The market’s needs with regard to source/measure units (SMUs) are changing. The battery life of portable devices is a critical design criterion. Battery life is extended by turning off unused subsystems, which means that system current draw can slew very rapidly. This in turn translates into measurement concerns. That dynamic current consumption must be measured accurately. Meanwhile, designers must be able to determine whether the circuit is saving power as predicted, which puts more demands on the SMUs’ ability to properly perform device parametric test.
Unfortunately, most traditional SMUs aren’t well suited for these kinds of functional testing. Functional test requires high accuracy, but the femtoamp-level accuracy of many traditional SMUs is overkill. Two-quadrant operation is really what’s called for, not the four-quadrant operation of most SMUs. But most four-quadrant SMUs are difficult to use and can suffer from negative-voltage output glitches that are large enough to damage the device under test (DUT).
So it’s the middle ground in the SMU landscape that is the sweet spot for power-management shakeouts in portable equipment. Until now, Agilent has not had an offering in this middle ground. But with the introduction of its N6781A and N6782A (see the figure) two-quadrant SMUs, the company hopes to address these emerging requirements.
In these modular SMUs, Agilent offers an all-in-one voltage source, current source, and electronic load. Both SMUs are two-quadrant sources that cannot go negative in voltage, eliminating the risk of damage to the DUT. Both the N6781A (intended for battery-drain analysis) and the N6782A (for functional test) offer glitch-free sourcing and measurement. The modules both include a built-in 18-bit digitizer that takes 200 ksamples/s.
A key feature of these SMUs is something Agilent calls seamless current measurement. This means that the modules can change measurement ranges in mid-measurement without glitching, losing no data in the process. As a dynamic waveform changes in current, the instruments always position themselves in the best range for the current at any point in time. Thus, the 18-bit digitizer effectively acts as though it were a 28-bit digitizer, says Bob Zollo, Agilent’s power-product planner.
“The benefit is in measuring dynamic currents in power-management schemes,” says Zollo. “These SMUs can measure a 1,000,000:1 dynamic range in current—from microamps to amps in a single scope trace.”
So who is likely to be interested in these SMUs? In the case of the N6781A battery-drain analyzer, anyone building a battery-powered wireless system (MP3 players, satellite radios, wireless computers, e-book readers, GPSs, or mobile handsets) would want to use it to determine battery lifespan. This can be accomplished either by removing the battery from the system and using the SMU in its place or by using the instrument as an ammeter to determine how long it runs on the actual system battery. Either way, the goal is stability and measurement of the dynamic current waveforms.
The N6782A functional-test SMU is for designers who are looking to create dynamic operating conditions. “You can use it to put sag on a dc-dc converter input and see how much of that sag reaches the output,” says Zollo. The SMU can be used to test power amplifiers, power-management circuits, or dc-dc converters, measuring dynamic loading on outputs and providing quick modulation of output voltages or currents.
The N6781A two-quadrant SMU for battery-drain analysis costs $5300 each, while the N6782A two-quadrant SMU for functional test sells for $4320 each. Agilent’s 14585A control and analysis software is bundled in at no charge. The N6705B dc power-analyzer mainframe for housing the modules costs $6908. All are available now.