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,

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)]

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.

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

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%

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 %

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 %

Multifunction DAQ and SCXI Signal Conditioning

Accuracy Specifications Overview