Measurement Computing miniLAB-1008 - Manual (Page 21)

miniLAB 1008 User's Guide Functional Details
Just connecting the miniLAB 1008 to your computer draws 20 mA of current from the USB +5V supply. Once
you start running applications with the device, each DIO bit can draw up to 2.5 mA, and each analog output can
draw 30 mA. The maximum amount of +5 V current available to the user is the difference between the total
current requirement of the PMD (based on the application), and the allowed current draw of the PC platform
(again, 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
miniLAB 1008 device's USB +5 V as follows:
(miniLAB 1008 @ 20 mA) + (4 DIO @ 2.5 mA ea) + (2 AO @ 30 mA ea ) = 90 mA
For an application running on a PC or powered hub, this value yields a maximum user current of 500 mA − 90
mA = 410 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
300-320 mA range.
Since laptop computers typically allow up to 100 mA, the miniLAB 1008 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
+5V by the load impedance of the pin in question.
Ground terminals
There are 9 identical ground connections that provide a common ground for all miniLAB 1008 functions. Refer
to the
pinout diagrams on page 3-5 for the location of the GND terminal pins.
Calibration terminal
The CAL connection on the output terminal provides a calibration reference voltage. This terminal should only
be used during calibration of the miniLAB 1008 device.
Calibration of the miniLAB 1008 is software-controlled via InstaCal.
Testing terminal
The TST terminal is reserved for factory testing only.
Counter terminal
The input connection to the 32-bit external event counter is made to the screw terminal labeled CTR . Refer to
the pinout diagrams on page 3-5 for the location of this pin. The internal counter increments whenever the CTR
input voltage changes from <1 volt to more than 4 volts. The counter is capable of counting frequencies up to
1 MHz.
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