Multifunction DAQ and SCXI Signal Conditioning
Accuracy Specifications Overview
Multifunction DAQ Accuracy Specifications
Every Measurement Counts
There is no room for error in your measurements. From sensor to
software, your system must deliver accurate results. NI provides
detailed specifications for our products so you do not have to guess
how they will perform. Along with traditional data acquisition
specifications, our E Series multifunction data acquisition (DAQ)
devices and SCXI signal conditioning modules include accuracy tables
to assist you in selecting the appropriate hardware for your application.
To calculate the accuracy of NI measurement products,
visit
ni.com/accuracy
Below is the Absolute Accuracy at Full Scale calculation for the
NI PCI-6052E DAQ device after one year using the ±10 V input
range while averaging 100 samples of a 10 V input signal. In all the
Absolute Accuracy at Full Scale calculations, we assume that the
ambient temperature is between 15 and 35 °C. Using the Absolute
Accuracy table on the next page, we see that that the calculation for
the ±10 V input range for Absolute Accuracy at Full Scale yields
4.747 mV. This calculation is done using the parameters in the same
row for one year Absolute Accuracy Reading, Offset and Noise +
Quantization, as well as a value of 10 V for the input voltage value.
You can then see that the calculation is as follows:
Absolute Accuracy = ±[(10 X 0.00037) + 947.0 µV + 87 µV] = ±4.747 mV
Absolute Accuracy
Absolute accuracy is the specification you use to determine the
overall maximum tolerance of your measurement. Absolute accuracy
specifications apply only to successfully calibrated DAQ devices
and SCXI modules. There are four components of an absolute
accuracy specification:
•
Percent of Reading
– is a gain uncertainty factor that is multiplied
by the actual input voltage for the measurement.
•
Offset
– is a constant value applied to all measurements.
•
System Noise
– is based on random noise and depends on
the number of points averaged for each measurement
(includes quantization error for DAQ devices).
•
Temperature Drift
– is based on variations in your
ambient temperature.
•
Input Voltage
– the absolute magnitude of the voltage input
for this calculation. The fullscale voltage is most commonly used.
Based on these components, the formula for calculating absolute
accuracy is:
Absolute Accuracy = ±[(Input Voltage X % of Reading) +
(Offset + System Noise + Temperature Drift)]
Absolute Accuracy RTI
1
= (Absolute Accuracy Input Voltage)
1
RTI = relative to input
In many cases, it is helpful to calculate this value relative to the input
(RTI). Therefore, you do not have to account for different input
ranges at different stages of your system.
Absolute Acuracy RTI = (±0.004747/10) = ±0.0475%
Data Acquisition and
Signal Conditioning
The following example assumes the same conditions except that the
ambient temperature is 40 °C. You can begin with the calculation
above and add in the Drift calculation using the % Drift per °C from
Table 2 on page 196.
Absolute Accuracy = 4.747 mV + ((40 – 35 °C) x 0.000006 /°C X 10 V) = ±5.047 mV
Absolute Acuracy RTI = (±0.005047/10) = ±0.0505%
Absolute Accuracy for SCXI Modules
Below is an example for calculating the absolute accuracy for the
NI SCXI-1102 using the ±100 mV input range while averaging
100 samples of a 14 mV input signal. In this calculation, we assume
the ambient temperature is between 15 and 35 °C, so Temperature
Drift = 0. Using the accuracy table on page 313, you find the
following numbers for the calculation:
Input Voltage = 0.014
% of Reading Max = 0.02% = 0.0002
Offset = 0.000025 V
System Noise = 0.000005 V
Absolute Accuracy = ±[(0.014 x 0.0002) + 0.000025 + 0.000005] V = ±32.8 µV
Absolute Accuracy RTI = ±(0.0000328 / 0.014) = ±0.234 %
Temperature drift is already accounted for unless your ambient
temperature is outside 15 to 35 °C. For instance, if your ambient
temperature is at 45 °C, you must account for 10 °C of drift. This is
calculated by:
Temperature Drift = Temperature Difference x % Drift per °C x Input Voltage
Absolute Accuracy for DAQ Devices
Absolute Device Accuracy at Full Scale is a calculation of absolute
accuracy for DAQ devices for a specific voltage range using the
maximum voltage within that range taken one year after calibration,
the Accuracy Drift Reading, and the System Noise averaged value.
The following example assumes the same conditions, except the
ambient temperature is 40 °C. You can begin with the Absolute
Accuracy calculation above and add in the Temperature Drift.
Absolute Accuracy = 32.8 µV + (0.014 x 0.000005 + 0.000001) x 5 = ±38.15 µV
Absolute Accuracy RTI = ±(0.00003815 / 0.014) = ±0.273 %
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