Our firmware applies a strong double notch filter at 50Hz and 60Hz to remove interference from the electrical power supply in most countries. The filter affects frequencies down to about 45Hz, so we specify 43Hz as the upper usable frequency limit where the spectral response is perfectly flat. The filters extend to about 66Hz, which is higher than the Nyquist cut-off frequency for 128Hz sample frequency. Our firmware also filters frequencies above about 85Hz in order to eliminate potential interference from harmonics of the electrical mains (100, 120, 150, 180Hz etc). Increasing the frequency response above 43Hz is not currently available.
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