Bridge Amp
Owner’s Guide
30
In comparison with a notch filter, this method produces little
waveform distortion and attenuates harmonics of the mains frequency
as well as the fundamental; it effectively removes non-sinusoidal
interference, such as that commonly caused by fluorescent lights.
The filter should not be used when:
•
the interference changes rapidly. The filter takes about 1 second to
adapt to the present level. If interference is present, and then is
suddenly removed, interference in the filtered signal will
temporarily be increased.
•
your signal contains repetitive components at frequencies close to
the mains frequencies.
•
your signal is already free from interference. If the signal-to-noise
ratio is greater than about 64 the mains filter introduces more noise
than it removes.
•
you are recording at close to maximum sampling rates. The mains
filter uses some of the PowerLab's processing power and therefore
reduces the maximum rate at which you can sample.
The template is not fully generated until about 1 second after
sampling starts and so the mains filter is not fully effective in the first
second of each data block.
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 can
be seen in older Western films: spoked wagon wheels appear to stop
or even go backwards when their rate of rotation matches the film
frame speed – obviously not 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.
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