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USB-1608FS User's Guide
Ground
The analog ground (
AGND
) terminals provide a common ground for all analog channels.
The digital ground (
GND
) terminals provide a common ground for the digital, trigger, counter, sync and power terminals.
Power output
The
PC +5V
terminal is a 5 volt output that is supplied by the computer. You can use this terminal to supply power to external devices or circuitry.
Caution! The PC +5V terminal is an output. Do not connect to an external power supply or you may damage the USB-1608FS and possibly the computer.
The maximum total output current that can be drawn from all USB-1608FS connections (power, analog, and digital outputs) is 500 mA. This maximum applies to most personal computers and self-powered USB hubs.
Bus-powered hubs and notebook computers may limit the maximum available output current to 100 mA.
Note
:
If the current requirement of the device exceeds the current available from the computer, connect to a selfpowered hub or power the computer with an external power adapter.
The USB-1608FS can draw up to 150 mA max of current from the USB +5V supply. When running applications with the device, each DIO bit can source up to 2.5 mA. With all outputs at their maximum output current, the total current requirement of the
PC +5V
terminal is calculated as follows:
Total current requirement = (USB-1608FS @ 150 mA) + (8 DIO @ 2.5 mA ea) = 170 mA
The maximum available excess current is the difference between the allowed current draw of the computer platform and the total output current requirement of the device. For an application running on a computer or powered hub, the maximum available excess current is calculated as follows:
Maximum excess current = 500 mA – 170 mA = 330 mA
Measurement Computing highly recommends that you figure in a safety factor of 20% below this maximum current loading for your applications. A conservative, safe user maximum in this case would be in the 250 mA to 300 mA range.
Accuracy
The overall accuracy of any instrument is limited by the error components within the system. Resolution is often used incorrectly 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 achievable with a USB-1608FS.
There are three types of errors which affect the accuracy of a measurement system:
offset
gain
nonlinearity
The primary error sources in the USB-1608FS are offset and gain. Nonlinearity is small, and is not significant as an error source with respect to offset and gain.
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USB-1608FS User's Guide
example of what you can expect when performing a measurement in this range.
Figure 7. Ideal ADC transfer function
The USB-1608FS offset error is measured at mid-scale. Ideally, a zero volt input should produce an output code
offset error. The typical offset error specification for the USB-1608FS 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.
Figure 8. 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
converted to voltage by multiplying the full-scale input (±10 V) by the error.
The accuracy plots in Figure 9 are drawn for clarity and are not drawn to scale.
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USB-1608FS User's Guide
Figure 9. ADC Transfer function with gain error
For example, the USB-1608FS exhibits a typical calibrated gain error of ±0.04% on all ranges. For the ±10 V range, this would yield 10 V × ±0.0002 = ±4 mV. This means that at full scale, neglecting the effect of offset for the moment, the measurement would be within 4 mV of the actual value. Note that gain error is expressed as a ratio. Values near ±FS (±10 V) are more affected from an absolute voltage standpoint than are values near midscale, which see little or no voltage error.
scale (±10 V). This plot is a graphical version of the typical accuracy specification of the product.
The accuracy plots in Figure 10 are drawn for clarity and are not drawn to scale.
Figure 10. Error band plot
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