Multifunction DAQ and SCXI Signal Conditioning

Accuracy Specifications Overview

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For both DAQ devices and SCXI modules, you should use the

Single-Point System Noise specification from the accuracy tables

when you are making single-point measurements. If you are

averaging multiple points for each measurement, the value for

System Noise changes. The Averaged System Noise in the accuracy

tables assumes that you average 100 points per measurement. If you

are averaging a different number of points, use the following

equation to determine your Noise + Quantization:

System Noise = Average System Noise from table x

(100/number of points)

For example, if you are averaging 1,000 points per measurement

with the PCI-6052E in the ±10 V (±100 mV for the SCXI-1102)

input range, System Noise is determined by:

NI PCI-6052E**

System Noise= 87.0 0 µV x

(100/1000) = 27.5 0 µV

NI SCXI-1102

System Noise= 5 µV x SQRT

(100/1000) = 1.58 µV

**The System Noise specifications assume that dithering is disabled for single-point

measurements and enabled for averaged measurements.

on the importance of calibration on DAQ device accuracy.

Absolute System Accuracy

Absolute System Accuracy represents the end-to-end accuracy

including the signal conditioning and DAQ device. Because absolute

system accuracy includes components set for different input

ranges, it is important to use Absolute Accuracy RTI numbers for

each component.

Total System Accuracy RTI = ±SQRT [(Module Absolute Accuracy RTI)2

+ (DAQ Device Absolute Accuracy RTI)2]

The following example calculates the Absolute System Accuracy

for the SCXI-1102 module and PCI-6052E DAQ board described in

the first examples:

Total System Accuracy RTI = ±

[(0.00273)2 + (0.000505)2] = ±0.278%

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:

1. Convert a typical expected value from the unit

of measure to voltage

2. Calculate absolute accuracy for that voltage

3. Convert absolute accuracy from voltage to the unit of measure

Note: it is important to use a typical measurement value in this

process, because many conversion algorithms are not linearized.

You may want to perform conversions for several different values in

your probable range of inputs, rather than just the maximum and

minimum values.

For an example calculation, we want to determine the absolute

system accuracy of an NI SCXI-1102 system with a NI PCI-6052E,

measuring a J-type thermocouple at 100 °C.

1. A J-type thermocouple at 100 °C generates 5.268 mV

(from a standard conversion table or formula)

2. The absolute accuracy for the system at 5.268 mV is ±0.82%.

This means the possible voltage reading is anywhere from

5.225 to 5.311 mV.

3. Using the same thermocouple conversion table, these values

represent a temperature spread of 99.3 to 100.7 °C.

Therefore, the absolute system accuracy is ±0.7 °C at 100 °C.

Benchmarks

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, 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.

We have performed some end-to-end calibrations for some typical

configurations and achieved the results in Table 1:

To maintain your measurement accuracy, you must calibrate your

measurement system at set intervals over time.

For a current list of SCXI signal conditioning products