Measuring Current Using the Analog Input of a DAQ Board Hardware: Multifunction DAQ (MIO)
Problem: Most data acquisition (DAQ) boards have analog input capabilities to measure voltages within a certain region, but I want to measure currents. How do I do this? Solution: Current cannot be measured directly using the analog input of a DAQ board. However, if a known resistance is placed in series with the load and the voltage across it can be measured, the current through the load can be calculated using Ohm's Law (V = I x R). For such applications, National Instruments provides the SCXI Resistor Kit (a set of 4 precision resistors at 249 Ohms with +/- 0.1% accuracy) for current measurement applications. Any resistors are acceptable, as long as the maximum expected current multiplied by the resistance does not exceed the input range of your DAQ board. When measuring current in this fashion, the smallest value resistor possible should be used as this will create the smallest interference with the existing circuit. However, smaller resistances create smaller voltage drops, so a compromise between resolution and circuit interference must be made. The figure below shows how to connect an external resistor to the DAQ board so current can be measured.
Also, placement of the shunt resistor in the circuit is important. If the external circuit shares a common ground with the computer containing the DAQ board, then the shunt resistor should be placed as close to the ground leg of the circuit as possible. If not, the common-mode voltage produced by the shunt resistor might be outside the specification for the DAQ board which could lead to inaccurate readings or even damage to the board.
Related Links: Online Catalog: SCXI Resistor Kit KnowledgeBase 2HL9BS2O: How Can I Do Current Measurements with NI Products? Attachments:   
Report Date: 12/19/1995 Last Updated: 05/15/2006 Document ID: 0GIBP403 |
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