DAQ BNC-2090 Manuel d'utilisateur télécharger pdf (Page 19)

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E Series DAQ Accuracy Speciﬁcations

Measurements

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 that you do not have to

guess how they will perform. Along with traditional data acquisition

speciﬁcations, our E Series multifunction data acquisition (DAQ)

devices also include accuracy tables to assist you in selecting the

appropriate hardware for your application. These tables are found

on the product pages and include speciﬁcations for both absolute

and relative accuracy.

Absolute Accuracy

Absolute accuracy is the speciﬁcation you use to determine

the overall maximum error of your measurement. Absolute accuracy

speciﬁcations apply only to a successfully calibrated DAQ device.

There are four components of an absolute accuracy speciﬁcation:

Percent of Reading is a percent of the actual input voltage.

Offset is a constant offset applied to all measurements.

Noise + Quantization is based on noise and depends on the

number of points averaged for each measurement.

Drift is based on variations in your ambient temperature.

Based on these components, the formula for calculating absolute

accuracy is:

Absolute Accuracy = ±[(Input Voltage X % of Reading) +

(Offset + Noise + Quantization + Drift)]

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:

Drift = Temperature Difference X % Drift per °C X Input Voltage

Absolute Accuracy at Full Scale is a calculation of absolute accuracy

for a speciﬁc voltage range using the maximum voltage within that

range taken one year after calibration, the Accuracy Drift Reading,

and the Noise + Quantization averaged value.

Below is the Absolute Accuracy at Full Scale calculation for the

PCI-MIO-16XE-50 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. You can see on

the next page that the calculation for the ±10 V input range for

Absolute Accuracy at Full Scale yields 1.443 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 0.0001) + 397.2 µV + 45.8 µV] = ±1.443 mV

The following example assumes the same conditions except that

the ambient temperature is 45 °C. You can begin with the calculation

above and add in the Drift calculation using the % Drift per °C from

the table on the next page (see Table 1).

Absolute Accuracy = 1.443mV + ((45 °C – 35 °C) X 0.000002 /°C X 10 V) = ±1.643 mV

If you are making single-point measurements, use the Single-Point

Noise + Quantization speciﬁcation from the accuracy tables. If you

are averaging multiple points for each measurement, the value for

Noise + Quantization changes. The Averaged Noise + Quantization

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:

Noise + Quantization for x averaged points =

Averaged Noise + Quantization from table X √

_____

100/X

For example, if you are averaging 1000 points per measurement

with the PCI-MIO-16XE-50 in the ±10 V input range, the Noise +

Quantization is determined by:

Noise + Quantization = 45.7 µV X √

________

= 14.5 µV100/1000

The Noise + Quantization speciﬁcations assume that dithering

is disabled for single-point measurements and enabled for

averaged measurements.

See page 24 or visit ni.com/calibration for more information

on the importance of calibration on DAQ device accuracy.

To calculate the accuracy of NI measurement products,

visit ni.com/measurements/accuracy

E Series DAQ Accuracy Speciﬁcations

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