Measurement Computing IOtech 640 Series - Manual (Page 58)

8-4 CE-Compliance & Noise Considerations 917093 640 & 650 Series
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. Ensure that the chassis of the primary data
acquisition device, e.g., 640/650 Series device 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 (for 640e and 650e). The use of short Ethernet cables will
reduce noise. The shorter the cable the better.
• Use short USB cables (for 640u and 650u). The use of short USB 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 analog input channel lines.