Olimex Active Electrode Design Questions

MichaelNDSUMichaelNDSU
in Electrodes
Hello,

I am working with a group building the 32bit Board and we are considering using the Olimex active electrode design, found here, but have some questions.

First, referencing this forum post, we are given two +2.5V, -2.5V and AGND pins to tap into, but when wiring them into the circuit design I don't see how we should be connecting them. Could you elaborate on where exactly the + and - 2.5V would be connected?  From what I can tell their design only has +5V and AGND, would we just go +2.5V where +5V is input, and change AGND to -2.5V?  In doing that how would that effect the HF interference?

Second, I know Jarek has another active electrode design on OpenEEG, here his design would require a virtual ground for the reference electrode and DRL electrode, what is he referring to?  Also would this be a better design for us to use?  Has anyone tried these designs yet?

Also referring to the other forum post, Jarek suggests to use the TLC2272 over the TL062CD from the circuit.  Has anybody considered using any of the following op amps?

MCP6L91/1R/2/4 : Current Supply 850 uA, Voltage Supply 2.4-6V, Input Noise Density  9.4 nV/square(Hz), Typical CMRR 91dB, Slew Rate 7 V/us

LMP7717 : Current Supply 1.15 mA, Voltage Supply 1.8-5.5V, Input Noise Density 5.8nV/square(Hz), Typical CMRR 94dB, Slew Rate 35 V/us

LME49721 : Current Supply 2.15 mA, Voltage Supply 2.2-5.5V, Input Noise Density 4nV/square(Hz), Typical CMRR 93dB, Slew Rate 8.5 V/us


Thanks for any help!
Michael

Comments

  • biomurphbiomurph Brooklyn, NY
    We have used the Olimex active electrodes with good success here.
    The connection should be

    Olimex -> OpenBCI
    +5V -> +2.5V
    AGND -> -2.5V

    That will give you the 5V potential. Then connect the Olimex output to one if the inputs of the openBCI board. We haven't seen any HF effect. 

    I'm not positive about Jarek's use of the term, but Virtual Ground is usually used to refer to V/2. That is, in a 5V system there is no negative voltage so all things being relative, a Virtual Ground is made out of 2.5V. So the 0V of the system 'looks' like -2.5V in reference to the virtual ground. VGND is usually made out of an op amp that is set up as a follower with a V divider on the +IN that sets the correct voltage. This is because the VGND needs to be low impedance.

    I have not looked into those op amps that you mention. The specs you call out sound good. It's also nice to know the 1/f noise, as well as the input bias current and input offset voltage. 

    Best way to know, as we say around here, is to try it!
  • MichaelNDSUMichaelNDSU
    edited May 2016
    Thanks for the reply biomurph, appreciate it!

    For anyone who is interested in possible replacement op amps for the ones that Jarek talked about I have created a short list for comparison.  The first two Jarek already mentioned the other three I found on http://www.digikey.com/, I'm sure there are others but I tried to keep it around the same price.  The links to each op amp's datasheet were provided in my previous post if anyone wants to delve deeper.
    image

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