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New Technician's Electroencephalogram Measurement Notes Extra Edition (Supplement)

Introduction
What is EEG? What is EEG? what do you know?
"Newcomer Technician's Electroencephalogram Measurement Notes", where newcomer engineers who started in such a situation post what they felt when they actually touched the EEG.
I'm going to tell you a raw voice that challenges without prior knowledge.
Concept of input impedance
We explained in "How to choose an electroencephalograph (EEG)" that the influence of contact resistance can be suppressed by having a large input impedance.
This time, I would like to take a break from brain waves and review electrical circuits.
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.
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