Programming instructions

Chapter 18 Things You Should Know about SCXI
LabVIEW Data Acquisition Basics Manual 18-4
© National Instruments Corporation
Note
You can minimize noise that lead wires pick up by using shielded cables or a
twisted pair of cables, and by minimizing wire length. Also, keeping signal wires
away from AC power cables and monitors will help reduce 50 Hz or 60 Hz noise.
If you amplify the signal at the DAQ device, the signal is measured and
digitized with noise that may have entered the lead wires. However if you
amplify the signal close to the signal source with an SCXI module, noise
has a less destructive effect on the signal. In other words, the digitized
representation is a better reflection of the original low-level signal. For
more information, consult
Application Note 025, Field Wiring and Noise
Considerations for Analog Signals.
You can access this note from the
NI Fax-on-Demand system as well as the BBS, World Wide Web, or
FTP site, the numbers for which are located in the front of this manual.
Isolation
Another common way to use SCXI is to isolate the transducer signals from
the computer for safety purposes. When the signal being monitored
contains large voltage spikes that could damage the computer or harm the
operator, you should not directly connect the signal to a DAQ device
without some type of isolation. Another reason for isolation is to make sure
that the measurements from the DAQ device are not affected by differences
in ground potentials. When the DAQ device and the signal are not
referenced to the same ground potential, a ground loop may occur. Ground
loops can cause an inaccurate representation of the measured signal. If the
potential difference between the signal ground and the DAQ device ground
is large, then damage may even occur to the measuring system. Using
isolated SCXI modules will eliminate the ground loop and ensure that the
signals are accurately measured.
Filtering
Signal conditioning systems can filter unwanted signals or noise from the
signal you are trying to measure. You can use a noise filter on low-rate
(or slowly-changing) signals, like temperature, to eliminate
higher-frequency signals that can reduce the accuracy of the digitized
signal. A common use of a filter is to eliminate the noise from a 60 Hz
AC power line. A lowpass filter of 4 Hz, which exists on several SCXI
modules, is suitable for removing the 60 Hz AC noise from signals sampled
at low rates. A lowpass filter eliminates all signal frequency components
above the cutoff frequency. The SCXI-1141 module has lowpass filters that
have software-selectable cutoff frequencies from 10 Hz to 25 kHz.