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A G R E AT E R M E AS U R E O F CO N F I D E N C E leads caused by high currents, excessive lead
resistance can still create measurement prob-
lems. When testing an LED using pulsed
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.
Minimizing cable inductance is also
crucial when testing LEDs with high test
Minimizing Cable-Induced
currents. Essentially, inductance resists
changes in current by creating a voltage drop
when the test current is changing. Although
Measurement Errors in
inductance in the test leads will have no
effect at the top and bottom of the pulse,
where the current is constant, it will cause
High Current Applications an additional voltage drop during the rising
and falling edges for which the SMU must
compensate. As with resistance, minimizing
lead inductance is important for obtaining
the fastest rise time.
Dave Wyban To learn more about how to ensure
higher source and measurement accuracy
In my job as an applications engineer, I've Two-wire mode uses a single set of test in LED test applications, I encourage you to
been fielding lots of calls about test and leads to both source and measure. Although download and view Keithley's latest webi-
measurement problems that engineers two-wire mode simplifies cabling, it cre- nar on this topic: Meeting the Electrical
encounter with high current applications ates measurement errors because voltage Measurement Demands of High Power,
like characterizing high power LED mod- measurements will not only include the volt- High Brightness LEDs at www.keithley.com/
ules and high brightness light-emitting age across the device under test (DUT) but events/semconfs/webseminars.
diodes (HBLEDs). At higher test currents, across the test leads as well. In high current
the appropriate cabling techniques are par- applications, even test leads with very little
ticularly important in obtaining repeatable resistance can cause large voltage drops. To About the Author
and accurate measurements and maximiz- compensate for this, SMUs include a four-
ing performance. wire measurement mode. In four-wire mode, David Wyban is an applications engi-
Source measurement unit (SMU) instru- one set of leads, known as the source leads, neer with Keithley Instruments, Inc.,
ments are among the most often-used types is used to source while a second set of leads, Cleveland, Ohio, which is part of the
of instruments for LED testing. SMU instru- known as the sense leads, is used to measure. Tektronix test and measurement port-
mentss, including Keithley's Model 2651A Because the measurement circuitry that the folio. He joined the company in 2006,
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