measurement application
Choosing a source measurement unit instrument
A source measurement unit (SMU) instrument integrates the capabilities of a precision power supply (PPS) with those of a high-performance digital multimeter (DMM) in a single
instrument.The high performance architecture allows using them as pulse generators,as waveform generators,and as automated current-voltage (I-V) characterization
systems.Mark A. Cejer, Marketing Director & Lishan Weng, Applications Engineer, Keithley Instruments, Inc discusses the benefits offered.
T
he real benefit of SMU instruments for test and measurement applications comes from their ability to source and measure signals simultaneously. 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 lets them protect the device under test (DUT) from damage due to accidental overloads, thermal runaway, and other dangers. It also makes SMU instruments ideal for characterizing and testing semiconductors and other non-linear devices and materials.
SMU vs. Power Supply
Given that an SMU instrument integrates the functions of a power supply with a digital multimeter, how exactly does the performance of an SMU’s source differ from that of a typical power supply? Greater 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. SMUs’ settling times are 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.
Wider operating range and better resolution: Because of their outstanding low current capability, 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.
Figure
1.Basic SMU instrument topology
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www.siliconsemiconductor.net Issue 2 2012
Four-quadrant rather than two-quadrant operation: As illustrated in Figure 2, a typical power supply can only source voltage and/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
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