Measurement Computing USB-1608FS-Plus User Manual page 14

USB-1608FS-Plus User's Guide
There are three types of errors which affect the accuracy of a measurement system:

offset

gain

nonlinearity
The primary error sources in a USB-1608FS-Plus are offset and gain. Nonlinearity is small, and is not
significant as an error source with respect to offset and gain.
Figure 7 shows an ideal, error-free transfer function. The typical calibrated accuracy of a USB-1608FS-Plus is
range-dependent. We use a ±10 V range as an example of what you can expect when performing a measurement
in this range.
The offset error is measured at mid-scale. Ideally, a zero volt input should produce an output code of 32,768.
Any deviation from this is an offset error. Figure 8 shows the transfer function with an offset error. The typical
offset error specification for a USB-1608FS-Plus on the ±10 V range is ±1.66 mV. Offset error affects all codes
equally by shifting the entire transfer function up or down along the input voltage axis.
The accuracy plots in Figure 8 are drawn for clarity and are not drawn to scale.
Gain error is a change in the slope of the transfer function from the ideal, and is typically expressed as a
percentage of full-scale. Figure 9 shows the USB-1608FS-Plustransfer function with gain error. Gain error is
easily converted to voltage by multiplying the full-scale input (±10 V) by the error.
Figure 7. Ideal ADC transfer function
Figure 8. ADC transfer function with offset error
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Functional Details