Measurement-computing miniLAB-1008 Manual do Utilizador descarregar pdf (Página 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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