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Test & Measurement Fig. 2: The Keysight N6781A SMU allows accurate measurements across dynamic current levels. However, when the device is in real operation, the battery life is typically shorter than the number you calculated. The most common comment is: “the battery quality is bad.” Representatives for big battery brands will offer detailed specifications and explain that among batteries of the same type, it is common to have capacity variations of 5 to 10 percent. But even using conservative battery capacity estimates, battery life typically falls short. The device dies before it is expected to. Why does this happen? Did we correctly estimate energy usage? Probably not. Let’s explore the problem. The complexity of measuring dynamic current drain In battery-powered devices like wireless sensors, to save energy the device sub-circuits are active only when required. Engineers design the device to spend most of its time in a sleep mode with minimum current drain. During sleep mode, only the real-time clock operates. The unit then wakes up periodically to perform measurements. The acquired data is then transmitted to a receiving node. The different operating modes result in a current drain that spans a wide dynamic range from sub-μA 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 μA, which is a 50% error on 10 μA or 500% error on a 1-μA 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 – 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 Fig. 4: Recorded current drain over 200 seconds of operation provides new insight into a device’s dynamic current drain. Fig. 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. 34 Electronic Engineering Times Europe October 2015 www.electronics-eetimes.com


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