First, thank you for your previous answer.
I'm wondering about the use of the reference and active electrodes in the recording of GSR.
To measure biopotentials, it is important to have a good common mode rejection ratio. I found some schemes on your website. One of them is about CMRR: http://www.biosemi.com/mark8_cmrr.htm
If I understand correctly, the common mode (floating common) is the mean between E1 and E2, respect to the CMS. If this mean (E1 and E2) is more positive than the CMS, electrical current (physical) is injected via the DRL. This will increase the potential near the DRL electrode (and near CMS as well, because of proximity). This decreases the common mode, because E1 and E2 are potentials respect to CMS. If the common mode is more negative than the CMS, current is "absorbed" by the DRL electrode. 1) Is that right
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About the GSR now. From what I have read, two electrodes are enough to record GSR. A constant voltage can be applied between the two electrode. Then, a small resistance is put in series with the skin and the voltage across this resistance is measured. In the Biosemi device, I have read that the current is a 16 Hz square wave with a 1 uA amplitude. So, measuring a voltage drop across a small resistance would be useless.
Instead, the potential between the two skin electrodes is measured, respect to the reference electrode. This voltage will be proportional to the resistance of the skin. But my concern is about the injected current. It must be constant through one of the electrodes only. Because the skin behaves like a capacitor and charges can accumulate there. The same about the rest of the body. 3) So... the currents across both electrodes are not exactly the same
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My third concern is about the frequency of the injected current between the GSR electrodes. I have read that in some modules, the frequency was 512 Hz and in other it was 16 Hz. 5) How to know the frequency of the injected current
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I know it's a long message, but I needed to write it. I feel like I have found some of the answers by myself while writing it.
Could you just confirm my guesses ? It would be nice !
Gordak
Ho and if the BBcode could be activated it would be really great !