Measuring AC and DC Current

Attach the test probes for measuring AC current or DC current as shown in the following figure:

Figure 18. AC Current and DC Current Connections

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About DC Current and AC Current Measurements

The PXIe-4081 allows you to measure currents ranging from a few picoamps to three amperes in the following modes:

  • DC Current mode has eight ranges that go from 1 µA to 3 A in decades.
  • AC Current mode has six ranges that go from 100 µA RMS to 3 A RMS in decades. AC Current mode uses the same fast measurement technique as the AC Volts DC Coupled mode.
Note AC current measurements are always DC coupled and therefore always offer low settle time. Keep in mind that the range chosen needs to include both the signal range and its offset, otherwise the input can get overloaded. For a list of the available ranges, refer to the PXIe-4081 Specifications.

Shunt Resistors

The PXIe-4081 uses internal shunt resistors with temperature coefficient and power ratings selected to reduce resistor self-heating errors. For the effect of temperature changes on the accuracy of the measurements, refer to the PXIe-4081 Specifications and the temperature coefficient.

Table 9. Shunt Resistor Values
Range Shunt Resistor Value
3 A 0.05 Ω
1 A 0.05 Ω
100 mA 0.5 Ω
10 mA 5 Ω
1 mA 50 Ω
100 µA 500 Ω
10 µA 50 kΩ
1 µA 50 kΩ
Tip The current measurement path within the PXIe-4081 utilizes an electromechanical relay that provides very low resistance to assure a low burden voltage. While this relay has an exceptionally long life, you can take special precautions to ensure reliable operation.

Whenever possible, switch the PXIe-4081 into the current measurement function before applying the current. Switching inductive current sources generally creates flyback voltages that stress the relay and, if done frequently, can shorten the reliability of the relay. Also, avoid interrupting the current by switching out of the current measurement function when currents are flowing through the circuit.

Input Protection

The current-measuring circuit is protected with a fuse rated at T 3.5 A 1000 V. The DMM can measure up to 3 A DC or 3 ARMS with up to 4.2 A peak current (sine wave only). Higher currents will blow the fuse.

If you apply higher current level to a range, the input protection circuitry of the DMM automatically engages, and you receive an overrange warning.

Also, keep in mind that exposing the current measurement connectors to voltage sources while the DC or AC current functions are selected may trigger the input protection circuitry or even blow the fuse.

Low-Current Measurement Considerations

You can use the DMM to measure low current levels.

Low-level measurements are susceptible to sources of error and noise that are often negligible for higher level measurements. While no material is an ideal insulator for low-level current measurements, some materials are better insulators than others. Selection of proper cables and interconnects becomes very important.

When making low-level current measurements, noise and error sources are more likely to affect measurements in the order of 10 µA or less. Take into account the recommendations listed in the following table to ensure measurement integrity:

Table 10. Noise and Error Sources
Error or Noise Source Cause Level of Induced Error Recommendations
Input bias current
  • Input bias current doubles with every 10 °C increase.
  • Input bias current results in an offset error that is temperature dependent on the 1 µA – 10 µA ranges.
 Typically 10 pA at 23 °C ambient, but it is externally calibrated to zero at that temperature.
  • Maintain an operating ambient temperature of less than 28 °C and enable Auto Zero to remove the internal DMM offset errors from the measurement.
  • If you need to operate above 28 °C, perform offset nulling before taking your measurements.
Leakage currents due to poor insulation When measuring low-level currents, leakage currents in insulators become relevant. Current flows between a high-impedance point and the nearby voltage sources. Depends upon the resistivity of the insulator material. Some examples include the following:
  • Glass Epoxy: >1012 Ω
  • Nylon: >1012 Ω
  • Polyethylene: >1015 Ω
  • Virgin Teflon: >1016 Ω
 Be sure that the test fixture for the DUT is constructed of a material appropriate for the desired test and that the test fixture is clean.
Noise currents due to triboelectrics Currents are generated by moving the cables because a transfer of charge occurs between the insulator and the conductor when they rub against each other. Hundreds of pA in a polyethylene cable (for example, RG-58)
  • Use cables designed for low noise.
  • If you are not using polyethylene cables, be especially mindful to do the following:
    • Secure the cables to non-vibrating surfaces.
    • Do not move or flex the cables during measurements.
Noise currents due to piezoelectrics Currents can be generated when the insulator is subjected to mechanical stress. Hundreds of pA for polyethylene
  • Remove mechanical stress from cables and insulators.
  • Make connecting structures rigid, especially when using Teflon insulated cable.
Error currents due to contamination and humidity Insulation resistance gets reduced with increasing humidity and contamination. These two combined could also generate small electrochemically-induced currents. On the order of pA to µA
  • Select insulators with low water absorption, such as Teflon.
  • Keep humidity at moderate levels. It is recommended to maintain a relative humidity less than 55% especially when the level of cleanliness in the environment is questionable.
  • Keep insulators and DUT connections clean.
Noise currents due to electromagnetic interference and electric fields Surrounding equipment (motors, power cables, vibrating equipment) can induce noise currents. Moving people and charged objects near measurement setup can induce electrostatic coupling. On the order of nA and tens of µA depending on proximity, device shielding, and magnitude of relative movement
  • Use shielded or twisted pair cables.
  • Shield the device under test.
  • Move power cables and other electrical equipment away from the measurement circuit.
  • Avoid vibration and movement around the circuit while the measurement is being taken.
  • Use shielded cabling, such as the Belden 83317E cable.
Burden voltage For the maximum burden voltage per range, refer to the PXIe-4081. Depends on the circuit being measured Calculate if the error caused by burden voltage is relevant when measuring the current in the circuit of interest. Refer to Burden Voltage for an example.
Leakage currents due to external switching External switches used for routing low level currents could be a source of leakage currents. Depends on the switch, temperature and humidity of the environment.
  • Consult your switch documentation on how to reduce leakage errors.
  • Refer to Switching Current for other recommendations for systems with switches. Maintain temperature and humidity conditions under 28 °C and 55% relative humidity unless the switch specifies leakage performance above this level.
Note For noise current due to triboelectrics, the Triboelectric Effect can be compared to the phenomenon of static electricity caused by rubbing certain insulators together. Teflon and silver in combination, for example, can create a very high level of triboelectric-induced current noise.
Note For noise currents due to piezoelectrics, the Piezoelectric Effect, although well-exploited in sensors which convert changes in pressure from sound waves or physical vibration into small voltage signals, is highly undesirable in cables and interconnects.

In addition to following these recommendations to reduce noise and errors, use software filtering and noise reduction techniques, such as increasing the measurement aperture, choosing the most appropriate type of DC noise rejection, or averaging several measurements by setting the Number of Averages property.

High-Current (Up to 10 A) Measurement Considerations

The PXIe-4081 can measure currents up to 3 A.

If you need to measure currents higher than the maximum range available, you can use the CSM-10A external shunt to measure currents up to 10 A. Refer to Measuring Current in the NI-DMM Help for more information about how to use the CSM-10A shunt with the DMM.

When measuring high current values, you could get errors due to resistor self-heating. The DMM uses internal shunt resistors with low temperature coefficients and good thermal resistance. Refer to the temperature coefficients (tempco) in the PXIe-4081 Specifications for the effect of temperature changes on the accuracy of these measurements.

The NI CSM-10A current shunts have excellent temperature coefficients of resistance. Refer to the NI Products Web site for more information on NI CSM-10A current shunts.

Check if the burden voltage can affect your measurement. Refer to the Burden Voltage section for an example on how to calculate the error due to burden voltage.

In applications where more than one current needs to be measured using the same DMM and the conditions require that the current cannot be interrupted across the load, use one of these two methods to measure the current:

  • Use a break-before-make switch in multiplexer topology and place external shunts across the channels. Connect the shunts in series with the circuit under test and then measure the voltage drop across the shunts.
  • Use make-before-break switches.

If the conditions allow interrupting the current flow through the circuit under test, it is still recommended to use external shunts. Switching currents above 1 A electromechanically can diminish the life of any switch. Refer to Switching Current for more recommendations for systems with switches.

Using Current Shunt Modules

When taking current measurements outside of the specified range on the DMM, you must use a current shunt module (CSM). NI offers the NI CSM-10A current shunt module, which has a 0.01 Ω sense resistor.

Note For valid current ranges, refer to the PXIe-4081 Specifications.

The CSM operates by passing the input current through a precise resistor. The following figure shows the internal circuitry of the CSM:

Figure 19. CSM Circuitry
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  1. Voltage Across Resistor (V OUT)
  2. Precision Resistor
  3. Fuse
  4. Current Source (I IN)

To configure current measurements using the NI CSM-10A, complete the following steps:

  1. Measure the voltage drop across the resistor of the CSM.
  2. Use this value to calculate the current using Ohm's Law, as follows:

    I IN = ( V OUT)/ R

    where

    I is the input current

    V is the voltage across the precision resistor

    R is the resistance of the precision resistor

    For example, assume that you are using the CSM-10A, which has a 0.01 Ω precision resistor, and the measured voltage is 50 mV. Apply these values to Ohm's Law to determine the current, as follows:

    5000 mA = 50 mV / 0.01 Ω

Note You can use the NI CSM-10A current shunt with any NI DMM for this method.