Chapter 5 Using the SCXI-1125
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National Instruments Corporation
5-3
Complete the following steps to calculate the overall temperature error
using the SCXI-1125 with an E Series MIO DAQ device:
1.
Based on the required temperature range and the type of sensor,
determine the gain to use. For example, using a K-type thermocouple
with a required temperature range of 0 to 100 °C, the corresponding
voltage range is –1.002 mV to 4.0962 mV (averaging 41.0 µV/°C in
this range). For this example, use a gain of 1000 for this temperature
range to get maximum temperature resolution.
2.
Next, look up the analog accuracy specifications from Appendix A,
, for the gain and filter settings you have chosen. You
must consider how offset, gain, and system noise affect your
measurement. You might also consider common-mode rejection,
temperature drift, and other specifications based on the operating
environment. For example, using a gain of 1000, the offset error is
± 0.2 µV, the gain error is ± 0.03% which corresponds to ± 1.43 µV at
full-scale temperature, and the system noise is 100 V
rms
(use peak
noise which is about 3 times this, or 300 nV
pk
) because of the 4 Hz
filter. In this example you might or might not be able to average out the
noise. The total error is ± 1.73 µV at the full-scale temperature range,
which gives a preliminary accuracy of ± 0.04 °C (1.73 µV divided by
41.0 µV/°C).
3.
Next, consider the accuracy of the cold-junction sensor you are using.
For example, using the SCXI-1328, which, at about room temperature
with little temperature gradient, has an accuracy of ± 0.5 °C. You must
convert this temperature accuracy back to a voltage corresponding to a
K-type thermocouple accuracy at 25 °C. This conversion produces
about ± 20 µV of error.
4.
Add the two voltages and determine the overall temperature error. For
example, the total error due to the SCXI portion of the system in this
example now becomes ± 21.73 µV. This total error corresponds to
about ± 0.53 °C (21.73 µV divided by 41.0 µV/°C) temperature error
using the K-type thermocouple at this range.
5.
Determine the contribution of DAQ device error. For example, if using
a 12-bit DAQ device, the DAQ device contributes a gain of 2, and
therefore the code width becomes 2.44 µV. As a result, the total system
error now becomes ± (21.73 µV + 2.44 µV), which corresponds to
about 0.59 °C. If you were to choose a 16-bit board, you can achieve a
code width of 0.153 µV, producing a total system error of 0.53 °C.
