentation amp would likely fry. A more serious threat would be the simultaneous failure of the aforementioned EMC filter capacitors, the PSU high frequency transformer or the isolation gap in the PSU, which could kill the user. Very unlikely to happen, but the risk is there \$\endgroup\$
    – jms
    Feb 16, 2016 at 17:10
  • \$\begingroup\$ @jms I wouldn't be so sure about the unlikely part. Some PC power supplies are real deathtraps. \$\endgroup\$
    – AndrejaKo
    Feb 16, 2016 at 17:14

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Yes, PCs have analog sound inputs (often both microphone inputs and stereo line level inputs). Normally these are on 3.5mm jack connectors though occasionally RCA connectors are seen.

be aware that while these are analog inputs they are not especially good analog inputs. Noise is often high (especially on laptops), the input is AC coupled so you lose low freqency content and even on soundcards with 96kHZ or 192kHz sampling (giving nyquist frequencies of 48kHz and 96kHz) there is often strong analog attenutation above 20 kHz or so.

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  • \$\begingroup\$ For EEG purposes, sampling frequency of around 512 Hz is usually sufficient. The main issue would be the loss of low frequency components from AC coupling. \$\endgroup\$
    – AndrejaKo
    Feb 16, 2016 at 16:55
  • \$\begingroup\$ Thanks @Peter Green (+1) - could you reduce the noise generated by the laptops by running the circuit through an ADC and then feeding into your laptop (perhaps via USB)? \$\endgroup\$
    – smeeb
    Feb 16, 2016 at 17:00
  • \$\begingroup\$ Yes, an external ADC is likely to be far less noisy than the one inside your laptop and if you are prepared to spend more (i.e. not buy "audio" equipment) you can also chose an ADC where both the ADC itself and the analog chain feeding it are more suitable for the task at hand. \$\endgroup\$ Feb 16, 2016 at 17:03
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The "DIY EEG" does not show using an RCA jack. The project uses 3.5mm plug (same as headphone plugs).

Here is RCA plugs :

enter image description here .

Here is what the DIY suggested :

enter image description here .

Also, the DIY stated

As we will be using our computer's sound card to get the data in.

. So they are using the sound card as the digitizer.

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