What’s the Difference Between An SMU and a DMM with a Power Supply?

This article first explains a couple of test situations where a design engineer might use a power supply and DMM to make measurments or else employ a source-measurement unit (SMU). Then it answers the question, &quot;How Do They Differ?&quot; and shows how one or the other&nbsp;test setup&nbsp;is more appropriate in a particular situation. &nbsp;

Many tests require supplying a varying voltage or current to a device under test or unit under test and making a voltage or current measurement. A typical example is plotting the I-V curve of a diode. One way to perform this test is to use a power supply to provide the source and a DMM to make the measurement. Another way to perform it is to use a source measurement unit instrument or SMU. As the name implies, this instrument is capable of both sourcing a voltage or current and making the required measurement. So, what’s the difference between the two approaches, and when is using one preferable to the other?

Using a Power Supply and DMM

One application that uses a precision power supply and DMM involves characterizing the current drawn by a device or circuit board at different voltage levels. Figure 1 shows a typical setup. Engineers typically use this kind of setup when testing prototype circuit boards. This setup shows a computer being used to control the test, but in many cases, the computer isn’t really necessary. The engineer or technician manually sets the output of the power supply and makes a current measurement with the DMM. Only if many units under test (UUTs) are going to be tested, or if the test results need to be recorded digitally, would a computer be used to automate the test.

Similarly, you could use a DMM and power supply to characterize semiconductors. Say you wanted to measure the turn-on voltage and leakage current of a diode. The test setup would be similar to the setup shown in Figure 1, and you would measure the current through the diode while stepping the voltage from some negative voltage less than the reverse breakdown voltage to a positive voltage greater than the turn-on voltage.

1. A typical test configuration includes a power supply and a DMM. When configured as shown, this system could be used to measure current flow through the external shunt resistor, but it’s also possible to use the DMM current jacks and function directly to measure current, depending on the level and sensitivity needed. An SMU instrument provides automatic switching between current and voltage sourcing and measurement, as well as selectable or automatic ranging across many decades of current and voltage.

You need to consider several things when using a power supply as a source for this test. The first is the resolution and accuracy of the power supply. When trying to determine the turn-on voltage for a diode accurately, you want high resolution and accuracy, so that you can determine precisely what that value is.

Another parameter that you have to take into account is settling time. This is the time it takes for a power supply’s output to settle to its final value. When characterizing diodes, you must wait until the power supply’s output settles before making the current measurement.

In most applications, power supply output changes are infrequent and settling time is not an issue. But, when you sweep the source voltage across a range, you’ll be making many different output changes. At some point, settling time becomes an issue because it increases test time.

For an application like semiconductor characterization, you probably will want to automate the test. If this is the case, a factor that can affect test time and certainly test development when using separate power supplies and DMMs is system integration. Basically, the more instruments that you have, the longer it will take you to develop tests.

Using a Source Measurement Unit (SMU)

To address some of the disadvantages of using precision power supplies and DMMs in some applications, test equipment manufacturers have developed an instrument called the source measurement unit instrument or SMU. This type of instrument integrates the capabilities of a precision power supply (PPS) with those of a high-performance digital multimeter (DMM) in a single instrument. SMUs can simultaneously source or sink voltage while measuring current, and source or sink current while measuring voltage (Fig. 2).

2. The basic SMU instrument topology contains a precision power supply and a high-performance DMM.

SMUs can be used as stand-alone constant voltage or constant current sources, as stand-alone voltmeters, ammeters, and ohmmeters, and as precision electronic loads. Their architecture also makes it possible to use them as pulse generators, waveform generators, and automated current-voltage (I‑V) characterization systems.

How Do They Differ?

Given that an SMU instrument integrates the functions of a power supply with a digital multimeter, how exactly does an SMU’s source differ from that of a typical power supply?

Speed and Precision: SMUs are optimized for both speed and precision, so they can offer significantly faster rise times and much lower measurement uncertainty than power supplies. An SMU’s settling time is measured in microseconds compared to the milliseconds that power supplies require to settle on their programmed value. Similarly, an SMU’s measurement uncertainty is measured in nanoamps vs. microamps for typical power supplies.

Operating Range and Resolution: SMUs are designed to offer better low current capability than power supplies. Because of this, SMUs typically offer much wider operating ranges with greater resolution than power supplies, so they are suitable for a wider range of test and measurement applications.

Four-Quadrant Operation: As shown in Figure 3, a typical power supply can only source voltage or current. In other words, it provides only two-quadrant operation (in quadrants I and III), but an SMU can provide full four-quadrant operation because it’s capable of sourcing and sinking power, acting as both power supply and an electronic load. During source or sink operation, the SMU can simultaneously measure voltage, current, and resistance. This operating flexibility can be especially valuable when characterizing batteries, solar cells, or other energy-generating devices.

3. A bi-polar output power supply (right) has two-quadrant operation; an SMU instrument (left) can source and sink power in all four quadrants.

Sweep Capability: A sweep is simply a series of points or I or V steps that the source outputs while measuring the DUT response.The various sweep capabilities SMUs offer can simplify programming a test’s source, delay, and measure characteristics, significantly boosting testing productivity. All sweeps can be configured for single-event or continuous operation to simplify the process of capturing the data needed to characterize and test a wide range of devices. Sweeps can also be used in conjunction with other throughput-enhancing features like Hi-Lo limit inspection and digital I/O control to create high-speed production test systems.

A fixed level sweep outputs a single level of voltage or current with multiple measurements. This is typically done to bias or stress devices. Various types of fixed level sweeps can be generated, depending on the needs of the application.

Pulsed sweeps are often used to limit the amount of power that goes into a material sample or device over time and to minimize self-heating effects that could otherwise damage semiconductors and light emitting diodes (LEDs), experimental materials such as graphene, or other fragile nanotechnology-based devices.

Custom sweeps simplify creating application-specific waveforms that can be linear, logarithmic or random.

Making measurements with an SMU is also different from making measurements with a DMM. Because it has built-in sourcing capabilities, an SMU can minimize overall measurement uncertainty in many applications. The first diagram in Figure 4 shows the basic voltmeter configuration for the SMU. Here, the built-in current source can be used to offset or suppress any system-level leakage currents (such as cable noise) that could cause unwanted errors in voltage measurement applications.

4. There are several configurations for making measurements with an SMU that are easily selectable from the front panel or via remote software interfaces.

For current measurements, the SMU’s built-in source and “feedback ammeter” design work together to keep voltage burden low, and enable low current measurements to sub-picoamp levels. DMMs do not have the built-in source, and usually have “shunt ammeter” designs that can limit low current capabilities to microamp or nanoamp levels.

Finally, for resistance measurements, the SMU architecture offers full flexibility over the amount of current or voltage sourced to the DUT. DMMs generally have fixed current source-only values that are pre-determined by the vendor and dependent on the range being used to measure resistance. SMUs offer fully programmable source I or V modes and values for measuring resistance. This can be valuable for protecting DUTs or for measuring extra high or extra low resistances. For high resistance measurements, the source voltage method is preferred; for low resistance measurements, the source current method is best. Some SMUs have a six-wire ohms feature that “guards out” the effects of unwanted parallel resistance paths in the circuit.

Summary

While a power supply and DMM are sufficient for many applications, SMUs are a better choice for applications where source precision and a wide dynamic range of voltage and/or current levels are necessary. This includes applications such as characterizing and testing semiconductors and other non-linear devices and materials for leakage, breakdown and forward operating characteristics.  Material resistance and resistivity are also important applications where the SMU’s capabilities are useful for fully characterizing operating performance.

They are also a better choice for tests that require automation. When compared with using separate instruments to handle each function, SMUs’ simultaneous operation provides for faster test times, simplified connections, improved accuracy, less complex programming, and a lower cost of ownership (COO). Their tight integration protects the device under test (DUT) from damage due to accidental overloads, thermal runaway, and other dangers.

References

1. Choosing the Optimal Source Measurement Unit Instrument for Your Test and Measurement Application,” Application Note 3161, Keithley Instruments, Inc., Cleveland, OH. www.keithley.com/data?asset=56423. April 2012.

2. “Source-Measure Units Increase Productivity and Accuracy In Automated Testing,” Article, Keithley Instruments, Inc., Cleveland, OH. www.keithley.com/data?asset=6058.

3. “Diode Production Testing with Series 2600 System SourceMeter® Instruments,” Application Note 2633, Keithley Instruments, Inc., Cleveland, OH. www.keithley.com/data?asset=50312. June 2005.

Charles Cimino has been with Keithley Instruments in Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio, for more than 20 years in various product and business development roles. Currently, he is the company’s marketing director, and is responsible for Keithley’s core instrument products and markets including SourceMeter® instruments, sensitive DMM and signal switching products sold to global research and production accounts. Cimino has an undergraduate electrical engineering degree and received his MBA from Case Western Reserve University's Weatherhead School of Management.