Time for Instrumentation Designers to “Step Up” to Help Engineers Characterize Low Power Devices
Because battery life has long been a major, competitive selling point for wireless products of all kinds, design engineers always strive to reduce the power consumption of the products they create. That includes extending the operating time of today’s ever-smaller products, including “wearables” like smartwatches, various physiological monitors, and wearable insulin pumps; “implantables” like pacemakers and defibrillators; and the myriad sensors and controllers that make up the Internet of Things (IoT).
Because battery life has long been a major, competitive selling point for wireless products of all kinds, design engineers always strive to reduce the power consumption of the products they create. That includes extending the operating time of today’s ever-smaller products, including “wearables” like smartwatches, various physiological monitors, and wearable insulin pumps; “implantables” like pacemakers and defibrillators; and the myriad sensors and controllers that make up the Internet of Things (IoT).
For engineers, the good news is that the power consumption of modern devices is lower than earlier ones by orders of magnitude. For example, the venerable 741 op amp, developed in the late 1960s, originally drew about 5–10mA of supply current; in contrast, the more modern LPV251 op amp draws just 351nA. The bad news is that characterizing the power consumption of today’s devices is significantly more challenging than in the past.
Meeting this challenge requires instrumentation capable of addressing two completely different sets of test requirements: (1) making highly accurate low-current measurements when the device is in its sleep and standby modes, (2) making extremely fast measurements during the short period when the device is in its active mode or when transmitting data. As an industry, however, instrumentation manufacturers haven’t done nearly enough to deliver equipment optimized for these test challenges. To understand what engineers need from their vendors, it’s important to understand the types of signals they must measure.
Reaching down into sleep mode
Today’s devices often have sleep or standby currents of a micro-amp or less. Characterizing some of these devices requires measuring these currents with a resolution of a tenth of a micro-amp or even less.
Producing stable, accurate measurements at such low levels generally requires making many measurements over a long measurement interval, then calculating their average as a way of digitally filtering the electrical noise created in the device and noise from the external environment. Good old analog filtering is another option for reducing noise and ensuring quality measurements. Using these techniques does extend measurement time, but the time trade-off is necessary to achieve a good quality low current measurement.
Capturing high-speed peak load currents
Making accurate low-current measurements is only half the battle. Wireless and medical devices draw relatively high currents in their active states or when transmitting data. A defibrillator, for example, may draw a peak load current of up to 5A, although only for a few hundred microseconds. To measure this current pulse, the instrument must respond quickly and make the measurement while the device is still active. Making this high-speed measurement requires sacrificing some degree of accuracy and resolution.
Figure 1 shows a typical load current profile for an IoT device. In sleep mode, the current is very low, but when the device is transmitting, the load current jumps sharply for a short time. To measure this current, the instrumentation must be capable of responding to a trigger control signal corresponding to the device’s transition into the active state.
Figure 1. A typical load current profile for an IoT device includes long periods of low current consumption interrupted by short bursts of high current consumption when the device is active or transmitting data. The instrumentation used to measure this load current must be able to respond quickly to a trigger signal indicating that the device is transitioning from its standby mode to its active mode, delay for an appropriate time to allow the overshoot to stabilize, then make the high current measurement quickly.
Typically, there will be some overshoot before the load current settles to a stable value. To avoid incorporating the overshoot into the measured value, the test system must be able to delay the start of the measurement. Once started, the measurement must be fast enough so that it complete before the load current starts to drop, which would also skew the measurement.
Because making accurate low-current measurements and making very fast high-current measurements are so different, many engineers think they need multiple instruments for these differing test requirements. To make the low-current measurement, they might connect a sense resistor in series with the test lead that connects a power supply to the device-under-test and measure the voltage across the sense resistor with a DMM. If the sense resistor were small enough, it would add only a small additional error in the load current measurement. However, choosing a load resistor with too small a value would result in a voltage that the DMM might not be able to measure with sufficient precision.
Here’s another problem with that scenario: measuring a fast, high-current, active mode load current accurately with a DMM is nearly impossible. It would require the DMM to make the measurement very quickly and start the measurement at just the right time, which would require an external trigger. The DMM would have to delay the start of the measurement to ensure it was measuring the appropriate portion of the load current pulse and be fast enough to measure that portion accurately.
Although an oscilloscope is an appropriate instrument to capture a waveform such as a short, active-mode load current pulse, the oscilloscope does not offer sufficient resolution to quantify the load current peak accurately nor can the oscilloscope measure low level values. To make all the necessary measurements, a power source, a DMM, and an oscilloscope might be required, but even that solution may not yield the best results.
A source measure unit (SMU) instrument could be another possible solution for this application. They can measure very low currents (down to pico-amps or less) accurately; unfortunately, they are not designed to capture short pulses. Also, they are generally low power instruments and so might not have sufficient total power to deliver the peak current necessary to characterize a device that draws a large amount of peak power such as an implantable defibrillator. In addition, because of their extraordinary sensitivity, SMU instruments can be relatively expensive.
Simplify the problem with a single-instrument solution
Although it would be possible to make all the necessary measurements using some combination of the instruments described above with some software to control triggering and timing, a single instrument that could do it all would be a far better solution. Unfortunately, few developers have stepped up to the challenge of creating an instrument that can provide sufficient power to energize a wide range of devices without sacrificing the ability to measure both very low load currents and much higher active load currents accurately and with high resolution. Such instruments are only now starting to emerge on the market in the form of power supplies with integrated precision DC measurement capabilities (Figure 2).
For anyone considering a precision power supply as a solution for this application, if the instrument is going to be used in an automated test system in addition to the designer’s bench, it’s essential that it offers the LAN, USB, or GPIB interfaces and digital inputs and outputs needed to integrate it with other equipment.
Figure 2. The Series 2280S Power Supplies can measure load currents from 100 nano-amps to 6 amps very accurately. A choice of four load current measurement ranges—10 amps, 1 amp, 100 milli-amps, and 10 milli-amps—allows measuring both full load currents and standby/sleep mode currents with 6½ digits of resolution.
What capabilities should instrument developers integrate into their products to satisfy the needs of engineers characterizing the next generation of low-power devices? To measure very low standby or sleep mode currents, they must be able to make DMM-quality measurements with up to 6½ digits of resolution. When making high current measurements, they must be able to capture current pulses as short as hundreds of microseconds. Also, because many of these devices have a power-up load sequence and a power-down sequence, similar to the one shown in Figure 3, the next generation of instruments must be able to make multiple, synchronized measurements at each state of the power-up or power-down cycle. Instrument suppliers are beginning to offer general-purpose power supplies with advanced display capabilities, including built-in graphing functions that simplify monitoring the stability of the load current, capturing and displaying a dynamic load current, or viewing a start-up or turn-off load current.
Figure 3. As a device is powered up, the load current increases as it cycles through the sleep, standby, and active modes. To characterize this start-up sequence, the test setup must be capable of making fast, synchronized current measurements.
