Every Measurement Counts
There is no room for uncertainty in your measurements. From sensor
to software, your system must deliver accurate results. National Instruments
provides detailed specifications for our products so that you don't have
to guess how they perform. Along with traditional specifications, our DAQ
device or signal conditioning products include accuracy specifications
to assist you in selecting the appropriate hardware for your application.
Absolute accuracy is the specification you must use to determine the
overall maximum possible error of your measurement. Absolute accuracy does
assume you have calibrated your DAQ device or signal conditioning equipment within the last year.
Main Components of Absolute Accuracy
There are four main components of an absolute accuracy specification:
1) Percent of Reading is a percentage of the actual input voltage.
2) Offset is a constant value applied to all measurements.
3) System Noise and Quantization is based on noise and depends on the
number of points averaged for each measurement.
4) Temperature Drift is based on variations in your ambient temperature.
Absolute Accuracy Formula
Based on these components, the formula for Calculating absolute accuracy
for a given module is:
Absolute Accuracy = ±((Input Voltage * % of reading) + Offset + System
Noise + Temperature Drift)
Absolute Accuracy with Respect to the Input (RTI) Formula
Absolute Accuracy RTI = ±(Absolute Accuracy / Input Voltage)
System Absolute Accuracy Formula
Since the uncertainty of the individual components is assumed to be independent, the combined standard uncertainty is the square root of the combined variance. This is given in the following equation:
The combined standard uncertainty is an estimated standard deviation and characterizes the dispersion of the values that could reasonably be attributed to the measurand. Refer to "Determining Combined Standard Uncertainty" in the Guide to the Expression of Uncertainty in Measurement (GUM) published by ISO or ANSI/NCSL.
Temperature Drift Formula
Temperature effects are already
accounted for unless your ambient temperature is outside of the 15 to 35° C
HERE range. For instance, if your ambient temperature is at 45° C,
you must account for 10° C of drift., This is calculated by:
Drift = ±(Input Voltage * % of Reading/° C
+ Offset/° C)
The calculations described above represent the
maximum error you should receive from any given component in your system,
and a method for determining the overall system error. However, you
typically have much better accuracy values than what you obtain from these
tables. If you need an extremely accurate system, you can perform an
end-to-end calibration of your system to reduce all system errors.
However, this system must be calibrated with your particular input type
over the full range of expected use. Accuracy depends on the quality and
precision of your source. We have performed some end-to-end calibrations
for some typical configurations and achieved the results below:
||±0.25 °C at 250 ° C or ±24 mV at 9.5 V
||±0.21 mV at 9.5 V
||±2.2 mV at 2 V