Measurement Computing USB-1616FS - Manual (Page 18)

USB-1616FS User's Guide Functional Details
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Power limitations when using multiple USB-1616FS devices
When daisy chaining additional MCC USB Series products to the USB-1616FS, you must ensure that you
provide adequate power to each board that you connect. The USB-1616FS is powered with a 9 VDC nominal,
3.0 A external power supply.
Voltage drop
A drop in voltage occurs with each board connected in a daisy chain system. The voltage drop between the
power supply input and the daisy chain output is 0.5 V maximum. Factor in this voltage drop when you
configure a daisy chain system to ensure that at least 6.0 VDC is provided to the last board in the chain.
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 "16-bits" or "1 part
in 65,536" does indicate what can be resolved, it provides little insight into the quality, or accuracy, of an
absolute measurement. Accuracy specifications describe the actual measurement that can be relied upon with a
USB-1616FS.
There are three types of errors which affect the accuracy of a measurement system:
 offset
 gain
 nonlinearity
The primary error sources in the USB-1616FS are offset and gain. Nonlinearity is small in the USB-1616FS,
and is not significant as an error source with respect to offset and gain.
Figure 6 shows an ideal, error-free, USB-1616FS transfer function. The typical calibrated accuracy of the USB-
1616FS is range-dependent, as explained in the "Usage note" in the Specifications chapter. We use a ±10V
range as an example of what you can expect when performing a measurement in this range.
The accuracy plot in Figure 6 is drawn for clarity and is not drawn to scale.
Figure 6. Ideal USB-1616FS transfer function
The USB-1616FS 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 7 shows the USB-1616FS transfer function with an
offset error. The typical offset error specification for the USB-1616FS 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.