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Connect Analog Output Signals to a Data Acquisition (DAQ) Device


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This document provides step-by-step instructions for wiring and configuring your NI data acquisition device for generating analog output signals. Before you begin using your NI data acquisition hardware, you must install your application development environment (ADE) and NI-DAQmx driver software. Refer to the Installing NI LabVIEW and NI-DAQmx document for more information. 

Many NI data acquisition devices have the ability to generate voltage signals. Multifunction DAQ devices typically have two or four analog output channels. Dedicated analog output devices or NI CompactDAQ may have more. You can generate single sample DC signals or time-varying multiple sample signals.

AO reference selection allows you to set the analog output range. The analog output range describes the set of voltages the device can generate. The digital codes of the digital-to-analog converter (DAC) are spread evenly across the analog output range. So, if the range is smaller, the analog output has better resolution; that is, the voltage output difference between two consecutive codes is smaller. Therefore, the analog output is more accurate.

The analog output range of a device is all of the voltages between:

–AO Reference and +AO Reference

The possible settings for AO reference depend on the device model. For some models the AO reference is always 10 V, making the analog output range equal to ±10 V. For other models, you may select from pre-determined voltages (such as 10 V or 5 V) or an external voltage reference (APFI or AO EXT REF terminals).

You can connect an external signal to APFI terminals to provide the AO reference. The AO reference can be a positive or negative voltage. If AO reference is a negative voltage, the polarity of the AO output is inverted. The valid ranges of APFI are listed in your device specifications. You can use one of your device’s AO channels to be the AO reference for a different AO signal. However, you must externally connect this channel to APFI.

Note that when using an external reference, the output signal is not calibrated in software. You can generate a value and measure the voltage offset to calibrate your output in software.

Before connecting any signals, locate your device pinout.

  1. Open NI Measurement & Automation Explorer (MAX) and expand Devices and Interfaces.
  2. Right-click on your device name, and select “Device Pinouts.”

Figure 1. Device Terminals Help

The following terminal types correspond with analog voltage generation:
  • AO x(+/-): Analog Output – This is usually followed by a number corresponding to a channel number. For instance, if your DAQ device has 8 analog output channels, your pinout diagram shows positive (+) and negative (-) terminals for AI 0 through AI 7 located throughout the connector.
  • AO GND: Analog Ground – This terminal is the ground reference for AO channels. On some multifunction DAQ devices AO GND may be electrically connected with AI GND and D GND.
  • COM: Common – Common ground terminal for all AO channels. This ground may or may not be isolated from earth ground, depending on your device.
  • APFI x: Analog Programmable Function Interface Channels – Each APFI signal can be used as AO external reference inputs for AOx or as an analog trigger input. APFIx are referenced to AI GND when they are used as analog trigger inputs. APFIx are referenced to AO GND when they are used as AO external offset or reference inputs. These functions are not available on all devices.
  • AO EXT REF – AO external reference input. (This is the same APFI on some devices.)

You can use NI Measurement & Automation Explorer (MAX) to quickly verify the accuracy of your measurement system setup. Using an NI-DAQmx Global Virtual Channel you can configure an analog output without any programming. A virtual channel is a concept of the NI-DAQmx driver architecture used to represent a collection of device property settings that can include a name, a physical channel, terminal connections, the type of measurement or generation, and scaling information.

Follow these steps to begin:
  1. With NI Measurement & Automation Explorer open, select Data Neighborhood and click Create New.
  2. Select NI-DAQmx Global Virtual Channel and click Next.
  3. Select Generate Signals » Analog Output » Voltage

Figure 2. Creating an NI-DAQmx Virtual Channel

  1. Select ao0 or whichever physical channel you intend to connect your voltage output. A physical channel is a terminal or pin at which you can measure or generate an analog or digital signal.

Figure 3. Device Physical Channels

  1. Click Next and enter a name for the global virtual channel or leave the default.
  2. Click Finish and you should see the following screen in MAX:

Figure 4. Setting up a Voltage Output Channel in MAX

  1. On the settings tab, type in the minimum and maximum voltage values you expect to generate from your measurement (-10 to 10 V by default).

Signal connections vary depending on your device, connector block, and signal conditioning module. The following figure shows how to make analog output connections for a typical NI device.

Figure 5. Analog Output Connections

  1. Connect the positive lead of the load to the AOx terminal. Connect the ground of the load to a COM or AO GND terminal.

NI-DAQmx global virtual channels allow you to test your outputs.

  1. With MAX still open, enter a value in the numeric field between your maximum and minimum voltage you specified.
  2. Click Apply Value to Channel. You should see the channel value appear in the table. (The default channel name is MyVoltageOutChannel.)

Figure 6. Updating a Virtual Channel in MAX

  1. This procedure will update your AO channel with a DC voltage value. You may connect a handheld DMM or another DAQ device to your AO channel to verify the voltage.

You also have the option of saving your NI-DAQmx Global Virtual Channel should you wish to refer to this configuration screen again in the future.

 
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