Measurement Computing USB-7204 - Manual (Page 18)

USB-7204 User's Guide Functional Details
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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.
The accuracy plots in Figure 10 are drawn for clarity and are not drawn to scale.
Figure 10. ADC transfer function with offset error
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 11 shows the USB-7204 transfer function with gain error. Gain error is easily
converted to voltage by multiplying the full-scale (
FS ) input by the error.
The accuracy plots in Figure 11 are drawn for clarity and are not drawn to scale.
Figure 11. ADC Transfer function with gain error
For example, the USB-7204 exhibits a typical calibrated gain error of ±0.2% on all ranges. For the ±10 V range,
this would yield 10 V × ±0.002 = ±20 mV. This means that at full scale, neglecting the effect of offset for the
moment, the measurement would be within 20 mV of the actual value. Note that gain error is expressed as a
ratio. Values near ±FS are more affected from an absolute voltage standpoint than are values near mid-scale,
which see little or no voltage error.
Combining these two error sources in Figure 12, we have a plot of the error band of the USB-7204 for the
±10 V range. This is a graphical version of the typical accuracy specification of the product.
The accuracy plots in Figure 12 are drawn for clarity and are not drawn to scale