How to deal with short battery life in wireless sensors

The DMM makes measurements by inserting a shunt in the circuit and measuring the voltage drop across it. Normally to measure low current, you choose a low range based on a shunt with high resistance; to measure high current you choose a high range based on a low-resistance shunt.

The voltage drop is also called burden voltage. Due to this voltage drop, not all the battery voltage reaches the wireless sensor. Most accurate low ranges for sleep current measurements have burden voltage during current peaks that may even cause the device to reset.

Practically, we end up compromising and using a high current range that keeps the device operating during current peaks. This compromise enables us to handle peak current and measure the sleep current, but at a high price. As the offset error is specified on range full scale, it heavily impacts measurements on low current levels.

Its error contribution can be 0.005% error on 100mA range = 5µA, which is a 50% error on 10µA or 500% error on a 1µA current level. This current level is where the device spends most of its time, so this error has a huge impact on the battery life estimation.

After measuring the sensor's low current level during sleep mode, we have to measure the active and transmission pulses. Measurements need to include both the current level and the time the sensor spends at that level.

Oscilloscopes are excellent tools for measuring signals changing over time. However, we need to measure current in the 10's of mA level, and current probes do not do a good job there due to their limited sensitivity and their drift. Good clamp probes have 2.5mArms noise, and the zero compensation procedure needs to be repeated often.

Current probes measure the electric field over a wire, so the trick to increase sensitivity is to pass the same wire multiple times so we multiply the magnetic field—this multiplies the current readout, enabling us to measure the current a bit better. With this approach, we can capture the current pulse of the activity and the transmission time.

Even within the activity and transmission, the current changes levels: it looks like a train of high and low levels. To properly calculate the average current the waveform needs to be exported and all the measured points need to be integrated to get the average value.

Oscilloscopes do a good job of capturing a single burst. However, the measurements are more complex if we want to verify how many times the sensor activates in a timeframe and how often it sends out a TX burst. Oscilloscopes can easily do a good job with measurements taken over the short term, but sensors may have operational cycles of minutes or hours, which can be complex to capture and measure.

Measurement innovations

The Keysight N6781A source/measure unit (SMU) for battery drain analysis overcomes the limitations of traditional measurements with two innovations: seamless current ranging and long-term gap-free data logging. The SMU is a module that can be used with the Keysight N6700 low-profile modular power system or N6705 DC power analyser.

The seamless current ranging is a patented technology that enables the SMU to change the measurement range while keeping the output voltage stable without any dropout due to ranging. This feature enables you to measure the peaks with high current ranges and measure the sleep current with the 1mA FS range, which has 100nA of offset error. This low offset error (100nA offset error is 10 per cent at 1µA or 1 per cent at 10µA), orders of magnitude better than a traditional DMM.

Figure 2: The Keysight N6781A SMU allows accurate measurements across dynamic current levels.

The seamless current ranging is combined with two digitizers to measure voltage and current with simultaneous sampling at 200kSa/s (5µs time resolution). Digitised measurements can be captured over 2 seconds and displayed with full time resolution and proportionally longer time with lower resolution.

However, for long-term measurements, the internal data logger in the Keysight N6705B modular DC power analyser integrates the 200kSa/s measurements over a user-specified integration period (20µs to 60 seconds) without losing any samples between the integration periods.

As the data logger is gap-free, all the samples fall in one integration period or in the next one—no samples are lost. With the data logger, engineers can now measure the current and energy drain performance of a wireless sensor for up to 1,000 hours of operation.

Figure 3: Data logger: all the samples are integrated in consecutive sample periods. No samples are lost. For every sample period, min and max values are also available.

Measuring the sleep current is just a matter of placing the markers and directly reading out the values provided. The measurement in Figure 4 is made with a single acquisition over a long period of time; we get the complete picture of the current drain as well as an accurate measurement of the sleep current at 599nA.

Figure 4: Recorded current drain over 200 seconds of operation provides new insight into a device's dynamic current drain.

With pan and zoom capability, it's possible to look at the current level and time spent at every power level. Details that traditional measurement tools do not see can now be identified and measured.

A clear example is the trailing pulses identified by "???" in Figure 4. The software revealed this surprise: the device drain pulsed energy at approx. 90µA peaks for 500ms for an average current of 3.3µA.

   1 • 2 • 3