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AD620 is commonly used for DIY EEG/EC/EMG projects. I've seen an instrumentation amplifier that is designed by using 3x TL081 opamp for EEG project.

For example : enter image description here

I've checked that AD620's input offset voltage is in uV and TL081's in mV. As you know input signals are in the range of micro volts for EEG.

1) Do you think using the figure above as instrumentation amplifier would just work fine or would have high error level ?

2) How can I choose what resistor value is best for me ? In the figure Rg = 220 ohm, for example. What would occur if Rg was 3.3k ?

3) As known, high and low pass filters are used in EEG project and those filters have opamp inside. What should be the gain value for them ?

Thanks in advance for your help. Have a nice day.

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  • \$\begingroup\$ Best if you use an integrated frontend for biopotentials like ADS1194. It is basically a body-to-digital converter. You can configure all gains and options via SPI. \$\endgroup\$ – filo Feb 15 '18 at 17:36
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To start with, the TL081 is far from the best op amp to use. Take a look at the Open EEG Project, specifically the schematics

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1) It depends on resistor matching. You should use at least 1% tolerance or even lower tolerance.

2) You can search the web for instrumentation amplifier to get the equations: wiki

3) The noninverting input of the opamp has a very high impedance in terms of giga ohms. You can add a network of resistors and capacitors to make a LPF at the input. Since the current into opamp is almost zero, the resistor would not affect the measuring even if they are in range of hundred kohm. You should put also some ESD protection on the input, like diodes, but those with very small leakage current or JFET transistors.

Olimex EEG

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  • \$\begingroup\$ What do you say about input offset voltage ranges mentioned in the first question ? \$\endgroup\$ – PIC16F84A Sep 4 '17 at 17:50
  • \$\begingroup\$ @PIC16F84A You have input offset voltage, not range. Each Opamp has this offset voltage (datasheet) that is amplified, you can calculate how much offset you will get on output. By range, it is only the common mode range CMR. \$\endgroup\$ – Marko Buršič Sep 4 '17 at 18:08

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