IOtech DaqBook/2000 Series, Руководство пользователя

Страница: 60 / 90

Noise Considerations
Controlling electrical noise is imperative because it can present problems even with the best measurement
equipment. Most laboratory and industrial environments suffer from multiple sources of electrical noise.
For example, AC power lines, heavy equipment (particularly if turned on and off frequently), local radio
stations, and electronic equipment can create noise in a multitude of frequency ranges.
Local radio stations are a source of high frequency noise, while computers and other electronic equipment
can create noise in all frequency ranges. Creating a completely noise-free environment for test and
measurement is seldom practical. Fortunately, simple techniques such as using shielded/twisted pair wires,
filtering, and differential voltage measurement are available for controlling the noise in our measurements.
Some techniques prevent noise from entering the system; other techniques remove noise from the signal.
While many techniques for controlling noise in signals provide a means of removing the noise that is
already present, the preferred solution is to prevent the occurrence of noise in the signal in the first place.
The following practices, some of which are required for CE compliance, should be employed to minimize
noise.
•
Make a solid earth ground connection . Required for CE Compliance . Ensure that the chassis of the
primary data acquisition device, e.g., DaqBook/2020 is connected to earth ground. This practice:
(a) keeps radiated emissions low by keeping the chassis electrically quiet, (b) keeps potential common-
mode voltages low, (c) improves user safety, and (d) provides a safe path for Electrostatic Discharge
energy back to earth ground.
•
Use short Ethernet cables . The use of short Ethernet cables will reduce noise. The shorter the cable
the better.
•
Use shielded cables . Loose wires are effective antennae for radio frequency pickup and can form
loops for inductive pickup. The use of properly connected shields will greatly reduce such noise.
•
Minimize ambient EMI . The lower the ambient EMI, the better. Sources of electromagnetic
interference include solenoids, motors, computer equipment, high power distribution wiring, etc.
•
Distance cables . Power supply switch transients can vary in strength and frequency. Ethernet cables
can radiate digital switching noise. For these reasons route the power supply and Ethernet cables such
that they are as far as possible from all analog lines. The analog lines include those that connect to P1
on the front panel, as well as those that connect to the rear panel via an expansion port, or through the
P4 connector.
•
Use ferrite inductive collars . A clamp-on ferrite collar can be secured at each end of the Ethernet
cable and at both ends of the power supply cable. The collars on the Ethernet cable will reduce digital
switching noise. Note that the collars will not reduce the integrity of the Ethernet channel if the
system is making use of shielded Ethernet cables, part number CA-242. Ferrite collars on the power
cable will reduce the effects of power transients.
This partial view of the host PC shows the
Ethernet cable with a ferrite collar. The
PC’s connection to the ground-line is also
visible.
This partial view of a data acquisition device
shows its connection to the ground-line. It also
shows two ferrite collars, the foremost of which is
clamped to the power cable. The background
collar is clamped onto the Ethernet cable.
7-4 CE-Compliance & Noise Considerations
897995
DaqBook/2000 and DaqOEM/2000 Series