How correct connect two AD8220 for measuring EEG signals?

Hi,

I'm looking into this and will get back to you soon.

Regards,

Kristine

Hi,

If you connect the red wire as shown, then the instrumentation amplifiers do not see the same impedance at their inputs, they will be somewhat unbalanced (see "Instrumentation Amplifier (In-Amp) Inputs" on page 6 in www.analog.com/.../mt-096.pdf ). The passive filter at the inverting input of the first amplifier is loaded with another passive filter asymmetrically. Also, the input of the passive filter at the inverting input of the second amplifier is not connected directly to the signal source, while the other is. It is certainly better to avoid this, so I think you should connect the red wire to point B. 

As a matter of fact, I think the passive filter values in the datasheet are wrong. I guess 2.2pF should be 220pF and 10pF should be 10nF, as you can see in another article:

https://www.analog.com/en/technical-articles/mitigation-strategies-for-ecg-design-challenges.html

Even the protecting diodes have larger parasitic capacitance than 2.2pF, and the datasheet says 5pF for the input of the AD8220. So I don’t believe that the values are correct. I would increase the resistor values too, in many ECG applications you can see values about 100k. This could reduce the dependence on the impedance of the signal source (the body), which can also cause unbalanced impedance seen by the differential inputs.

I hope I could help you.

Hi Zoltan, thank you so much, your answer is very helpful and very professional!!!