Maintaining low and stable electrode contact impedances is critical for obtaining high quality signals in out-ofthe-lab EEG units. Current EEG units measure the impedance of the electrode contacts by injecting an out-of-band, typically 1 kHz, current into the head. This high frequency component is easily isolated from the true EEG to avoid introducing artifacts, but does not give direct information on the contact impedance at the wanted cortical frequencies, typically at 5–30 Hz. This paper investigates two techniques for allowing simultaneous impedance measurements at 5–30 Hz for the first time. One method uses digital processing for removing the EEG artifact that continuousin-band impedance monitoring produces. The other uses a new 36 nW notch filter for removing the interference. Both are shown to allow impedance monitoring at 5–30 Hz while leaving minimal residual artifacts in the collected EEG traces.
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