ADInstruments Bio Amp, Owner'S Manual

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Chapter 2 Setting Up
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High-Pass Filtering . The
High Pass
pop-up menu gives the choice of 0.1, 0.3,
1, 3, and 10 Hz high-pass filters. Dual and Octal Bio Amps have a 0.02 Hz
filter as well. The high-pass filter removes frequencies below the chosen
frequency and allows high frequencies in the signal. These filters are useful for
removing slowly moving baselines, such as motion or respiration artifacts,
particularly in ECG (EKG) recordings.
Low-Pass Filtering . The
Low Pass
pop-up menu gives the choice of 50, 100,
200, and 500 Hz, and 1 and 5 kHz low-pass filters. Dual and Octal Bio Amps
have a 2 kHz filter as well. The low-pass filter removes frequencies above the
chosen frequency and allows low frequencies in the signal. These filters are
useful for removing high-frequency signals, such as noise, and to prevent
aliasing in the recorded signal.
Notch Filter . Select or deselect the
Notch
checkbox to turn the notch filter on
and off. The notch filter is automatically set to either 50 or 60 Hz, depending
on the power line voltage frequency being used by the PowerLab (the mains
frequency). It provides approximately 32 dB of attenuation, thus reducing the
effect of the 50 or 60 Hz signals that can easily be picked up by long leads.
Mains Filter . Select or deselect the
Mains filter
checkbox to turn the mains
filter on and off. The mains filter allows you to remove interference related to
the mains frequency (both fundamental and harmonic frequencies). This is
an adaptive filter. It adjusts to filter the interference by tracking the input
signal for a second. Because of this, in general, using the mains filter is better
than using the notch filter but the mains filter does have some limitations.
More details on the mains filter can be found in the LabChart Help Center.
Anti-alias . Click the
Anti-alias
checkbox to turn anti-aliasing on and off.
Aliasing is distortion caused by frequencies of the incoming biological
waveform that are more than half the sampling frequency.
If you monitor physiological signals with a low-pass filter setting of 100 Hz,
but you are only sampling at 100 Hz, aliasing may cause the recorded
waveform to be quite different from the actual signal. An analogy to aliasing
can be seen in older Western films: spoked wagon wheels may appear to stop
or even go backwards when their rate of rotation matches the film frame
speed — obviously not showing an accurate record of the wheels’ motion.
To prevent aliasing, the sampling rate must be at least twice the rate of the
highest expected frequency of the incoming waveform. For example, if
monitoring an ECG with maximum frequency components of 100 Hz, the
sampling rate needs to be at least 200 Hz to provide an accurate signal. The
sampling rate could be increased further if fast spikes or peaks (such as in the
QRS complex of an ECG) must be accurately recorded. A high sampling rate,
however, will use more computer memory and may limit recording time.