National Instruments SCXI-1125 - page 40

Chapter 5 Using the SCXI-1125
© National Instruments Corporation 5-3 SCXI-1125 User Manual
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,
Specifications, 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 Vrms (use peak
noise which is about 3 times this, or 300 nVpk) 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.