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Supplying an Output Greater Than 20 mA with a Current Output Device

Hardware: Signal Sources (AWG/FG/AO)>>Analog Output Devices>>PCI-6704

Problem:
I am using an analog output device such as NI PCI-6704 or NI 9265 that is capable of outputting current between 0.1 and 20.2 mA, according to the specifications in the User Manual. Can I send more than 20 mA from these devices?

Solution:
Yes, you can supply more than 20 mA from the NI PCI-6704 and the NI 9265.
Both of these devices are limited to supplying 20 mA current outputs on a per channel basis; however, you can output greater than 20 mA of current by wiring two or more outputs in parallel. Refer to the respective User Manuals, for more information on outputting current.

Although you can increase the current supplied from the analog output device, it is still limited to a maximum voltage generation. The NI PCI-6704 can supply a maximum voltage of 10V, and the NI 9265 can supply up to 12 V. According to Ohm’s Law (V = I*R), as you increase the current supply to a constant resistive load the amount of voltage that must generated increases. Since the voltage generation is limited, the amount of load resistance that you are able to drive current through is limited to a maximum value.

For example, if you supply 40 mA of current through your load, and the maximum voltage that the NI PCI-6704 can generate is 10V, then the load is limited to a maximum resistance of 250 ohms because 40 mA /10V=250ohms(V/I=R). If the load is greater than 250ohms the device must generate more than 10V to drive the set current level, which is not possible.

When you wire current sources in parallel, the resulting current is the added values of the sources.

Follow the directions below to wire current outputs in parallel.
  1. Wire two or more current output channels together. For example, when using channels 16 and 17 of the NI PCI-6704, wire pins 67 and 32 together.

  2. Wire the two or more respective analog ground pins together. When using channels 16 and 17 on the NI PCI-6704 this corresponds to pins 68 and 33.

  3. If you are using LabVIEW, you can use the attached Current Output VI to send output current on multiple channels.
    • Open the example program.
    • Select the correct device number of your current output device and appropriate channels for that device.
    • Assign between 0 and 20 mA to each current source that you are using.
    • Run the example program. The current you measure between any of the current output channels and analog grounds is the sum of all the current sources that you have wired together.


Related Links:
Product Manuals: NI 6703/6704 User Manual
Product Manuals: NI 9265 Operating Instructions and Specifications

Attachments:


Outout Current.vi

Report Date: 11/10/2003
Last Updated: 04/21/2008
Document ID: 3398F13E

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