I will try out the FabECG circuit.

• R1: Gain set resistor for a gain of 49400 (constant) / 2200 (gain set resistor) = 22.45

• R2/R3: Voltage divider to set the virtual-ground at 1/2 Vcc.

• C6: Bypass capacitor, used to prevent noise from entering the system by bypassing it to the ground.

• C1/R4: High-pass filter with a cut-off frequency iof 0.16 Hz (I assume to stop any DC voltage passing through). Or maybe R4 is a pull-down resistor to prevent undesired signals when the inputs are floating?

• R5/R6/RV1/C2: Setting the gain of U2.

• C2: There are several explanations that I found:

  • "improves stability properties of the whole feedback circuit - provided the value of this cap is selected properly. It is a kind of lag compensation"
  • "at high frequencies the cap acts as a short, so the gain for those frequencies is reduced/cut to unity. When this is done to prevent oscillation it's sometimes called lead compensation"

• RV1: I assume it functions here as a form of adjustable level-shifter.

• R7/C3, R8/C4, R9/C5: Passive low-pass filters with a cut-off frequency of 28.94 Hz.

• U2 is configured as an inverting amplifier.

• Perhaps the opamp (U1) is because the AD620 does not seem to go rail-to-rail on the output? (not sure if this is the case but I thought I saw that in my earlier experiment)

This circuit is based on the one on this page which may have more info, but is apparently based on Chipstein's design as well.

Next steps:

• :white_check_mark: Make a test sinewave generator (not connected to mains ground).
• :white_check_mark: Amplify signal from test sinewave generator with AD620 (done with 2x amplification).
• :white_check_mark: Read the signal using an Arduino connected to a laptop.
• :white_check_mark: Step 2: Lower signal from test sinewave generator to levels about the same as an EEG would be (simple voltage divider?).
• :white_check_mark: Step 3: Amplify that signal to a level in the volt- something range and get a clear signal.
• Step 4: Build the opamp part and test and analyse that.
• Step 5: Add in the extra opamp to the AD620 circuit and get a clear signal.

Huh, I thought I read that EEG scalp signals were in the millivolt range? Apparently not:

A typical adult human EEG signal is about 10 æV to 100 æV in amplitude when measured from the scalp (source)

EEG activity is quite small, measured in microvolts (mV) (source)

That changes things...now I understand why the opamp is set to such high amplification.