Small-Current, Low-Noise, High-Accuracy DMM Measurements
2020-07-02 | 9 min read
Many applications, especially portable devices, operate in very low power to extend the battery. When designing and testing such devices, you need an instrument that can measure very small currents, likely in the microampere range and picoampere resolution. It also should not aggravate or introduce noise to the measurements.
When measuring very low currents, particularly in the microampere range, you need to be aware of three main concerns that affect measurement accuracy:
- Burden voltage – This is the change in potential created when current flows through the shunt resistor of a digital multimeter (DMM).
- Noise – When measuring small current, undesired external or internal noise to the measurement circuit affects accuracy.
- Dynamic range – Ensuring proper range settings for optimal accuracy when measuring dynamic current is always a challenge.
There are many types of current measurement instruments, including oscilloscopes with current probes, source measure units, DC power analyzers, and dedicated current waveform analyzers. However, when budget or flexibility is a concern, you can use a DMM to accomplish many of these measurements.
Tips to Solve Top Concerns Affecting Current Measurement Accuracy
These tips address the three concerns when using a DMM to measure small current accurately.
In-circuit current measurement requires users to open the circuit under test and place the measurement instrument leads in series. Ideally, the DMM would appear to the circuit as a dead short. The reality is that there is always some small but potentially significant resistance from the current shunt, switches, fuses, and meter leads. The burden voltage is the voltage drop that occurs across the DMM as a result of this additional impedance. When measuring low-level currents on highly sensitive components, you must be aware of the effects of burden voltage.
Figure 1 shows the DMM placed in series in the return path of the circuit. By adding a small voltage above the low of the power supply, the low of your device might well be above your design tolerance.
Consider moving the DMM in series to the positive side of your power supply. If you can increase the voltage to accommodate the burden voltage, you can supply the correct voltage to your device and measure current.
When measuring low currents, your measurements are more susceptible to noise in the environment. Some DMM brands may introduce injected current noise into the circuit of your measurements.
These types of external noise sources can affect your measurements:
- Magnetic field noise – This noise can come from common high-current electric appliances such as electric motors, generators, televisions, and computer monitors. Avoid making measurements or shield your circuit and DMM cables from these magnetic field sources.
- Ground loop noise – A ground loop forms when measuring small signals in circuits where the multimeter and the device under test (DUT) are both referenced to a common earth ground. This can be significant enough to cause measurement errors. One way to avoid this problem is to isolate your measurement circuit from the earth ground. If you cannot avoid making an earth ground isolated measurement, put the DMM and DUT as close as possible in terms of circuit ground to minimize ground loops.
- Thermoelectric noise – This type of noise happens when circuit connections with dissimilar metals get subjected to higher temperature variations. Thermal noise gets more pronounced with high temperature variations. Best practice is to use similar copper-to-copper crimped circuit connections.
Unfortunately, not all measuring instrument designs prevent injected current noise. Figure 2 shows that the injected current is dependent upon power-line configuration and frequency. Residual capacitances in the multimeter’s power transformer can cause small currents to flow from the LO terminal to earth ground.
Learn more about how to eliminate noise from this white paper: Eliminate Measurement Errors and Achieve the Greatest Accuracy Using a DMM.
Figure 3 shows the typical current profile of a portable radio transceiver. As you can see, the current draw is complex because of a wide range of sleep, standby, and active modes. The dynamic range of the current is broad because the operating currents are drawing approximately 30 to 40 mA, while the standby currents are only 1 to 10 μA.
To get accurate readings for both ranges with a DMM, you need to take multiple reading sweeps with different ranges. One method for capturing the current profile is to run the DUT multiple times to capture the sleep and standby modes separately and then the operating mode currents.
In the first capture, set the DMM to the 100-mA range and 0.001 PLC (20 μs per sample). This setting captures the complete current signal, including the active mode values from 30 to 40 mA, but provides less resolution on the lower-current measurements. Once you have captured the readings, you can save the data to memory and analyze it on a PC.
Next, you can set the DMM to a lower current range for standby and sleep currents. Doing this lets you capture very-low-level currents from approximately 2 to 10 μA. Anything measured above 120 percent of the range will result in an overload condition.
How Can Keysight Truevolt Series DMMs Help You Measure Low-Level Currents?
- Keysight’s 34465A and 34470A Truevolt DMMs can measure very low current, in the range of 1 µA with pA resolution, allowing you to make measurements on very-low-power devices.
- Keysight’s 34465A and 34470A Truevolt DMMs also have advanced triggering and digitizing capabilities to enable you to capture current waveforms at the exact timing events you need. You can digitize your captured current waveform at up to 50,000 samples / sec and store your waveform up to 1,000,000 samples per trigger.
- Keysight’s Truevolt technology accounts for measurement errors created by these real-world factors, so you can be confident in your measurements. It is available only on Keysight DMMs.
In a rack or on a bench in the real world, signals are never flat. They have some level of AC signal riding on top from power-line noise, other environmental noise, or injected current from the meter. How well your meter deals with these extraneous factors and eliminates them from the true measurement makes a big difference in your accuracy. Figure 4 compares a Truevolt DMM against similar DMM brands in terms of noise and injected current. Truevolt DMMs contribute less than 30% of the injected current compared with alternatives. Compared with some lower-cost alternatives, Truevolt DMMs offer almost 100% less noise.
Keysight’s 34465A and 34470A Truevolt DMMs can effectively characterize dynamic currents with their very low 1-µA range, advanced triggering, and digitizing capabilities.
For more information about the Keysight Truevolt digital multimeters, go to www.keysight.com/find/dmm.