Multifunction DAQ and SCXI Signal Conditioning
Accuracy Specifications Overview
For both DAQ devices and SCXI modules, you should use the 1. Convert a typical expected value from the unit
Single-Point System Noise specification from the accuracy tables
of measure to voltage
when you are making single-point measurements. If you are 2. Calculate absolute accuracy for that voltage
averaging multiple points for each measurement, the value for 3. Convert absolute accuracy from voltage to the unit of measure
System Noise changes. The Averaged System Noise in the accuracy
tables assumes that you average 100 points per measurement. If you
Note: it is important to use a typical measurement value in this
are averaging a different number of points, use the following process, because many conversion algorithms are not linearized.
equation to determine your Noise + Quantization:
You may want to perform conversions for several different values in
your probable range of inputs, rather than just the maximum and
minimum values.
System Noise = Average System Noise from table x (100ꢃnumber of points)
For an example calculation, we want to determine the absolute
For example, if you are averaging 1,000 points per measurement system accuracy of an NI SCXI-1102 system with a NI PCI-6052E,
with the PCI-6052E in the ± 10 V (± 100 mV for the SCXI-1102) measuring a J-type thermocouple at 100 °C.
input range, System Noise is determined by:
1. A J-type thermocouple at 100 °C generates 5.26ꢂ mV
NI PCI-6052E**
(from a standard conversion table or formula)
System Noise= ꢂ7.0 0 ꢁV x (100ꢃ1000) = 27.5 0 ꢁV
2. The absolute accuracy for the system at 5.26ꢂ mV is ± 0.ꢂ2%.
NI SCXI-1102
This means the possible voltage reading is anywhere from
5.225 to 5.311 mV.
System Noise= 5 ꢁV x SQRT (100ꢃ1000) = 1.5ꢂ ꢁV
3. Using the same thermocouple conversion table, these values
represent a temperature spread of ꢀꢀ.3 to 100.7 °C.
**The System Noise specifications assume that dithering is disabled for single-point
measurements and enabled for averaged measurements.
Therefore, the absolute system accuracy is ± 0.7 °C at 100 °C.
See page 21 or visit ni.com/calibration for more information
on the importance of calibration on DAQ device accuracy.
Benchmarks
The calculations described above represent the maximum error you
Absolute System Accuracy
should receive from any given component in your system, and a
Absolute System Accuracy represents the end-to-end accuracy method for determining the overall system error. However, you
including the signal conditioning and DAQ device. Because absolute typically have much better accuracy values than what you obtain
system accuracy includes components set for different input from these tables.
ranges, it is important to use Absolute Accuracy RTI numbers for
each component.
If you need an extremely accurate system, you can perform an
end-to-end calibration of your system to reduce all system errors.
However, you must calibrate this system with your particular input
type over the full range of expected use. Accuracy depends on the
quality and precision of your source.
Total System Accuracy RTI = ± SQRT ±(Module Absolute Accuracy RTI)2
+ (DAQ Device Absolute Accuracy RTI)2]
We have performed some end-to-end calibrations for some typical
The following example calculates the Absolute System Accuracy configurations and achieved the results in Table 1:
for the SCXI-1102 module and PCI-6052E DAQ board described in
the first examples:
To maintain your measurement accuracy, you must calibrate your
measurement system at set intervals over time.
Total System Accuracy RTI = ± ±(0.00273)2 + (0.000505)2] = ± 0.27ꢂ%
For a current list of SCXI signal conditioning products
with calibration services, please visit ni.com/calibration
Units of Measure
In many applications, you are measuring some physical phenomenon,
such as temperature. To determine the absolute accuracy in terms of
your unit of measure, you must perform three steps:
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