Measurement Computing USB-1408FS - Manual (Page 16)

USB-1408FS User's Guide Functional Details
16
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.
+2.5VREF output
The +2.5VREF connection is an output terminal that supplies 2.5 volts. You can use this pin as the voltage
source for another analog channel. For example, to configure the +2.5VREF pin as the voltage source for
channel 0 in SE mode, connect +2.5VREF to CH0 IN . In differential mode, connect +2.5VREF to CH0 IN HI , and
connect CH# IN LO to AGND .
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 +5 V terminal is an output. Do not connect to an external power supply or you may damage
the USB-1408FS and possibly the computer.
The maximum total output current that can be drawn from all USB-1408FS 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-1408FS to your computer draws 80 mA of current from the USB +5 V supply. Once
you start running applications with the USB-1408FS, 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-1408FS, 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-1408FS USB +5 V supply as follows:
(USB-1408FS @ 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
280 mA.
Since laptop computers typically allow up to 100 mA, the USB-1408FS 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 used to quantify the performance of a measurement product. While "14-bits" or "1 part in
16383" 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-1408FS
There are three types of errors which affect the accuracy of a measurement system:
 offset
 gain
 nonlinearity
The primary error sources in the USB-1408FS are offset and gain. Nonlinearity is small in the USB-1408FS,
and is not significant as an error source with respect to offset and gain.