This document provides step-by-step instructions for wiring and configuring your NI DAQ device for current measurements. Before you begin using your DAQ hardware, you must install your application development environment and NI-DAQmx driver software. Refer to the Installing NI LabVIEW and NI-DAQmx document for more information.
NI DAQ devices measure current by means of an internal, precision shunt resistor. Most devices can measure ± 20 mA, which maps to the output range of most current transducers and 4 mA to 20 mA current loops. Before making signal connections, check the specifications of your device. When making signal connections, you must be careful with the voltage level of the signal going into the device. You can make two types of connections.
Devices capable of different current measurements have two terminals per measurement channel: AI+ and AI-. Channels on most devices are isolated from earth ground and each other. You can connect ground-referenced or floating current sources to your device. Connect the positive side of the current source to the AI+ terminal, and connect the negative side of the current source to the AI– terminal. If you make a ground-referenced connection between the current source and the DAQ device, make sure the voltage on the AI+ and AI– connections are in the channel-to-earth safety voltage range to ensure proper operation of your device.
Figure 1. Connecting a Differential Current Measurement
If your device does not have channel-to-channel isolation, verify that the voltage levels on the AI+ and AI- side do not exceed the common-mode input range. Common-mode input range is the voltage input range with respect to the device’s ground reference (AI+ versus AI GND, AI- versus AI GND, or AI+ versus AI-).
Devices capable of single-ended measurements have one terminal per measurement channel (AI) and a common ground terminal (COM). Connect the positive lead of the current signal to the AI terminal. Connect the negative lead to the COM terminal. Refer to Figure 2 for an illustration of how to connect single-ended current signals to your device.
Figure 2. Connecting a Single-Ended Current Measurement
Environmental noise can seriously affect the measurement accuracy of the device if you do not take proper care when running signal wires between signal sources and the device.
Minimize noise pickup and maximize measurement accuracy by using individually shielded, twisted-pair wires to connect AI signals to the device. With this type of wire, the signals attached to the positive and negative input channels are twisted together and covered with a shield. You then connect this shield only at one point to the signal source ground. This kind of connection is required for signals traveling through areas with large magnetic fields or high electromagnetic interference.
Refer to the white paper "Field Wiring and Noise Considerations for Analog Signals" for more information.
Before connecting any signals, locate your device pinout.
Figure 3. Device Terminals Help
The following terminal types correspond with current measurements:
You can use MAX to quickly verify the accuracy of your measurement system Set Up. Using an NI-DAQmx Global Virtual Channel you can configure a current measurement 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, input terminal connections, the type of measurement or generation, and scaling information.
Follow these steps to begin:
Figure 4. Creating an NI-DAQmx Virtual Channel
Figure 5. Device Physical Channels
Figure 6. Setting Up a Current Channel in MAX
The next step is to physically connect the current signal to your DAQ device.
Figure 7. Current Connection Diagram
The connection diagram above indicates which pins on your DAQ device should be wired according to the physical channel you selected. In this example, a singled-ended input connection uses pins 0 and 9, corresponding to AI0 and COM on an NI 9203 C Series module.
Use NI-DAQmx global virtual channels to preview your measurements.
Figure 8. Previewing a Current Measurement in MAX
You can choose to view the signal in tabular form or as a graph by selecting Graph from the Display Type dropdown. 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.