PowerLab Teaching Series - Owner’s Guide
37
When devices that transfer a lot of information, such as scanners and video cameras,
are connected to the same USB tree and are used at the same time as a PowerLab,
sampling rates may be limited considerably (in LabChart) . All computers (both PC and
Macintosh) usually have several independent USB ports. Using these, rather than a hub,
to connect multiple devices will avoid them competing for capacity (bandwidth).
You can safely turn on or o , or disconnect or reconnect, a USB-connected PowerLab
while the computer remains on, as long as the application program is o when you do it.
Earthing and Ground Loop Noise
The prime function of earthing is safety, that is, protection against fatal electrocution.
Safety concerns should always override concerns about signal quality. Secondary
functions of earthing are to provide a reference potential for the electrical equipment
and to mitigate against interference.
The earthing (grounding) stud provided on the back panel of the PowerLab is a
potential equalization post and is compatible with the DIN 42801 standard. It is directly
connected to the earth pin of the power socket and the PowerLab chassis. The earthing
stud can be used where other electronic equipment is connected to the PowerLab, and
where conductive shields are used to reduce radiative electrical pick-up. Connection to
the stud provides a common earth for all linked devices and shields, to reduce ground-
loops.
The earthing stud can also be used where a suitable ground connection is not provided
with the mains supply by connecting the stud to an earthed metal infra-structure,
such as a metal stake driven into the ground, or metal water piping. This may also be
required in laboratories where safety standards require additional grounding protection
when equipment is connected to human subjects. Always observe the relevant safety
standards and instructions.
Note that magnetically-induced interference in the recorded signal can be reduced by
minimizing the loop area of signal cables, for example by twisting them together, or by
moving power supplies away from sensitive equipment. This can reduce the inductive
pick-up of mains frequency fields. Please consult a good text for further discussion of
noise reduction
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