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This is a circuit for EEG. How do I make the connections labelled as Virtual Ground? Also, how do I connect the 1M trim pot in the top right? Schematic for EEG

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  • \$\begingroup\$ If you found this somewhere, there must have been explanatory comments and instructions. I suggest you go back to those. \$\endgroup\$ – pipe Feb 7 '16 at 11:39
  • \$\begingroup\$ Nah, there was no mention of how to wire virtual ground. \$\endgroup\$ – Swastik Mohapatra Feb 7 '16 at 11:50
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That is really a horrible schematic. They should at least have used a different ground symbol for the virtual ground. It's actually a completely different net from ground. And it looks like they used a resistor symbol for the rheostat-connected trimpot.

Such a schematic will have no (useful) connectivity information and using it for board layout will be a bad idea.

So, connect all the virtual ground 'ground' modes together, and the other 'microcontroller ground' nodes together. If you are pouring a ground plane it should be virtual ground in the analog section and microcontroller ground in the digital section. Adding bypass capacitors, though typically a good idea, will probably cause problems with op-amp oscillation in this particular 'design'.

The pot can only adjust the output voltage lower than 2.5V, down to the saturation of the op-amp. Since the 3140 output will swing from a couple hundred mV up to Vcc - 2V or so, it is asymmetrical too.

They show the wiper on the trimpot connected to +5 through a 1M resistor, and one of the ends of the element goes to R5/C2/R2 etc. Either end will work, it will just reverse the effect of pot rotation. Tie the otherwise unused end of the element to the wiper, not as shown, for a bit better design.

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  • \$\begingroup\$ Thanks for the advice. I agree that it is confusing for beginners like me to understand this schematic. So you are suggesting that I short the unused end of the trim pot and the wiper? \$\endgroup\$ – Swastik Mohapatra Feb 7 '16 at 14:14
  • \$\begingroup\$ Yes, exactly. It's slightly better in a couple of ways. \$\endgroup\$ – Spehro Pefhany Feb 7 '16 at 14:23
  • \$\begingroup\$ Thanks a lot!! Because of you I can now easily assemble the circuit. Tons of thanks! :) \$\endgroup\$ – Swastik Mohapatra Feb 7 '16 at 14:30
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Okay, I got a clear idea of how this works after some research. Here's help for those who need it in the future.

The Opamp in the bottom half of the image actually produces a voltage of 2.5V when connected with an Arduino.(5v) The output of the second Op-Amp, therefore, is the Virtual ground. All connections labelled as virtual ground must be connected to the pin 6 of this Op-Amp

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