How Source Meters Can Streamline Your Testing

Testing complex systems like low-power integrated circuits or automotive systems at a high degree of reliability and qualification is far from trivial. The number of test parameters, voltage and current ranges, test sequences, and criteria set by compliance standards make such testing extremely complicated and laborious.

To streamline and automate such tasks, engineers use instruments like source meters.

In this article, learn the ins and outs of source meters, their benefits, and their wide range of applications.

What is a source meter?

A source meter (or sourcemeter) is an instrument that measures direct current (DC) or voltage while simultaneously sourcing/supplying or sinking/receiving them. It's also known as a source measure unit (SMU).

In addition to simultaneous measurement with high precision, source meters are unique because of the following features:

• Many instruments in one package: An SMU combines the functionality of a power supply (or power source), electronic load, voltmeter, and ammeter.
• Both sourcing and sinking capabilities: A source meter can both source and sink power. This is termed as operating in all four quadrants of the voltage-current plane, a concept we'll explain in depth later. It's an essential feature for testing many applications.
• Rapid settling times: Compared to other instruments, SMUs can rapidly reach set voltages or currents. The typical settling times of good SMUs are of the order of tens to hundreds of microseconds.
• High-speed transitions between modes: A critical capability of an SMU, which is essential for many characterization tests, is its ability to rapidly transition between different source and sink modes to simulate real-world conditions.

How does a source meter facilitate both sourcing and measuring voltage and current simultaneously?

What problems are source meters trying to solve? In this section, let's understand the need for source meters better with an example.

A typical diode's current-voltage (IV) characteristic is complex, like this:

• When forward-biased, the diode blocks current till the voltage goes above the forward voltage.
• When reverse-biased, the diode behaves like an open circuit. But a small leakage current does flow back through it.
• However, if the negative voltage goes beyond a threshold breakdown voltage, the diode behaves like a wire, allowing high amps to flow in the opposite direction.

Due to their complex non-linear behaviors, engineers must characterize diodes by supplying a series of positive and negative voltages and measuring the current through the circuit. At a minimum, they'd need to use a power supply and an ammeter.

Moreover, for accurate measurements, they must carefully synchronize the current measurement and the voltage sweep with minimal latency. One approach is to use dedicated software to program the synchronization of the two instruments and drive the entire test.

Clearly, all this requires several pieces of equipment, considerable time, and careful experimentation.

Instead, you can improve expenses, time, and accuracy by using just one instrument, an SMU. You just have to set up its voltage source to sweep the range of required voltages in steps. The SMU quickly sweeps through the range thanks to its fast settling time, measures the forward and reverse currents through the diode at each voltage, records the values, and makes them available for data export.

For characterizing a device under test (DUT), a source meter supports four basic concurrent source-and-measure modes:

• Source a set of voltages: In voltage sourcing mode, the SMU is configured to supply a specific output voltage to the DUT or sweep rapidly through a range of voltages. The SMU simultaneously measures and records the current drawn by the DUT.
• Source a set of currents: When sourcing current, the SMU is configured to supply a specific current to the DUT or rapidly sweep through a series of current values. The SMU concurrently measures the accurate voltage at the DUT.
• Sink a set of voltages: Instead of sourcing, the SMU is configured to receive power from a sourcing DUT (like a battery, power supply, or solar cell) at a particular voltage or while sweeping through a voltage range. It simultaneously measures the current drawn from the DUT.
• Sink a set of currents: Similarly, the SMU is set up to draw a specific current from the DUT or sweep through a current range. Simultaneously, it measures the voltage across the DUT.

What are the advantages of this dual capability?

The ability to measure voltage or current while sourcing or sinking power provides several technical benefits:

• Low measurement errors: Since the sourcing and measurement circuits are intimately coupled, the measurement error and latency are very low compared to using multiple instruments.
• One instrument in place of two or three: Setting up just one SMU is far easier and faster than setting up a computer, a power supply, and an ammeter or multimeter. An SMU simplifies test setups by avoiding the need to synchronize multiple instruments, reducing cable mess, and freeing up power outlets.
• Highly stable voltage and current sourcing: Since SMUs produce high-accuracy readings of their own current and voltage outputs, they have many advantages over conventional power supplies. All SMUs have internal feedback loops that provide instantaneous feedback to the sourcing circuitry, which in turn allows the SMU output to remain accurate and stable even if the load conditions change unexpectedly. Also, voltage and current limiting features prevent device damage.
• Accurate timing control of source and measurement: The integration of the source and measurement resources in an SMU allows much tighter synchronization than would be possible with separate instruments. Some SMUs provide very flexible triggering options that allow measurement points to be configured independently from the sourced current or voltage waveform.

The business and logistical advantages of SMUs include:

• Lower expenditure: Purchasing one instrument is typically cheaper than purchasing two to three separate instruments.
• Lower maintenance and calibration costs: Similarly, maintaining and calibrating one instrument is typically cheaper, with less overall downtime, than separate instruments.
• Less lab space: Fewer instruments take up less lab space and possibly reduce rental or leasing costs.

What is compliance voltage in source meters, and why is it crucial in testing semiconductor devices and other electronic components?

The compliance voltage is the configurable maximum limit on the measured voltage. If the voltage goes above this high-voltage limit, the SMU can be configured to quickly cut off power and shut down to protect both the instrument and the DUT.

Similarly, there's also a compliance current limit to protect the SMU and DUT from high currents.

How does a source meter differ from a power supply, power analyzer, and multimeter?

A source meter combines the functionalities of a power supply and a digital multimeter.

Unlike a power analyzer, an SMU typically does not have any oscilloscope or signal generator features to generate waveforms and view them in real time. However, some specialized SMUs, like the PZ2100 series, have built-in digitizers to characterize response waveforms and pulsers to generate pulsed waveforms.

In terms of performance, SMUs are very high-speed instruments with very fast settling times compared to other instruments.

What does the four-quadrant operation of a source meter mean?

As mentioned earlier, source meters can both supply and receive power and switch quickly between them. In fact, these capabilities can be described more accurately by using the concept of a voltage-current plane that's divided into four quadrants as shown below.

Fig 1. Voltage current plane and quadrants

What does positive and negative voltage or current mean? In a regular DC power supply mode, the voltage set across the instrument is considered positive and the DUT is assumed to reach that same voltage. To avoid confusion, we'll name the instrument's test leads as "output terminal" and "return terminal." (Note: Most SMUs actually label them "High/Hi" and "Low/Lo.")

Fig 2. Naming convention for SMU output terminals used in this article to reduce confusion

Regular power supply mode is considered a first-quadrant operation where both the voltage and current are positive. Conventional current is considered as flowing out from the SMU's output terminal, into the DUT's input terminal, out the DUT's output terminal, and back into the SMU via its return terminal, as shown below:

Fig 3. Voltage and current flow sign conventions

The other three quadrants are shown below.

Fig 4. Voltage polarity and current flows in the four quadrants

The working of the SMU and DUT in each quadrant is as follows:

• In the first quadrant, the instrument is a source with a positive voltage across it and is supplying a positive current from its output terminal.
• In the second quadrant, the instrument acts as a load with a positive voltage across it. It sinks, or receives, current at its output terminal from the DUT.
• In the third quadrant, the instrument is again a source supplying current but its polarity is the opposite of conventional polarity, and hence negative. That means the return terminal is set at a higher voltage than the output terminal but without changing the wiring to the DUT.
• In the fourth quadrant, the SMU again acts like a load. Voltage is negative, which means the return terminal is at a higher potential than the output terminal. But the current flows conventionally.

Which electronic testing and manufacturing processes use source meters, and What specific parameters do they measure?

Some common SMU test applications are described below.

Low-power IC testing in multiple domains

Low-power integrated circuits (ICs) are used in multiple domains like:

• General electronics: They're used in components like microcontrollers and sensors.
• Telecommunications: Low-power ICs are used in consumer devices like smartphones and wearables.

Low-power ICs operate at extremely low current and voltage levels, but accurate testing of such small current and voltage levels can be challenging due to limited equipment sensitivity. Additionally, the complex transitions between sleep and active modes require dynamic and static DC IV characterization. As circuits become more complex, test sequences also become more complicated and require additional test instruments.

SMUs like the Keysight PZ2100 high-channel densitySMU solves these challenges. At a lower cost and smaller footprint, it supports broad IV characterizations of highly integrated low-power ICs in various applications.

Optical device testing in data center operations

Testing highly integrated optical devices requires a significant number of precision bias sources. For instance:

• Integrated tunable lasers need precision current sources for the laser diodes and precision bias sources for each heater to adjust wavelength precisely.
• Coherent optical receivers need multiple precision bias sources to properly convert the optical signal to an electrical signal by measuring the photocurrent of photodiodes. Typically, an integrated coherent assembly with a laser diode requires as many as 20 or more precision bias sources for testing.
• Highly integrated optical devices have more test ports and components, requiring a large number of high-precision power supplies and significant space.
• Their evaluation requires extensive testing across numerous test ports, leading to complex test sequences and connections.

A high-channel-density solution like the PZ2100, an all-in-one SMU with integrated digitizer, significantly reduces the necessary test instruments and system footprint.

Automotive applications

Low-power ICs are common in automated driver assistance systems, infotainment, and lighting. As described above, SMUs are used for IV characterization of their low-level voltage and current levels.

SMUs are also used for testing battery charging in electric vehicles. This is an example of second-quadrant operation with the battery as the DUT, positive voltage in the SMU, and negative current because current enters the SMU from the battery.

Another automotive example is regenerativebraking where the SMU is used as a load for receiving power from the DUT (the regenerative braking circuit that converts the vehicle's kinetic energy to electrical energy). This is an example of the fourth quadrant operation. The voltage is negative because the counter electromotive force produced at high kinetic energies goes above the battery voltage. The current is positive to simulate the positive current through the battery when the generator charges it.

Diodes and LED tests

SMUs are used for measuring the leakage current and breakdown voltage of reverse-biased diodes and low-current light-emitting diodes (LEDs).

Semiconductor characterization

Specialized SMUs like the Keysight B1500A with all-in-one device characterization analyzer capabilities are available for comprehensive semiconductor testing. They have features like:

• multiple measurement techniques, including IV, capacitance-voltage (CV), and dynamic IV
• the ability to switch between capacitance and current voltage measurements seamlessly without re-cabling
• ultra-fast transient phenomena surpassing conventional test instruments
• ultra-low current DC transistorcharacterization at the wafer level

How do source meters handle dynamic and transient conditions, and why is this capability essential in certain testing scenarios?

In sensitive wafer-level characterization, fast transient conditions can occur due to phenomena like dielectric absorption. To handle such conditions, SMUs provide fine-grained control on the voltage sweeps, like hold time at the beginning of a sweep and delay time between each measurement point in the sweep to provide enough time for transient voltages to decay.

What considerations should be taken into account when selecting a source meter for a particular application?

Pay attention to these aspects and specifications when selecting an SMU for your use case:

• required quadrants of operation and quadrants supported by the SMU
• voltage sweep range
• current sweep range
• settling times
• transition times between modes
• number of SMU channels required for complicated test sequences with plug-in modules

How can source meters be integrated into automated test systems?

To integrate source meters easily with automated test equipment, look for source meters with connectivity options like:

• ethernet connectivity to local area networks (LAN) with LAN extensions for instrumentation (LXI) protocol support
• universal serial bus (USB) connectivity
• general purpose interface bus (GPIB) connectivity

Such connectivity enables you to program SMU instruments for running complicated test sequences on demand.

Another essential element for efficient automated testing is software that allows remote control of SMUs for quick measurements without programming.

For example, the PathWave BenchVue Power Supply Control App supports up to 20 channels of SMUs, simplifies their configuration, monitors and records their outputs, and helps to visualize IV measurements.

Fig 5. PathWave BenchVue Power Supply Control App

Similarly, the PathWave IV Curve Measurement Software enables engineers to conduct synchronous IV measurements on up to 20 channels of SMUs and analyze the results without any programming.

Fig 6. PathWave IV Curve Measurement Software

Source meters for efficient and economical test setups

In this article, we learned how source meters streamline the productivity, repeatability, costs, and footprints of test setups.

Contact us for expertise in helping you select the best SMU for your use case from our wide range of general-purpose and specialist instruments.

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