Ground; Power Output; Accuracy - Measurement Computing usb1608fs User Manual

USB-1608FS User's Guide

Ground

The analog ground (
AGND
The digital ground (
GND
terminals.

Power output

The terminal is a 5 volt output that is supplied by the computer. You can use this terminal to supply
PC +5V
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.
If the current requirement of the device exceeds the current available from the computer, connect to a self-
powered 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
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:
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.
) terminals provide a common ground for all analog channels.
) terminals provide a common ground for the digital, trigger, counter, sync and power
PC +5V
Maximum excess current = 500 mA – 170 mA = 330 mA
terminal is calculated as follows:
14
:
Note