Shunlongwei Co. ltd.
IGBT Module / LCD Display Distributor
Customer Service
+86-755-8273 2562
IGBT Module / LCD Display Distributor
Current meters are used to measure electric current and the readings are given in amperes (A). There are two types of current meter structures: shunt-type current meters and feedback-type current meters.
Shunt-type current meters are commonly used and applied in many situations. Feedback-type ammeters are more suitable for measuring small currents, and their usage has been increasing as the need for measuring smaller currents grows. However, choosing the correct type of current meter depends not only on the magnitude of the measured current but also on the characteristics of the Device Under Test (DUT), typically referring to its impedance.
The common form of shunt-type current meters is found in almost all Digital Multimeters (DMMs). The current being measured forms a voltage across the input resistance of the DMM, which is proportional to the measured current.
▲ Figure 1: Shunt-Type Current Meter
The main drawback of shunt-type current meters lies in the input impedance of the meter. This becomes more evident as the input current decreases because a larger input resistance is required to generate a higher measured voltage. Shunt-type current meters typically work well in the following two situations:
The voltage across the terminals of the current meter is referred to as the voltage burden. When the current meter is connected in series with the measured current circuit, the voltage burden causes a decrease in the measured current, making it inaccurate.
An ideal current meter should have zero input impedance and zero voltage burden on the current circuit. However, a practical current meter will have a certain level of voltage burden. Generally, the measurement error caused by the voltage burden of the current meter is equal to the voltage burden divided by the output resistance of the device being measured. The voltage burden generated by typical current meters is on the order of a few hundred millivolts.
Compared to the shunt-type ammeter, the feedback-type ammeter is closer to an ideal ammeter and is suitable for measuring currents below the microampere level or when there are particularly stringent requirements for input impedance.
The feedback-type ammeter converts the input current into voltage through a high-gain operational amplifier in a feedback loop, where the voltage is proportional to the measured current. This voltage no longer appears at the input terminal of the ammeter but is amplified by the operational amplifier and output to the subsequent measurement circuit. The input voltage of the ammeter (also known as the voltage burden mentioned earlier) is equal to the output voltage divided by the voltage gain of the operational amplifier (typically around 100,000), so the voltage burden at the input terminal of the ammeter is reduced to the microvolt level. Due to the lower voltage burden, the feedback-type ammeter generates smaller errors when measuring small currents or when the output impedance of the device being measured is small. Keithley electrometers and picoammeters use feedback-type ammeter technology.
Image 2: Feedback-type ammeter
The diagram below illustrates the impact of voltage burden on measuring the emitter current of a transistor. Although a digital multimeter can measure the emitter current of the transistor well, the voltage burden of the digital multimeter significantly reduces the voltage of the measured quantity, resulting in a smaller measurement result. If a picoammeter or electrometer is used, the measurement error can be negligible.
There are several factors that can affect the accuracy of current measurements. All ammeters will produce a small current flowing through the input terminal, even when the input terminal is open-circuited. This current is called the bias current and can be nullified through device current suppression. External leakage current is an additional measurement error that can be reduced by proper shielding and protection. The output impedance of the device being measured affects the noise performance of the ammeter. Additionally, there are other types of currents that can superimpose on the measured current and cause errors. The following discusses some types of current and how to minimize their impact on measurements.
(1) Frictional Current
Frictional current is caused by the imbalance of charges between conductors and insulation layers, as shown in the diagram below. Choosing suitable conductors, such as Keithley low-noise cables, can reduce the impact of frictional currents. These cables have a layer of graphite on the outer polyethylene insulation, which acts as a lubricant between the internal insulation and the outer shielding, creating a cylindrical equipotential protection layer that balances charge distribution and reduces charge generation.
Image 4: Impact of frictional charging on current measurements
(2) Piezoelectric Current
Piezoelectric currents occur when certain special crystal structures generate current under mechanical pressure, typically at insulation interface terminals and internal connection hardware. In some plastics, there may be stored charges that can generate piezoelectric-like effects. The diagram below shows the piezoelectric current generated by a terminal with piezoelectric effects. The following methods can be used to reduce piezoelectric currents:
Image 5: Interface terminal with piezoelectric effects
(3) Pollution and Moisture
Pollutants combined with moisture can cause electrochemical effects, resulting in error currents. This is because pollutants containing ionizable chemical substances can generate a weak “battery” effect between the two conductors of a printed circuit board, such as epoxy resin. Without thorough cleaning of corrosive solutions, flux residues, oil stains, salts (usually from fingerprints), or other pollutants, several nA currents can be generated between the two conductors. To avoid errors caused by pollutants and moisture, choose non-absorbent insulation materials or maintain the humidity at an appropriate level. Keep all insulation materials clean and free from contamination.
▲ Figure 6: Error current generated by pollutants between circuit conductors
When measuring high impedance (typically greater than 1 GΩ), a constant voltage is applied to the resistor under test, and the resulting current is detected using a series ammeter. The resistance value can be calculated using Ohm’s law (R = V/I). This method of applying voltage to measure current is more suitable for high impedance measurements because the characteristics of some high-resistance resistors may be influenced by high currents. Of course, a controlled voltage source capable of generating an appropriate voltage range is crucial for the measurement. This method usually requires the use of electronic ammeters or pA meters capable of measuring small currents. The content mentioned earlier about small current measurement and error analysis is also applicable to high impedance measurements.
When measuring high resistance, leakage current is a common source of error, which occurs in the high impedance channel (leakage resistance) between the circuit under test and nearby voltage sources. Measures to reduce the influence of leakage resistance include using appropriate protective features, cleanliness, high-quality insulation materials, and reducing humidity.
The following figure shows the resistance values of common insulation materials. If moisture is absorbed, the resistance of some insulation materials may change by several orders of magnitude.
▲ Figure 7: Resistance of different insulation materials
The table below provides a quantitative description of moisture absorption and other effects.
▲ Figure 8: Resistance values of different insulation materials
▲ Figure 9: Moisture absorption, piezoelectric, and triboelectric properties of materials
When measuring substances with high impedance, background currents can cause significant measurement errors, which may be caused by stored charges, static electricity, triboelectric effects, piezoelectric effects, etc., in the insulation materials.
The alternating polarity measurement method can eliminate the influence of background currents on measurement results. This method is also straightforward: apply a positive voltage to the resistor under test, measure the current after a delay, then change the polarity of the voltage, and measure the current after a delay. Calculate the magnitude of the resistor under test by dividing the change in voltage by the change in current. This method can be repeated several times, and the weighted average of the measurement values can be taken to reflect the value of the resistor under test.