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When To Use A Source Measure Unit

When To Use A Source Measure Unit

Using an integrated Source Measure Unit (also known as SMU or source meter) offers numerous advantages over assembling separate components. This article highlights the benefits of using an SMU, including speed, synchronization, programmability, cost-effectiveness, the ability to handle negative voltages, and its superiority over traditional multimeters and power supplies. Discover how an SMU can enhance efficiency and accuracy in various applications.

SMUs are valued for their versatility and capability to perform both sourcing and measuring functions in a single device. They can be used for in various applications, including:

  • Semiconductor testing such as taking current-voltage (I-V) measurements
  • Materials characterization
  • Electronic component testing
  • Any measurement where accurate and controlled electrical measurements are essential
Ossila Source Measure Unit with the Ossila Solar Simulator to test solar cells.
Ossila Source Measure Unit measuring current and voltage throgh a resistor.

Benefits of a Source Measure Unit


You could use multiple small components together to perform all the functions of an SMU. For example, you could use a bench top power supply to supply voltage, and combine this with an ammeter and a voltmeter to measure current and voltage. However, there are several benefits that come with using an integrated precise SMU.

Speed and Synchronization

If you are using individual components, you will need to make sure your voltage supply and subsequent measurements are synchronized. You should do this using an automated process involving a third device such as a computer. The relaying of information between these devices will add a delay to your measurement and limit the speed of your system.

Source measure units are designed to communicate this information within the integrated system. This means a source measure unit can supply voltage and measure current/voltage quickly with little latency.

The Ossila Source Measure Unit has varying modes allowing various measurement speeds between 7-3.5 kHz with little/no delay. In its fastest mode, it can measure 252 data points taken per second.

Another benefit of using a source measure unit is the with this improved transmission speed, the current limit information is fast enough that it should prevent damage to the SMU or your circuit from over current conditions, better than cheap power supply.

Programmable

Tasks like graph plotting, performance monitoring over time, and high-throughput experiments need extensive data collection. A single I-V sweep can often contain hundreds of individual measurements. Doing this manually will be time consuming, difficult and leave lots of space for human and systematic error.

Many experiments require automated data collection for fast, reliable measurements and accurate, precise results. These need to be logged, exported and controlled using a programmable system. You can automate individual components to do these functions, but it will take a significant amount of time to write and debug this code.

Separate measurement channels allow you to apply multiple different voltages and take different measurement simultaneously. Source measure units also provide current limiting functions to protect equipment and other circuit components during testing.

Most source measure units (including the Ossila SMU) are design to be controlled using simple command functions. Many resources are available to help you control your SMU with code. The Ossila Source Measure Unit is compatible with our free downloadable software for more specific uses. This allows you to take measurements straight away with no coding involved.

The Ossila Source Measure Unit

Compact and Low Cost

It is generally less expensive to buy a complete source measure unit rather than sourcing the individual components needed to take your measurement. Designing, configuring and synchronizing these components will also take significant time.

Additionally, a single source measure unit takes up significantly less bench space compared to the individual components and the necessary connecting wires.

Negative to Positive Voltages

SMUs can easily source negative voltage. This is important for testing many electronic devices such as diodes and photovoltaic devices. To fully understand how these devices work, you need to test them under both positive and negative voltages.

You can artificially measure negative voltages using a generic power source. This is done by reversing the input and output channels, or by turning the component you are testing around manually. However, you will need to measure this once the right way round (under positive voltage), then once the “wrong way round” (under negative voltage) and manually combine these results.

Source measure units can apply a range of voltages across a device, from negative to positive– and can do this in quick succession. You can sweep from negative bias to positive bias, and back again in one measurement. This is particularly useful for testing diodes – or devices with diode-like behaviour such as solar cells.

SMU vs. Multimeter


A standard multimeter can perform some of the same functions as a source measure unit. Multimeters will apply a voltage between two points and display the current flowing, or vice versa. A good handheld multimeter will be able to measure voltage with an accuracy of a few hundred micro-volts and current to an accuracy of a micro-amp. This is much better than a bench-top power supply.

However, multimeters can’t measure voltage and current simultaneously. They also lack automated data extraction or recording capabilities. As a result, handheld multimeters are primarily used for quick checks rather than precise data collection.

With a Source Measure Unit (SMU), you can perform quick checks as you can with multimeters. You can also design a programme to simultaneously measure both voltage and current for large data collection.

You could potentially build a source-measure unit with moderate accuracy by using a bench-top power supply to output a voltage/current and several good quality multimeters — one to measure voltage and the other to measure the current. However, this wouldn't be easily programmable and also wouldn't allow negative voltages to be measured very easily (both of which are important for many applications).

Source Measure Unit vs Power Supply


Source measure units are similar in some ways to bench-top power supplies. Both can output a voltage and measure the current that flows. However, SMUs are orders of magnitude more precise, are fully programmable, and allow the user to sweep the voltage over a defined range.

With a bench-top power supply, you use a dial to select the desired voltage, and the display shows how much current is flowing through your circuit. Usually a bench-top power supply will output a voltage from 0-12 or 0-24 Volts and measure the current to the nearest milliamp. This is great when measuring the current used by motors, light bulbs or high power devices.

However, for precise scientific measurements, a milliamp is actually a huge amount of current. You will likely need much more precise measurements to characterize many electronic devices (microamps or nanoamps). For this, you can use a source measure unit.

Source Measure Unit

Source Measure Unit

Contributing Authors


Written by

Dr. Mary O'Kane

Application Scientist

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