Accurately measure nanoampere current using Tektronix DMM4020 multimeter

In current electronic devices, measuring standby power or current leakage is a common task in design verification and debugging. Because consumers need products with longer battery life and higher energy efficiency, design engineers must carefully manage the energy lost in current leakage, which requires accurate measurements. The device is usually designed with standby current leakage, which is

Side-effect products, consumer electronics with always-on clock displays, and power conversion equipment such as chargers. As energy is power times time, even if the standby power level is low, it may It consumes a lot of energy, consumes AC power, and consumes dry batteries. Accurately measuring nano-level low-level leakage currents is a challenge faced by most digital multimeters. The Tektronix DMM4020 multimeter provides a dedicated circuit that can measure 1 nA Resolution measurement of leakage current solves this challenge.

Figure 1. The shunt multimeter circuit is used as an ammeter.

Measuring standby current leakage

Measuring standby current seems like an easy task. It only needs to connect the leads of a high-quality multimeter (DMM) to the corresponding terminals and directly measure the current amperage. But in fact, this task is not so simple, because the current leakage is generally very low, only a few nanoamperes, the measurement using traditional DMM may be inaccurate. The DMM measures current by applying a known resistance in the form of a shunt resistor in series with the circuit under test, allowing current to flow through the circuit. When current flows, the DMM measures the amount of voltage drop across the shunt resistor and uses Ohm's law to calculate the current. This shunt resistor method introduces a sag voltage in the shunt device, called the burden voltage, as shown in Figure 1. The burden voltage becomes a source of error, because according to Kirchhoff's voltage law (KVL), from

Subtracting this burden voltage from the voltage provided in the circuit, an error of more than 50% may occur. By using a lower shunt resistor, the number of errors can be reduced, and the high-end DMM provides a variable shunt value, allowing selection of current range. However, using a low shunt resistance value will increase the measured voltage sensitivity, making the measurement inaccurate and unstable.

Figure 2. The circuit in the feedback DMM is used as a low current ammeter. The Tektronix DMM4020 multimeter is one such tool. By using the current-to-voltage amplifier (op amp) conversion technique in series with the circuit, using DMM as an ammeter in low-current applications can

Greatly improve the accuracy, as shown in Figure 2. For example, for a circuit with a 1.2 V dc power supply and a 100 kÙ test load device, the calculated current is 12 μA. However, due to the addition of a series meter shunt resistor (10 kÙ), the measured current through the device under test will drop to 10.909 μA. In order to improve the sensitivity of low current measurement, the ammeter design engineer will increase the shunt resistance; as the shunt resistance increases, the error will also increase. The Tektronix DMM4020 digital multimeter uses current-to-voltage operational amplifiers in two low DC ranges: 2000 μA and 200 μA. In these ranges, the operational amplifier introduces low impedance into the circuit, putting the unknown input current

Converted to voltage without the need for a low resistance shunt device, thereby eliminating the burden of voltage. As a result, the instrument provides a resolution of up to 1 nA and an accuracy of 0.03% for the low current measurement of the specified range, with minimal impact on the measurement load. Now, by using the Tektronix DMM4020 desktop multimeter, you can accurately measure the standby current. Figure 2. The circuit in the feedback DMM is used as a low current ammeter. The Tektronix DMM4020 multimeter is one such tool. Device under test mA input R feedback LO input V out signal conditioning and analog-to-digital converter

Pay attention to sources of error

When making low current measurements, it is necessary to understand the possible sources of measurement error and how to prevent these sources of error from affecting the measurement. Common error points include: external leakage current, such as leakage current caused by pollutants such as dust, grease, and flux. Whether it is the device under test, the test instrument itself, or the test cable or connector, contaminants may provide other flow paths for the current, thereby introducing errors in the measurement. When conducting low-current tests, alcohol should be used to clean all potentially contaminated surfaces. Any type of noise may introduce errors in low current readings:-AC power supply noise may overwhelm sensitive amplifiers and cause inaccurate readings. Filtering is helpful, using coaxial cables or shielded twisted pair test cables can reduce false readings. -Any type of acoustic noise may introduce vibration into the measurement process. Vibration will cause the conductor to move relative to the insulator, causing noise in the circuit, which in turn will cause errors. -When the thermal energy of the shunt device or the device under test stimulates the random movement and collision of electrons in the circuit, thermal noise will be generated. The resulting voltage and current are proportional to the square of the resistance in the circuit (from the device under test and the measurement circuit). It is helpful to use a shunt device with lower resistance.

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