Analysis of the causes of error current in current measurement

　　Any additional current generated in the test system will be added to the current being measured and cause errors. This current can be generated internally, such as the input bias current of the instrument, or it can come from the outside, such as from insulators and cables. The following will discuss the various sources of current generation.

　　1. Bias current

　　Bias current can be generated internally in the instrument (input bias current) or by an external circuit (external bias current).

　　① Input bias current

　　An ideal ammeter should read zero when the inputs are open circuited. However, real ammeters do have some small current when the inputs are open circuited. This current is called input bias current and is caused by the bias current of the active devices and the leakage current flowing through the insulators inside the instrument. The bias current generated in picoammeters, electrometers, and SMUs is given in the instrument's specifications. The input bias current is added to the current being measured, so the meter measures the sum of the two currents: IM = IS + IOFFSET

　　The input bias current can be determined by capping the input connector and selecting the lowest current range. Allow approximately 5 minutes for the instrument to stabilize and then take the reading. The reading should be within the instrument's specifications.

　　Another way to subtract the input bias current from the measurement is to use the relative (REL or ZERO) function of the ammeter. Allow the reading to stabilize with an open circuit, then turn on the REL function. Once the REL value is established, all subsequent readings will be the difference between the actual input value and that REL value.

　　②External bias current

　　External bias current can be generated by ionic contamination on the insulator connected to the ammeter. This bias current can also be generated externally by reasons such as triboelectric effect and piezoelectric effect. As shown in Figure 2-18, the external bias current is also added to the source current, and the meter displays the sum of the two.

　　The external bias current can be eliminated by using the instrument’s current suppression function (if it has this function) or by using a relatively stable and quiet external current source, as shown in Figure 2-19. When this method is used, the current measured by the instrument is: IM = IS + IOFFSET - ISUPPRESS

　　If IOFFSET and ISUPPRESS are equal in magnitude and opposite in polarity, then: IM = IS

　　The advantage of using an external current source is that IOFFSET can be equal to or even greater than the full-scale value of the instrument, as long as IOFFSET - ISUPPRESS is small.

　　2. Triboelectric effect

　　Triboelectric current is generated by the charge created by friction between a conductor and an insulator. In this case, friction causes free electrons to break away from the conductor, creating a charge imbalance and thus generating an electric current.

　　"Low-noise" cables can greatly reduce this effect. Such cables usually use an inner layer of polyethylene insulation and a graphite coating underneath the outer shield. The graphite acts as a lubricant and forms a conductive equipotential cylinder to balance the charges and minimize the charges generated by the friction effects of cable movement. However, even low-noise cables will generate some noise when subjected to vibration or expansion and contraction. Therefore, all connections should be kept as short as possible, free from the effects of temperature changes (which create thermal expansion forces), and preferably the cable should be secured to a non-vibrating surface such as a wall, tabletop, or other solid structure.

　　There are several ways to address motion and vibration issues:

　　* Eliminate mechanical coupling to vibration sources. Motors, pumps, and other electromechanical equipment are common sources of vibration.

　　* Make the test fixture stable. Securely fix or bundle electronic components, wires and cables. Shielding should be secure and complete.

　　Triboelectric effects can also occur when other insulators and conductors rub against each other, so it is also important to minimize contact between insulators and conductors when building test fixtures and making low-current and high-impedance connections.

　　3. Piezoelectric effect and stored charge effect

　　Certain crystalline materials, when used as insulating terminals and connections, generate piezoelectric currents when mechanical stress is applied. In some plastics, the storage of electrical charges causes the material to behave like piezoelectric materials. The terminals of a piezoelectric insulator are shown in Figure 1.

　　To minimize the current generated by this effect, it is important to eliminate mechanical stress on the insulator and use insulating materials that minimize piezoelectric and stored charge effects.

　　This effect has nothing to do with the change in capacitance between the metal plate and the terminals. The movement of charges creates an electric current.

　　In practice, it can be quite difficult to distinguish between the effects of stored charge (in an insulator) and the piezoelectric effect. For either phenomenon, it is important to choose a good insulating material and make the connecting structure as strong as possible.

　　4. Pollution and humidity

　　Electrochemical effects produce error currents when ionic chemicals form a weak chemical battery between two conductors on a circuit board. For example, a commonly used epoxy printed circuit board, if not thoroughly cleaned of etching solutions, flux, or other contaminants, can produce several nanoamperes of current between two conductors (see Figure 2).

　　High humidity and ionic contamination can significantly reduce insulation resistance. High humidity occurs when condensation or water absorption occurs, while ionic contamination can be caused by body oils, salts, flux, etc.

　　The main result of these contaminations is reduced insulation resistance. Under the combined effects of high humidity and ionic contamination, conductive paths can also be formed, and even chemical batteries with high series resistance can be formed. The batteries formed in this case can output currents in the picoamp or nanoamp range for a long period of time.

　　To avoid the effects of contamination and humidity, choose insulation materials that resist water absorption and keep humidity levels at appropriate levels. Also, make sure all insulators are clean and free from contamination.

　　If the insulators are contaminated, clean all interconnects with a clean solvent such as methanol. It is important to completely rinse the contaminants away after they have dissolved into the solvent so that they do not settle again. Only pure solvents should be used for cleaning; lower grade solvents may contain contaminants that can leave an electrochemical film after cleaning.

　　5. Dielectric absorption

　　When voltage is applied to an insulator, the positive and negative charges inside the insulator are polarized due to the movement of various polar molecules at different rates, and dielectric absorption occurs in the insulator. When the voltage is removed, these separated charges will produce a decaying current in the circuit connected to the insulator when they recombine.

　　To minimize the effects of dielectric absorption on current measurements, avoid applying voltages greater than a few volts to insulators used when making sensitive current measurements. When this is unavoidable, it can take several minutes, and sometimes hours, for the current caused by dielectric absorption to dissipate.