Accuracy - Measurement Computing USB-7204 User Manual

USB-7204 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-7204 are offset and gain. Nonlinearity is small in the USB-7204, and is
not significant as an error source with respect to offset and gain.
Figure 9 shows an ideal, error-free, USB-7204 transfer function. The typical calibrated accuracy of the USB-
7204 is range-dependent, as explained in the Specifications chapter on page 21. We use a ±10 V range here as
an example of what you can expect when performing a measurement in this range.
The accuracy plots in Figure 9 are drawn for clarity and are not drawn to scale.
The USB-7204 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-7204 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.
Figure 9. Ideal ADC transfer function
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Functional Details