ADInstruments BIO, Owner'S Manual

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Owner’s Guide
14
Similarly, if the high-pass filter was set too high, the amplitude of the T-wave sections
may be reduced. The
Input Amplifier... dialog allows you to examine ECGs and similar
slowly changing waveforms to fine-tune filter settings before recording.
Saturation
Saturation occurs when the range is set too low for the signal being measured (the
amplification, or gain, is too high). As the signal amplitude exceeds the allocated range,
the recorded waveform appears as if part of the waveform had been cut off , an eff ect
referred to as clipping.
Clipping can also be caused by excessive baseline off set: the off set eff ectively moves
the whole waveform positively or negatively to an extent that causes all or part of it to
be clipped. This problem is overcome by selecting a higher range from the Range menu
in the front-end-specific
Input Amplifier... dialog. In the case of excessive baseline
off set, you may wish to apply a high-pass filter with a higher frequency cut-off .
Ground Loops
Ground loops occur when multiple connected pieces of recording equipment are
connected to mains power grounds. For safety reasons,
all electrical equipment
should have a proper connection to the mains power grounds, or to a primary earth
connection in situations where a mains ground connection is not available. Connecting
linked electrical equipment to a common earth connection (equipotential connection
point) – such as the earthing (grounding) stud provided on the rear of all PowerLabs –
can prevent ground loops.
The electric fields generated by power lines can introduce interference at the line
frequency into the recorded signal. Electromagnetic fields from other sources can also
cause interference: fluorescent tubes, apparatus with large transformers, computers,
laptop batteries, network cables, x-ray machines, microwave ovens, electron
microscopes, even cyclic air conditioning.
Reasonable care in the arrangement of equipment to minimize the ground loop area,
together with proper shielding, can reduce electrical frequency interference. For
example, use shielded cables, keep recording leads as short as possible, and try twisting
recording leads together. For sensitive measurements, it may be necessary to place the
subject (the biological source) in a Faraday cage.
Interference should first be minimized, and then you can turn on the Mains filter in the
front-end-specific
Input Amplifier... dialog.
Mains filter
The Mains filter (/20, /25, /30, /35 and 26T PowerLabs) allows you to filter out interference
at the mains frequency (typically 50 or 60 Hz). The mains filter is an adaptive filter which
tracks the input signal over approximately 1 second. A template of mains-frequency
signal present in the input is computed from the signal. The width of the template is
the mains power period (typically 16.6 or 20 ms) as determined from zero-crossings of