Measurement Computing DaqScan 2000 Series - Manual (Page 50)

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., DaqLab/2001 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 a shielded Ethernet cable, e.g., CA-242, to connect the DaqLab or DaqScan
chassis to the host computer’s chassis.
• 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 the 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 959195 DaqLab/2000 Series and DaqScan/2000 Series