Avoid Cable-related Errors In High Current Smu Measurements

Instrument Selection

Instrument Selection. Selecting the appropriate test equipment is the first step in achieving accurate, repeatable results at the highest speed. When it comes to electronic component testing, source measurement units (SMUs) have become the workhorses for high throughput testing. An SMUs is a special DC measuring instrument that can work as a constant current source or constant voltage source. It simultaneously sources to a pair of terminals and measures (reads back) the current or voltage at those terminals. Typically when an SMU sources constant voltage it takes a current measurement through the terminals. When it sources constant current through the terminals, it takes a voltage measurement across them.

In SMU testing the instruments are usually set up to perform time varying sweeps of current or voltage amplitudes that are used to determine current-voltage (I-V) properties during device characterization. There are at least four terminals on an SMU: the SOURCE terminal pair and the SENSE terminal pair, which are used to apply a stimulus to the DUT and measure its response.

Lead Configuration. However (this is where cabling becomes a factor), most SMUs can be used for either 2-wire or 4-wire measurements. This feature is important when testing high brightness LEDs and other components that draw large currents. These applications may also require an SMU with pulse output capabilities up to 50A or more, such as that available in the Keithley Model 2651A High Power System SourceMeter instrument.

When setting up an SMU measurement, keep in mind that 2-wire mode uses a single set of test leads to both source and measure. Although 2-wire mode simplifies cabling, it creates measurement errors because voltage measurements will not only include the voltage across the device under test (DUT), but also the voltage drop across the test leads. In high current applications, even test leads with very little resistance can cause large voltage drops and measurement errors.

To avoid this type of error, SMUs include a 4-wire measurement mode. In 4-wire mode one set of leads (the SOURCE leads) supply the voltage or current to the DUT, while a second set (the SENSE leads) measure the DUT response to the source stimulus. Because the measurement circuitry associated with the SENSE leads has very high input impedance, very little current will flow through these leads. Therefore, the voltage drop across them will be insignificant and the voltage measurement will accurately portray the voltage across the DUT.

Although 4-wire mode is designed to avoid excessive voltage drop in test leads due to high currents, excessive lead resistance can still create measurement problems. For example, when characterizing a high brightness LED using pulse mode testing rather than DC currents, fast rise and settle times are critical to obtaining the shortest pulse widths to minimize device self-heating. Minimizing lead resistance makes it possible to obtain the fastest rise times.

Cable Inductance. Minimizing cable inductance is crucial in pulse mode testing of any device. Essentially, inductance resists changes in current by creating a voltage drop when the test current is changing. Although inductance in the test leads will have no effect at the top and bottom of the pulse where current is constant, it will cause an additional voltage drop during the rising and falling edges. Therefore, its important to use short SENSE cables that have not only low resistance, but also low inductance. Minimizing lead resistance and inductance is important for obtaining the shortest, most distortion-free rise time, and allows high-speed, high-accuracy measurements.

by: David Wyban

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