Why is it difficult for a multimeter to measure very small currents?
Standard multimeters perform well when measuring currents in the milliampere (mA) to ampere (A) range. However, when the value to be measured drops to the tens of microamperes (µA) or nanoamperes (nA) level, conventional measurement methods begin to show their limitations.
Suppose the circuit under test has a current of approximately 2 µA. A device with a resolution of 0.1 mA would be unable to display this value. Even if the meter features a µA range, the reading might still fluctuate due to electrical noise, leakage current within the measurement circuit, or environmental conditions.
This explains why the displayed value for the same circuit may fluctuate continuously when switching to a lower measurement range. The issue lies not in the measurement technique itself, but in the fact that the measurement requires a device with higher sensitivity.
Where do small electric currents typically occur?
This is a common scenario in electronics, particularly in circuits requiring strict control over power consumption or leakage current:
+ Measuring leakage current in semiconductor components
+ Testing leakage current in insulating materials
+ Measuring dark current in sensors or electronic circuits
+ Checking standby power consumption
+ Evaluating component characteristics during R&D
+ Monitoring minute currents generated by materials or test samples
In these measurements, a difference of just a few microamperes (µA) can be technically significant. If the current to be measured falls within the nanoampere (nA) or picoampere (pA) range, the requirements for equipment and circuit layout become far more stringent.
What is the difference between 1 µA and 1 nA?
1 µA = 1,000 nA
When the current drops from the µA range to the nA range, the value being measured is a thousand times smaller; the difference is even greater when reaching the pA range.
Some modern low-current measurement devices can operate across very low ranges. For example, the KEITHLEY 6487 (available at EMIN) offers measurement scales from 2 nA to 20 mA with a minimum resolution of 10 fA. The KEYSIGHT B2981C is designed for femtoampere and picoampere measurements, with a specified range of 0.01 fA to 20 mA, and includes capabilities for data storage and PC-based monitoring. However, simply needing to measure a low current does not automatically mean one should choose the device with the lowest possible range; the actual current range of the sample under test is the determining factor for selecting the appropriate equipment.
Can a shunt resistor be used to measure small currents?
Yes. A common method involves passing the current through a resistor of a known value, measuring the voltage across it, and calculating the current using Ohm's Law.
For example, with a 1 kΩ resistor and a measured voltage of 1 mV, the current flowing through the resistor is approximately 1 µA.
This approach is suitable for many standard testing applications. However, when measuring very low currents, factors such as the resistor itself, the wiring, the environment, and the voltage-measuring equipment become critical variables that must be managed. Leakage currents in the measurement circuit or external interference can cause significant deviations from the actual signal. Therefore, this method is not a complete substitute for specialized current-measuring instruments.
When should you switch to a picoammeter?
A picoammeter should be considered in the following situations:
- The value to be measured falls below the practical range of a standard DMM. The meter lacks sufficient resolution to distinguish small changes in current.
- Readings fluctuate significantly. Even though the circuit remains virtually unchanged, the displayed value drifts or jumps continuously, making the results difficult to interpret.
- Leakage current measurement is required. Testing for leakage current in components, insulating materials, or electronic circuits often requires a much lower measurement range than standard electrical measurements.
- Current needs to be recorded over time. Some tests require monitoring how the current changes over minutes or hours—or under specific test conditions—rather than simply capturing a single reading at a specific moment.
EMIN’s portfolio of low-level current and voltage measurement and sourcing instruments encompasses a wide range of devices to meet diverse needs, spanning from picoammeters and nanovoltmeters to DC/AC current sources and combined source-measure units.
Notably, the KEYSIGHT B2981C is well-suited for ultra-low current measurements requiring data monitoring and logging. Meanwhile, the KEITHLEY 6487 combines low-current measurement with voltage sourcing capabilities, making it ideal for component and circuit testing that requires control over both the measurement and power supply aspects.
Conclude
While a multimeter performs well for standard current measurements, the device's limitations begin to significantly affect results when signals drop to the µA, nA, or pA range. For applications involving leakage current, dark current, semiconductor components, or materials research, a picoammeter or low-level current measurement instrument is more suitable.
