Source Measure Units tame challenges of assessing battery drain, functional test

Santa Clara, Calif.—Measuring the performance of battery-powered units used to be easy: measure the current via a resistive or inductive shunt pick-off while monitoring the terminal voltage, end of story. Since the drain was fairly steady, this method would work reasonably well. For checking power supplies, you needed just some line/load variation (sags, droops, and similar) to assess their performance.

But today's battery-operated devices and their supplies don’t allow for a "simple" approach, as they constantly and quickly shift in and out for various power-down, idle, sleep, inactive, and active modes. Result: you have to measure the current drain over an extremely wide dynamic range and with fast transients.

In a complementary vein, when testing DC/DC converters, power management units (PMUs), even RF power amplifiers (PAs), you need to stimulate the input stage of the device under test (DUT) with various sourcing waveforms; and/or load the output stage using its electronic load capability. These are challenging requirements.

Two source measure units (SMU) from Agilent Technologies address both situations:

• The N6781A 2-quadrant unit is designed for battery-drain analysis and capturing of power-consumption data.
• Its sibling, the N6782A, is a 2-quadrant unit for functional test which lets you modulate its output at up to 100 kHz, suitable for testing DC/DC converters, power management units (PMUs), even RF power amplifiers (PAs) (which it does by stimulating the input stage of the device under test (DUT) with various sourcing waveforms; and/or loading the output stage using its electronic load capability.

The 2-quadrant operation allows the SMUs to act as voltage or current sources, or constant-voltage or constant-current loads. (And if you need a "curve tracer" for device parametric test, the N6782A can do that, as well.) Both units have 4-channel capability, color-coded to minimize unpleasant cabling and channel-identification errors (to which power-related tests often react badly).

 

Figure 1: Agilent N6781A Source Measure Unit
(Click on image to enlarge)
Key to the operation of these units is the glitch-free sourcing and input-digitizing measurement function. The latter can seamlessly sweep over a 28-bit dynamic range using an 18-bit, 200 kHz (5 μsec) digitizer, but not incur any reading glitches nor lose readings. This range is needed because wireless handset may quickly switch from full-transit mode (roughly 3 A drain) to standby mode (100 mA) to sleep mode (1 mA) and you want to measure with meaningful precision in each zone (think " GSM pulse").

 

Figure 2: Wide-dynamic-range measurement with seamless ranging, needed for sleep-to-active mode shifting (and reverse)
(Click on image to enlarge)
I saw the N6781A unit and had a hands-on demonstration, and here's one more important take-away: in addition to the easy-to-use front panel and user interface, it has a big, red "emergency stop" button on the front panel's upper-right corner. So as your MOSFET begins to smoke, you hit this button, and all output power is shut off, while the remainder of the instrument's functionality remains active and provides all data it has captured.

While both units can operate as standalone instruments, they also integrate with control and analysis software from Agilent which allows for control of multiple instruments (up to 16 supplies), data analysis, waveform creation for downloading, data logging, and networking.–Bill Schweber

Pricing and availability: the units are available now. The N6781A is $5300; the N6782A is $4230, not including power models. Agilent offers over twenty DC power modules (N6700 family), ranging up to 20 A.

For more information:

• On the Agilent N6781A and N6782A SMUs: http://www.agilent.com/find/n6780
• For a video describing the seamless measurement ranging (28 bits from an 18-bit converter, with no missed readings or glitches): http://www.agilent.com/find/n6780_video