Source measure units (SMUs) are precision instruments that you can use to simultaneously source and measure both current and voltage on the same channel. This functionality makes SMUs ideal for a broad range of applications from providing high-precision DC power to characterizing semiconductor components such as discrete components or ICs.
Certain SMUs are optimized for a specific application or requirement, such as leakage testing, high-power IV sweeps, or powering mobile devices with a fast transient response. While these instruments are ideal for serving a specific function, you may need multiple SMUs to fully characterize devices that require a variety of different stimulus or functionality. Alternatively, system SMUs combine high-power, high-precision, and high-speed source measure capability into a single instrument. This gives you the versatility to perform many different functions and test a broad range of devices on the same pin. This not only simplifies connectivity, but also reduces the number of different instruments, the test system footprint, and overall cost.
An NI system SMU combines the power and precision of a traditional box SMU with NI technology that makes it faster, more flexible, and more compact. This combination gives these instruments the ability to perform functions beyond traditional SMUs and turns it into a multipurpose instrument you can use as a:
Combining this functionality into a single instrument makes them ideal for testing a broad range of devices, including those that require:
With the modularity and compact size, you can integrate a small number of SMU channels into mixed-signal automated test applications, or build high-density semiconductor test systems with up to 17 SMU channels in a single 4U PXI chassis.
The broad power boundary and high-precision specs of NI system SMUs allow you to address a majority of DC test requirements using a single instrument. Additionally, they expand on traditional SMU functionality by incorporating a fast sampling rate, a fast update rate, and NI technology for customizing the SMU response. This opens up a variety of applications for SMUs that previously required external instruments such as oscilloscopes, function generators, or highly specialized power supplies.
Extended Range Pulsing
These SMUs can generate pulses up to 500 W, which gives you a way to quickly test the IV characteristics of high-power devices without cascading multiple SMUs. Testing in the extended pulse boundary gives you the ability to not only operate the SMU outside of its normal IV boundary, but also minimizes the amount of heat dissipated through the device under test (DUT) and reduces the need for thermal management infrastructure.
NI SMUs are equipped with a digital control loop technology called SourceAdapt, which gives you the ability to customize the control loop of the SMU and create the optimum step response for any given load—even highly capacitive or inductive loads. Removing overshoots helps protect the DUT while eliminating oscillations that are critical for system stability. Additionally, minimum rise and fall times help you achieve the fastest possible test times.
This feature is critical for powering highly capacitive loads without damaging the DUT or instrument. Traditional SMUs that use fixed, analog control loops can switch from “normal” to “high-capacitance” mode to prevent the SMU from becoming unstable. However, when operating in high-capacitance mode, the rise time of the SMU is dramatically throttled and the load capacitance is often still limited to 50 uF. Because SourceAdapt gives you direct access to the digital control loop of the SMU, you can optimize the SMU response for any load, even a 2,500 uF capacitor, as in the figure below.
The SMU in the figure above is sourcing voltage into a DUT with a 2,500 uF capacitor in parallel with a 2 Ohm resistor. The voltage rises to 3.3 V with a 10 A current limit, and settles around 1 ms. Without SourceAdapt technology, typical SMUs would become unstable in the presence of such a reactive load, and potentially damage the DUT or instrument.
Testing PMICs, such as the following step-down converter from Texas Instruments, requires two SMUs in the following setup. The first SMU provides input power to the DUT at a specific voltage, and the second SMU acts as a load by sweeping current in incremental steps.
By measuring the voltage and current on the input and output of the DUT, you can calculate the power efficiency of the DC-DC converter, as shown in the graph below. The wide IV boundary and hardware timed sequencing engine of the NI PXIe-4139 make it ideal for quickly sweeping from low to high power.
Additionally, the fast sampling rate of the NI PXIe-4139 gives you the ability to capture line and load transients that would typically require an external oscilloscope. This transient behavior is important for characterizing DC components and something that traditional SMUs are unable to test.
The above LED from CREE has a typical forward voltage of 37 V, and handles up to 2.5 A. Even though these limits are beyond the 20 W DC boundary of the NI PXIe-4139, you can still characterize the device by taking advantage of the extended pulsing range. Pulsing gives you the ability to not only characterize the LED with a single SMU, but it also minimize the amount of heat dissipated through the DUT and avoids the need for building custom heat sinks.
The above graph shows an IV characterization of the high-power LED by using a sequence of 50 us pulses and gradually increasing the current to its maximum rating. Using a narrow, 50 us, pulse width not only minimizes the self-heating effects of the LED, but reduces the overall test time. Creating accurate, narrow pulses would not be possible without NI SourceAdapt technology, which optimized the SMU response for this specific DUT and provided the fastest possible rise time with no overshoots or oscillations.
NI system SMUs provide up to 500 W of pulsing power in a compact PXI form factor, while still offering current sensitivity down to 100 fA. Not only do these instruments provide exceptional DC performance, but they also overcome the limitations of traditional SMUs with the addition of a built-in digitizer function and customizable SMU response. In addition, you can achieve unprecedented channel density of up to 17 system SMU channels in 4U, 19-inch rack space. The combination of power, precision, and speed, along with excellent density makes an NI system SMU a must-have for your next test system.