The Instruments and Ways to Optimize Low-Current Device Measuring

As we know. Measuring devices at low current is quite difficult, which requires professional knowledge and skilled testing as well as the proper test equipment. You may ask, what is the standard of low current? Generally speaking, low current is a relative variable data depends on the device. Therefore, a current level considered low for one application, such as 1mA, may be high for a device operating at 10nA. In general, an instrument’s noise level is related to its low-level sensitivity, with low-current measurements referring to those made near an instrument’s noise level. However, even with knowledge, skill, and test equipment, the measurement is still a challenge, because the current level is usually below the noise level. We should have a better understanding to the types of test equipment to use, and estimate the error could be occurred, and know how to minimize these errors. Below we will introduce some test equipments including digital multimeter (DMM) and ammeters.

DMM

In fact, there are a number of instruments available for low-current device measurements. How to choose the instrument depend on the features of the device and the current level under test. Generally, people use the digital multimeter (DMM) to test the current, voltage, resistance, and temperature. The most sensitive DMMs available can measure current levels as low as about 10pA.

Ammeter

The next one is ammeter. DMM instrument may not test the result with high precision. At thay time, when higher precision is demanded, ammeters are available to achieve this goal. The testing instruments can vary from simple devices that measure current flow through a coil to newer digital ammeters that use an analog-to-digital converter (ADC) to measure the voltage with a shunt resistor and then determine and display the current from that reading. The shunt resistor usually has a low value to minimize the voltage drop across it. That voltage, which can impact low-level measurements, is often referred to as the voltage burdens, which can also be realized using a feedback resistor, are referred to as picoammeters when designed specifically for low-current measurements. Picoammeters are available in various configurations, including high-speed models and logarithmic units capable of displaying a wide current range.

There are also additional current-measurement instruments include electrometers and source-measure units (SMUs).

Minimize noise

All the testing instruments are useful to low current device, though the noise will influence the results. How to minimize the noise? The device to be tested can create Johnson current noise, which can be expressed in terms of either current or voltage, is essentially the voltage noise of a device divided by the device resistance. The Johnson current noise is affected by the temperature and noise bandwidth. If we change the temperature or reduce the noise bandwidth will also reduce the Johnson current noise. Current noise is from several unwanted sources, including the coaxial cables used to interconnect test instruments to each other or to the device. Therefore, it produces the triboelectric effect.

The triboelectric effect can be minimized by using low-noise cable, with an inner insulator of polyethylene coated with graphite underneath the outer shield. The graphite reduces friction, and provides a path for the displaced electrons to return to their original locations, eliminating random electron motion and their contribution to the additional noise level. Excess current flow from the triboelectric effect can also be minimized by reducing the length of the test cables as much as possible. The test setup should be isolated from vibration to minimize unwanted movement of the test cables, by positioning test cables on top of vibration-absorbing material, such as foam rubber. Test cable movement can also be minimized by taping the cables to a stable surface, such as the test bench.