Electroencephalograph seen from the electric circuit

The schematic diagram of the electroencephalograph seen from the electric circuit is as follows.
Electrical signals generated by millions of neurons activated by brain activity propagate to the scalp and are captured by the sensor module of the electroencephalograph attached directly above.

R1 is the neuron to electrode impedance (fixed value + contact impedance) and R2 is the input impedance (resistance in the electroencephalograph).

Figure 1. Schematic diagram of the electrical circuit of an electroencephalograph

let's calculate

Let's recall Ohm's law and combined resistance that we learned in junior high school.

current = voltage / resistance
Combined resistance = (Resistance 1 x Resistance 2) / (Resistance 1 + Resistance 2)

It was
Let's apply it concretely.
Assume that the potential difference of the electrical signal generated from the neuron group (the potential difference between the reference point and the measurement point) is 10uV.
The impedance of the human body is about 1KΩ, and this time the contact resistance is calculated as 0Ω.
The current flowing between the electrodes at this point is

10uV/1KΩ = 0.01uA

becomes.
If the input impedance of the electroencephalograph is 10MΩ,
The combined resistance is

(1K x 10M) / (1K + 10M) = 999.9Ω

and the observed voltage is

0.01uA x 999.9Ω = 9.999uV

Therefore, almost no error occurs.

Figure 2. When input impedance is large

Now, let's do the same calculation when the input impedance is 10KΩ and 1KΩ.

(1K x 10K) / (1K + 10K) = 909Ω
(1K x 1K) / (1K + 1K) = 500Ω

That is, the observed voltage is

When the input impedance is 10KΩ:
0.01uA x 909Ω = 9.09uV 

When the input impedance is 1KΩ:
0.01uA x 500Ω = 5uV

becomes.
I hope you have realized that the error increases when the input impedance is small.

Figure 3. When the input impedance is 10kΩ

Figure 4. When the input impedance is 1kΩ

Summary

I hope you can understand the results.

・If the input impedance is sufficiently large compared to the impedance (fixed value + contact resistance) from the neuron to the electrode,
Measurement error drops to negligible levels
・If the input impedance is sufficiently large, the degree of influence can be suppressed even if the contact resistance increases somewhat.
・The contact resistance of the impedance from the neuron to the electrode is a variable, so if the input impedance cannot be increased,
Efforts must be made to reduce contact resistance

can be said.

Related information

New Technician's Electroencephalogram Measurement Record - Electrode Arrangement Method -

Electroencephalogram measurement record artifact of a new engineer ① ~ power supply noise edition ~

How to choose an electroencephalograph (EEG)

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