Low-Current Measurement Considerations

Low-Current Measurement Considerations

You can use the NI 4070/4071/4072 to measure low current levels.

The NI 4071 offers more current measurement ranges than the NI 4070/4072. In the 1 µA range of the NI 4071, you can resolve currents down to picoamperes.

Low-level measurements are susceptible to sources of error and noise that are often negligible for higher level measurements. In the low-level signals world, no material is an ideal insulator. 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 10µA or less. Take into account the recommendations listed in the table below to ensure measurement integrity.

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 factory 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 triboelectrics1	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 low-noise cables made from polyethylene. 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 piezoelectrics2	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 uA 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. Refer to the Belden CDT Incorporated Web site at www.belden.com for more information about these cables.
Burden voltage	For the maximum burden voltage per range, refer to the Related Documentation for the NI 4070/4071/4072 specifications documents.	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.
1The 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. 2The 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.

Refer to Switching Current for more recommendations for systems with switches.