Accuracy - Measurement Computing USB-1208LS User Manual

USB-1208LS User's Guide

Accuracy

The overall accuracy of any instrument is limited by the error components within the system. Quite often,
resolution is incorrectly used to quantify the performance of a measurement product. While "12-bits" or "1 part
in 4096" does indicate what can be resolved, it provides little insight into the quality of an absolute
measurement. Accuracy specifications describe the actual results that can be realized with a measurement
device.
There are three types of errors which affect the accuracy of a measurement system:
offset

gain

nonlinearity.

The primary error sources in the USB-1208LS are offset and gain. Nonlinearity is small in the USB-1208LS,
and is not significant as an error source with respect to offset and gain.
Figure 9 shows an ideal, error-free, USB-1208LS transfer function. The typical calibrated accuracy of the USB-
1208LS is range-dependent, as explained in the
example of what you can expect when performing a measurement in this range.
The USB-1208LS offset error is measured at mid-scale. Ideally, a zero volt input should produce an output code
of 2048. Any deviation from this is an offset error. Figure 10 shows the USB-1208LS transfer function with an
offset error. The typical offset error specification on the ±10 V range is ±9.77 mV. Offset error affects all codes
equally by shifting the entire transfer function up or down along the input voltage axis.
Specifications
Figure 9. Ideal ADC transfer function
14
chapter. We use a ±10 V range here as an
Functional Details