Chapter 4
Connecting and Using Accessories and Transducers
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National Instruments Corporation
4-9
Noise and Filtering
Because thermocouples inevitably pick up electrical noise, National
Instruments recommends that you sample thermocouples at a higher rate
than you actually need and average the data to reduce its noise variation.
Try averaging at least 30 samples per reading. In addition, averaging over
one power cycle helps reject power line noise; for example, averaging
100 samples per reading at a 6 kHz scan rate rejects 60 Hz noise in your
SCXI system.
Hardware filtering is another way to reduce noise. The SCXI-1102,
SCXI-1120, SCXI-1121, and SCXI-1125 have lowpass noise filters on
every input channel that you can apply as detailed in Chapter 2,
SCXI Modules and Connecting the SCXI System to the DAQ Device
. These
filters are commonly used in thermocouple applications.
The SCXI-1100 includes a single selectable filter following the
multiplexer. Therefore, enabling the 4 Hz filter limits sampling rates to less
than 4 S/s—attempting to sample faster produces erroneous data. This is
not a problem on the SCXI-1102, SCXI-1120, SCXI-1121, and SCXI-1125
because each channel has its own filter. Refer to Table 4-4 for SCXI-1100
maximum recommended sampling rates.
Strain Gauges
The strain gauge is another common transducer. The strain gauge requires
excitation; therefore, an SCXI-1121 or SCXI-1122 module is suitable for
measuring this transducer.
First, establish whether the strain gauge is a quarter-bridge, half-bridge, or
full-bridge configuration and determine the level and type of excitation the
strain gauge requires.
For example, a half-bridge configuration commonly requires a voltage
excitation of 3.333 V. Because the strain gauge requires 3.333 V of
excitation, you do not need to change the settings of the excitation mode or
level jumpers because 3.333 V is the factory-default setting. Because you
have a half-bridge strain gauge configuration, you must enable the
half-bridge completion network. Set jumpers W17 and W18 (for channel 1)
to the positions shown in Table 3-15.
Warning
If the strain gauge has a high common-mode voltage, refer to Appendix A,
Specifications, of the SCXI-1121 User Manual to verify that the common-mode voltage is
within the specified limits before connecting the signal to the SCXI-1121.
