Measurement Computing USB-1208FS - Manual (Page 16)

USB-1208FS User's Guide Functional Details
16
 Configure as an output to pace conversions on a second USB-1208FS device and acquire data from
16 channels. For more information synchronized operations see page 18.
Calibration output
The CAL terminal is an output used only to calibrate the device. Calibration is software-controlled via InstaCal.
Ground
The analog ground ( AGND ) terminals provide a common ground for all analog channels. The ground ( GND )
connections 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-1208FS and possibly the computer.
The maximum total output current that can be drawn from all USB-1208FS connections (power, analog and
digital outputs) is 420 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.
Just connecting the USB-1208FS to your computer draws 80 mA of current from the USB +5 V supply. Once
you start running applications with the USB-1208FS, each DIO bit can draw up to 2.5 mA, and each analog
output can draw 15 mA. The maximum amount of +5 V current available for experimental use, over and above
that required by the USB-1208FS, is the difference between the total current requirement of the USB (based on
the application), and the allowed current draw of the PC platform (500 mA for desktop PCs and self-powered
hubs, or 100 mA for bus powered hubs and notebook computers).
With all outputs at their maximum output current, you can calculate the total current requirement of the USB-
1208FS USB +5 V as follows:
(USB-1208FS @ 80 mA) + (16 DIO @ 2.5 mA ea) + (2 AO @ 15 mA ea ) = 150 mA
For an application running on a PC or powered hub, the maximum available excess current is 500 mA
−150 mA
= 350 mA. This number is the total maximum available current at the PC +5 V screw terminals. 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 350-380 mA range.
Since laptop computers typically allow up to 100 mA, the USB-1208FS in a fully-loaded configuration may be
above that allowed by the computer. In this case, you must determine the per-pin loading in the application to
ensure that the maximum loading criteria is met. The per-pin loading is calculated by simply dividing the +5 V
by the load impedance of the pin in question.
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 "12-bits" or "1 part in
4,096" does indicate what can be resolved, it provides little insight into the quality of an absolute measurement.
Accuracy specifications describe the actual measurement achievable with a USB-1208FS.
There are three types of errors which affect the accuracy of a measurement system:
 offset
 gain
 nonlinearity
The primary error sources in the USB-1208FS are offset and gain. Nonlinearity is small in the USB-1208FS,
and is not significant as an error source with respect to offset and gain.