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Managing battery life of IoT wireless sensors

Posted: 12 Aug 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Wireless sensors? battery? RF power amplifier? DMM? oscilloscope?

The different operating modes result in a current drain that spans a wide dynamic range from sub-?A to 100 mA, which is a ratio on the order of 1:1,000,000.

Traditional measurement techniques and their limitations
A well-known method for measuring current is to use the ammeter function of a DMM. The accuracy of current measurements made with modern digital DMMs looks good, but specifications are defined for fixed ranges and relatively static signal levels, which isn't exactly the situation on a wireless sensor due to its dynamic current drain.

The DMM is connected in series between battery and device to measure the current. From time to time we see some reading instabilities due to the sensor's active cycle or even the transmit mode.

We know that DMMs have multiple ranges, and with auto range it should be able to select the most appropriate range and give the best accuracy. However, DMMs aren't ideal. The auto range takes time to change range and settle the measurement results. Time to auto-range is often 10 to 100 ms, longer than transmission or active modes times. For this reason, the auto-range function needs to be disabled and the user needs to manually choose the most appropriate range.

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 100 mA range = 5?A, which is a 50% error on 10?A or 500% error on a 1-?A 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 2.5-mArms 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 C 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 1-mA FS range, which has 100 nA of offset error. This low offset error (100-nA offset error is 10% at 1?A or 1% at 10?A), orders of magnitude better than a traditional DMM.

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

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