Abrade DRL site

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sponlab
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Joined: Wed Jul 10, 2013 4:16 pm

Abrade DRL site

Post by sponlab »

Will abrading the DRL site ALONE help reduce excess 60hz noise?

Coen
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Re: Abrade DRL site

Post by Coen »

The impedance of the DRL electrode is one of the factors that determines the amount of Common Mode interference (the type of interference that is reduced a factor of 1000 when switching from unreferenced to referenced display, see http://www.biosemi.com/faq/cms&drl.htm). With the ActiveTwo system, Common Mode is relatively low in almost all situations - even with high DRL impedances - because of the system setup with a CMS/DRL feedback loop, compact battery powered front-end close to the subject, and a fiber-optic data transfer to the computer. In cases where the common mode level is high enough to be a problem with the DRL electrode on the unabraded skin, the amount of leakage currents will almost certainly cause also a lot of differential 50 Hz interfernce, see http://www.biosemi.com/faq/battery_and_fiber.htm. Since reducing the DRL impedance will only influence Common Mode, it is in practice more effective to attempt the usual methods to reduce interference leakage currents right away (locate AD-box close to the subject, use non-conductive chairs and tables, etc.).

Although we do not recommend abrading the DRL site, it should still be noted that the contact quality of the DRL electrode is a bit more critical than of all other electrodes (including CMS). The DRL has to conduct the small interference currents between AD-box and subject (may be up to the microAmp range), whereas all other electrodes merely have to conduct the extremely small input bias currents of the active electrdes (in the 100 picoAmp range per electrode). Therefore, we recommend to pay a bit more attention to the DRL electrode: use sufficient gel and allow the gel some time (a few minutes) to penetrate the upper layers of the skin.

Best regards, Coen (BioSemi)

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