ADInstruments POWERLAB TEACHING SERIES Owner's Manual page 31

Mains Filter. The mains filter allows you to remove interference at the mains frequency
(typically 50 or 60 Hz). This is an adaptive filter and should only be used when the signal
to mains noise ratio is less than 36 dB, that is the mains noise amplitude is greater
than 1/64 of the signal amplitude. More details on the mains filter can be found in the
LabChart Help Center.
Anti-alias.
Select Anti-alias to apply a sample rate-dependent low-pass filter that prevents aliasing
artifacts. The input signal is sampled and digitized at 100 kHz. The AD converter
incorporates a low-pass filter of fixed cutoff frequency (49 kHz). When LabChart samples
at 100 kHz, the converted samples are passed on unchanged. At all other LabChart
sampling rates, an additional (digital) filter is optionally applied to the AD converter's
100 kHz output stream. This is a decimating FIR filter whose cutoff frequency is set to
the Nyquist value: half the LabChart sampling frequency. It thus acts as an automatic
anti-aliasing filter, providing optimum noise rejection in the recorded data. Attenuation
in the stop band is at least 40 dB. Note that anti-aliasing is disabled at sampling rates
below 100 Hz so as to prevent the delay that otherwise occurs as samples pass through
the FIR filters.
The anti-aliasing filter characteristics are chosen to give the fastest step response
without overshoot (corresponding to an analog Bessel filter). Such a response is suited
to most types of physiological recordings. In the frequency domain, the filter response
starts to roll off well below the cutoff frequency. In certain (relatively uncommon) types
of measurement, a maximally flat response is desirable; this can be obtained by turning
anti-aliasing off .
Inverting the Signal
Selecting Invert allows you to invert the signal on the screen. It provides a simple way
to change the polarity of the recorded signal without having to swap the connections to
the recording electrodes.
PowerLab Teaching Series - Owner's Guide
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